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Slowconformationaldynamicsofthe guanine
nucleotide-binding proteinRascomplexedwiththe GTP
analogue GTPcS
Michael Spoerner
1
, Andrea Nuehs
1
, Christian Herrmann
2
, Guido Steiner
1
and
Hans Robert Kalbitzer
1
1 Universita
¨
t Regensburg, Institut fu
¨
r Biophysik und physikalische Biochemie, Germany
2 Ruhr Universita
¨
t Bochum, Physikalische Chemie I, Germany
Guanine nucleotide-binding proteins oftheRas super-
family function as molecular switches, cycling between
a GDP-bound ‘off’ and a GTP-bound ‘on’ state. They
regulate a diverse array of signal transduction and
transport processes.
It has been shown using
31
P NMR spectroscopy
that Ras (rat sarcoma) protein occurs in two con-
formational states (state 1 and 2) when complexed
with theGTP analogues guanosine-5¢-(b,c-imido)tri-
phosphate (GppNHp) [1] or guanosine-5¢-(b,c-methy-
leno)triphosphate (GppCH
2
p) [2]. These two states
interconvert with rate constants in the millisecond
time scale. They are characterized by typical
31
P NMR chemical shifts, with shift differences up to
0.7 p.p.m. NMR structural studies have shown that
this dynamic equilibrium comprises two regions of
Keywords
conformational equilibria; GTP analog;
GTPcS; Ras
Correspondence
H. R. Kalbitzer, Institut fu
¨
r Biophysik und
physikalische Biochemie,
Universita
¨
tsstraße 31, Regensburg,
D-93040, Germany
Fax: +49 941 943 2479
Tel: +49 941 943 2595
E-mail: hans-robert.kalbitzer@biologie.
uni-regensburg.de
(Received 28 July 2006, revised 13 Novem-
ber 2006, accepted 8 January 2007)
doi:10.1111/j.1742-4658.2007.05681.x
The guaninenucleotide-bindingproteinRas occurs in solution in two
different conformational states, state 1 and state 2 with an equilibrium
constant K
12
of 2.0, when theGTPanalogue guanosine-5¢-(b,c-imido)tri-
phosphate or guanosine-5¢-(b,c-methyleno)triphosphate is bound to the
active centre. State 2 is assumed to represent a strong binding state for
effectors with a conformation similar to that found for Rascomplexed to
effectors. In the other state (state 1), the switch regions ofRas are most
probably dynamically disordered. Ras variants that exist predominantly in
state 1 show a drastically reduced affinity to effectors. In contrast, Ras(wt)
bound to theGTPanalogue guanosine-5¢-O-(3-thiotriphosphate) (GTPcS)
leads to
31
P NMR spectra that indicate the prevalence of only one con-
formational state with K
12
> 10. Titration withthe Ras-binding domain of
Raf-kinase (Raf-RBD) shows that this state corresponds to effector binding
state 2. In theGTPcS complex ofthe effector loop mutants Ras(T35S) and
Ras(T35A) two conformational states different to state 2 are detected,
which interconvert over a millisecond time scale. Binding studies with Raf-
RBD suggest that both mutants exist mainly in low-affinity states 1a and
1b. From line-shape analysis ofthe spectra measured at various tempera-
tures an activation energy DH
|
1a1b
of 61 kJÆmol
)1
and an activation entropy
DS
|
1a1b
of 65 JÆ K
)1
Æmol
)1
are derived. Isothermal titration calorimetry on
Ras bound to the different GTP-analogues shows that the effective affinity
K
A
for the Raf-RBD to Ras(T35S) is reduced by a factor of about 20 com-
pared to the wild-type withthe strongest reduction observed for the GTPcS
complex.
Abbreviations
GppCH
2
p, guanosine-5¢-(b,c-methyleno)triphosphate; GppNHp, guanosine-5¢-(b,c-imido)triphosphate; GTPcS, guanosine-5¢-O-(3-
thiotriphosphate); ITC, isothermal titration calorimetry; Raf-RBD, Ras-binding domain of Raf-kinase; Ras, protein product ofthe proto
oncogene ras (rat sarcoma).
FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS 1419
the protein called switch I and switch II [1,3,4]. Solid-
state NMR shows that even in single crystals or crys-
tal powders of Ras(wt)•Mg
2+
•GppNHp the two
conformational states can be observed to be in dyna-
mic equilibrium at ambient temperatures [5,6].
A threonine residue located in the effector loop
(Thr35 in Ras) is conserved in all members of the
Ras superfamily and seems to play a pivotal role in
the conformational equilibrium. It is involved, via its
side-chain hydroxyl, in the coordination ofthe diva-
lent metal ion and, via its main-chain amide, in a
hydrogen bond withthe c-phosphate ofthe nucleo-
tide when complexed to the effector [7,8]. The same
coordination pattern is most probably preserved in
state 2 of free Ras. Replacing this threonine in Ras
with an alanine or serine residue leads to a complete
shift ofthe equilibrium towards state 1 in solution,
when Ras is bound to theGTP analogues GppNHp
[9] or GppCH
2
p [2]. These Ras variants, previously
used as partial loss-of-function mutants in cell-based
assays, show a reduced affinity between Ras and
effector proteins without Thr35 being involved in
any interaction. X-Ray crystallography [9] on
Ras(T35S)•Mg
2+
•GppNHp and EPR investigations
[10] suggest that switch I and switch II exhibit high
mobility in state 1. Recently, X-ray structures of
M-Ras [11] and ofthe G60A mutant of human
H-Ras [12], both in the GppNHp-bound form, were
published. These Ras variants seem to exist in
conformational state 1, as shown using
31
P NMR
spectroscopy. In the X-ray structure the contacts of
Thr35 (Thr45 in M-Ras) withthe metal ion and the
c-phosphate group do not exist.
31
P NMR data indi-
cate that state 2 corresponds to the conformation of
Ras found in complex withthe effectors. State 1,
characteristic ofthe mutants Ras(T35S) and
Ras(T35A) in the GppNHp form, represents a weak-
binding state oftheprotein [9,13]. Upon addition of
the Ras effector Raf-kinase, the
31
P NMR lines of
Ras(T35S) but not Ras(T35A) shift to positions cor-
responding to the strong binding conformation of
the protein [9].
A conformational equilibrium in the interaction site
with effectors seems to be a general property of Ras
and other small GTPases [14]. The equilibrium is influ-
enced not only by specific mutations but also by the
nature oftheGTPanalogue bound (GppNHp or
GppCH
2
p). In this study we investigate the dynamic
behaviour ofRas in complex with guanosine-5¢-O-(3-
thiotriphosphate) (GTPcS), another commonly used
GTP analogue that is hydrolysed slowly to find more
evidence for the biological importance ofthe conform-
ational equilibria.
Results
Chemical shifts ofthe nucleotide analogue
GTPcS in the absence and in the presence of
magnesium ions
Chemical shift values for the phosphates and the thio-
phosphate group ofthe nucleotide depend strongly on
the degree of protonation of their oxygens. Further-
more, chemical shifts and pK values are influenced by
Mg
2+
binding to the protein–nucleotide complex. For
a better interpretation ofthe chemical shifts of the
protein-bound nucleotide analogue we first studied
GTPcS in the presence and absence of Mg
2+
ions
within a pH range of 2–13. The rate of exchange
between Mg
2+
and the nucleoside triphosphate is slow
enough to observe the resonances ofthe metal-free
form separately from the metal-complexed form at
lower temperatures. Therefore, experiments were per-
formed at 278 K to ensure that over the whole pH
range a significant contribution of metal-free nucleo-
tide, if existing, could be directly detected by addi-
tional resonance lines. At a magnesium concentration
of 3 mm the nucleotide is completely saturated with
the divalent ion in the pH range studied since further
increase ofthe Mg
2+
concentration does not influence
the observed chemical shifts (also see Experimental
procedures).
Figure 1 shows the titration curves for GTPcSin
the absence and presence of Mg
2+
. Separation of the
three phosphate signals by more than 60 p.p.m. is
rather large. Particularly in case ofthe c-phosphorus
(Fig. 1A,B) two pK values are necessary in order to
describe the observed dependence of chemical shifts in
the pH range studied. The corresponding pK values
and chemical shifts are summarized in Table 1 together
with the data for the analogues GppNHp and
GppCH
2
p [2]. As expected, the apparent pK values
decrease substantially in the presence ofthe metal ion.
By far the largest effect on the chemical shifts is found
for the b- and c-phosphate group, but a slight shift of
0.6 p.p.m. is also seen for the a-phosphorus resonance
in the Mg
2+
•GTPcS complex. In agreement with pre-
vious studies on ATP [15], our data suggest a mixture
of different metal complexes in solution with a high
population of complexes where the b- and c-phosphate
is involved, as shown previously for theGTP ana-
logues GppNHp and GppCH
2
p [2]. The pK
3
values in
GTPcS are much smaller than those reported for
GppNHp and GppCH
2
p. The value of pK
2
does not
depend much on theanalogue when a relatively large
error is taken into consideration. pK
2
and pK
3
are
usually associated withthe first and the second
Conformational dynamicsofRas bound to GTPcS M. Spoerner et al.
1420 FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS
deprotonation step at the c-phosphate group of the
nucleotide for the transition from the threefold negat-
ively charged state to the fourfold negatively charged
state. In line with this suggestion the largest shifts are
observed for the c-phosphate group for the first
deprotonation step for the three analogues. However,
the second deprotonation step is associated with larger
changes in the b-phosphate shifts in GppNHp and
GppCH
2
p, indicating a more complex pH perturbation
of the electronic system in these analogues.
Fig. 1. Titration curves of free and Mg
2+
bound GTPcS. (A,C)
31
P chemical shift val-
ues ofthe a-, b- and c-phosphate groups
were determined on a 2.5 mL of a 1 m
M
GTPcS solution in 100 mM Tris, 95% H
2
O
and 5% D
2
O containing 0.1 mM 2,2-dimeth-
yl-2-silapentane-5-sulfonate for indirect refer-
encing. The pH was adjusted by adding HCl
or NaOH. Measurements were performed in
a 10-mm sample tube at 278 K. (B,D) Meas-
urements on the Mg
2+
complexes were per-
formed in the presence of 3 m
M MgCl
2
. The
dependence of chemical shifts on the pH
values was fitted to Eqn (7). The
31
P reso-
nances were assigned by selective
1
H- and
31
P-decoupling experiments.
Table 1. pH dependence of chemical shifts of different GTP analogues. Data were recorded at 278 K in solutions of 1 mM nucleotide in the
absence or presence of 3 m
M MgCl
2
in 95% H
2
O ⁄ 5% D
2
O. In a first approximation d
2
, d
3
, and d
4
correspond to the chemical shifts of two-,
three-, and fourfold negatively charged nucleotide. pK
2
and pK
3
are the corresponding pK
a
values ofthe three phosphates ofthe nucleotide.
d
2
values are given in parentheses the titration up to pH 1.5 does not allow the precise estimation of this value. For d
3
and d
4
the estimated
error is ± 0.05 p.p.m.
Nucleotide
Phosphate
group d
2
⁄ p.p.m. pK
2
d
3
⁄ p.p.m. pK
3
d
4
⁄ p.p.m.
GTPcS a ()11.3) ) 11.30 ) 11.04
b ()24.0) 2.8 ± 0.1 ) 24.0 5.78 ± 0.02 ) 23.06
c (40.8) 39.70 33.91
Mg
2+
•GTPcS a ()11.2) ) 11.27 ) 10.67
b ()24.2) 1.7 ± 0.5 ) 23.78 ) 20.51
c (41.6) 40.38 4.11 ± 0.02 36.85
GppCH
2
p
a
a ()10.86) ) 10.93 ) 10.82
b (7.14) 3.2 ± 0.15 8.74 8.96 ± 0.02 13.22
c (17.85) 14.63 6.57 ± 0.02 11.23
Mg•GppCH
2
p
a
a ()10.83) ) 10.47 ) 10.33
b (9.50) 2.3 ± 1.5 9.93 14.93
c (16.98) 14.29 11.46
GppNHp
a
a ()10.95) ) 10.80 8.79 ± 0.02 ) 10.55
b ()12.27) 3.4 ± 0.04 ) 10.91 ) 7.76
c (0.20) ) 1.64 ) 0.91
Mg•GppNHp
a
a ()11.17) ) 10.34 6.56 ± 0.02 ) 10.01
b ()9.36) 2.0 ± 0.8 ) 8.95 ) 5.46
c ()1.38) ) 2.16 ) 1.02
a
Data from Spoerner et al. [2].
M. Spoerner et al. ConformationaldynamicsofRas bound to GTPcS
FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS 1421
Conformational states ofRascomplexed with
Mg
2+
•GTPcS
Figure 2 shows
31
P NMR spectra of Ras(wt) in com-
plex withthe slowly hydrolysable GTP analogue
GTPcS at various temperatures. Assignment ofthe res-
onance lines was confirmed by a 2D
31
P–
31
P NOESY
experiment on Ras(wt)•Mg
2+
•GTPcS (data not
shown). Binding ofGTPcS to theRasprotein leads to
rather large chemical shift changes. In contrast to the
observations made for theGTP analogues GppNHp
and GppCH
2
p [1,2] only one set of resonances can be
observed for the wild-type protein in the temperature
range 278–308 K (Fig. 2). This most probably means
that wild-type Ras occurs predominantly in one state
when GTPcS is bound. It is reasonable to assume that
a second structural state also exists and is character-
ized by different chemical shift values, as observed in
the GppNHp and GppCH
2
p complexes [1,2]. When
this second state has clearly different chemical shifts
compared withthe first state then two scenarios are
consistent withthe observed spectrum. If fast exchange
conditions prevail over the whole temperature range,
then only one averaged resonance signal per phosphate
group would be observed. If slow exchange conditions
prevail, a second conformational state, characterized
by clearly different chemical shifts, must have a rather
low population because no signals can be detected
above noise level. In this case, from the signal-to-noise
ratio the equilibrium constant for the two states can
be estimated to be > 10. Analysing the temperature
dependence ofthe line width, particularly of the
c-phosphorus resonance, slow exchange conditions are
more likely. At lower temperatures the line width
decreases with increasing temperature due to the
decrease ofthe rotational correlation time. At higher
temperatures the line width increases again (51 Hz at
298 K, 57 Hz at 303 K). Chemical shift also changes
within the temperature range of 278–308 K by
+0.26 p.p.m. At higher temperatures, theGTP ana-
logue hydrolyses, and resonances of Ras-bound GDP
are thus detected. In principle, one would expect to
observe thiophosphate and Ras-bound GDP as result
of GTPcS hydrolysis. In contrast, with all the meas-
urements performed in this study, inorganic phosphate
could be observed only using
31
P NMR. In addition,
H
2
S could be detected by its smell after a time. The
exact mechanism of thio phosphate decay could not be
clarified. It is dependent on the presence of Ras, but
may be also due to other protein impurities occurring
in low concentrations in theRas preparations. In con-
trast to the situation observed for wild-type protein in
the complex ofGTP cS withthe mutant Ras(T35S) or
Ras(T35A), additional
31
P NMR lines are found at
low temperature (Fig. 3A). With increasing tempera-
ture, the lines initially become broader before coales-
cing again at higher temperature (Fig. 4A). From our
studies with GppNHp and GppCH
2
p we expect that
the effector interaction state 2 becomes destabilized by
replacing Thr35 with a serine or an alanine residue,
and therefore at least one ofthe new lines seen in the
mutant is likely to correspond to state 1. Because no
component ofthe two sets of resonances of Ras(T35S)
and Ras(T35A) has a chemical shift that corresponds
to that of Ras(wt) it is not clear whether the two sets
of resonance lines correspond to state 1 and state 2 or
if they represent two substates of state 1 (see below).
In the following, we call them state 1a and state 1b.
The equilibrium constant K
1a1b
¼ [1b] ⁄ [1a] between
these two states is 0.5. In the case ofthe serine mutant,
a weak third line ofthe c-phosphorus signal with a
similar chemical shift to the resonance of wild-type
Ras seems to exist (Fig. 3A); this is not visible in the
spectrum ofthe T35A mutant. The chemical shifts are
summarized in Table 2.
With knowledge ofthe resonance positions corres-
ponding to state 1a and 1b, we investigated whether
these states also exist in wild-type Ras bound to
GTPcS. Separation ofthe chemical shift values
between state 1b and state 2 of more than 4 p.p.m.
allowed us to perform a saturation-transfer experiment
with presaturation at frequencies around the signal
corresponding to state 1b. If exchange occurs over a
Fig. 2.
31
P NMR spectra of wild-type Rascomplexed with
Mg
2+
•GTPcS at various temperatures. The samples contained
1m
M Ras(wt)•Mg
2+
•GTPcSin40mM Hepes ⁄ NaOH pH 7.4,
10 m
M MgCl
2
, 150 mM NaCl, 2 mM 1,4-dithioerythritol and 0.1 mM
2,2-dimethyl-2-silapentane-5-sulfonate in 5% D
2
O, 95% H
2
O,
respectively. The absolute temperature was controlled by immer-
sing a capillary with ethylene glycol and measuring the hydroxyl-
methylene shift difference [28].
Conformational dynamicsofRas bound to GTPcS M. Spoerner et al.
1422 FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS
timescale < T
1
a decrease in the integral ofthe reson-
ance corresponding to state 2 should be observed, even
when state 1 is too sparsely populated to be detectable
directly. Some results are shown in Fig. 3B. A mini-
mum ofthe resonance integral of state 2 is obtained at
a presaturation frequency of 32.7 p.p.m., which corres-
AB
Fig. 3. Conformational equilibria of wild-type Ras and Ras mutants complexedwith Mg
2+
•GTPcS. (A) The sample contained 1 mM
Ras(wt)•Mg
2+
•GTPcS (lower), 1.2 mM Ras(T35S)•Mg
2+
•GTPcS (middle), and 1 mM Ras(T35A)•Mg
2+
•GTPcS (upper) in 40 mM Hepes ⁄ NaOH
pH 7.4, 10 m
M MgCl
2
, 150 mM NaCl, 2 mM 1,4-dithioerythritol and 0.1 mM 2,2-dimethyl-2-silapentane-5-sulfonate in 5% D
2
O, 95% H
2
O,
respectively. Data were recorded at 278 K. The assignment was determined by a
31
P–
31
P NOESY experiment on Ras(wt)•Mg
2+
•GTPcS.
31
P
resonances assigned to Ras–nucleotide complex in conformation of state 1a or state 1b are coloured in red, the resonances assigned to
state 2 are coloured green. (B)
31
P NMR saturation transfer experiment on Ras(wt)•Mg
2+
•GTPcS. The integrals ofthe resonance correspond-
ing to the c-thiophosphate group in state 2 of Ras(wt) are given in dependence ofthe frequency of presaturation d. For presaturation a weak
rectangular pulse of 1 s duration and a B
1
-field of 18 Hz were used. A Lorentzian function was fitted to the data. The integral ofthe c-phos-
phorus signal without presaturation is set to 100%.
Fig. 4. Experimental and simulated
31
P NMR data of Ras(T35S)•Mg
2+
•GTPcS at different temperatures. The sample contained 1.2 mM
Ras•Mg
2+
•GTPcSin40mM Hepes ⁄ NaOH pH 7.4, 10 mM MgCl
2
,2mM 1,4-dithioerythritol and 0.1 mM 2,2-dimethyl-2-silapentane-5-sulfonate
in 5% D
2
O, 95% H
2
O. The absolute temperature was controlled by immersing a capillary with ethylene glycol and measuring the hydroxyl–
methylene shift difference [28]. (A) Experimental spectra; (B) simulated spectra. Experimental data were filtered by an exponential filter lead-
ing to an additional line broadening of 5 Hz. Total number of scans per spectrum were 1600–5400. The rate constant for the transition
state 1a to state 1b are indicated. Data were simulated as described in Experimental procedures. The transverse relaxation rates 1 ⁄ T
2
at
278 K (in the absence of exchange) obtained from the data analysis are 251 s
)1
for both state 1a and state 1b ofthe a-phosphate group of
bound GTPcS, 236 s
)1
and 204 s
)1
for the b-phosphate group of bound GTPcS in state 1a and state 1b, respectively, and 189 s
)1
for
state 1a and 1b ofthe bound c-thiophosphate group (values are given with an estimated error of ± 15 s
)1
).
M. Spoerner et al. ConformationaldynamicsofRas bound to GTPcS
FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS 1423
ponds to the frequency of state 1b detected for the two
Thr35 mutants. These results indicate the existence of
state 1b in wild-type Ras, but with a very sparse popu-
lation. A more detailed analysis including calculation
of exchange rates was not possible because ofthe lim-
ited signal-to-noise.
Dynamics oftheconformational exchange
By analysing the temperature dependence of the
31
P NMR data from Ras(T35S)•Mg
2+
•GTPcS
(Fig. 4B) for the transition between substates 1a and 1b
the Gibb’s free activation energy DG
|
, the activation
enthalpy DH
|
and the activation entropy DS
|
can
be determined (Table 3) using a full-density matrix
analysis. The exchange rates obtained are somewhat
higher than that found between states 1 and 2 of
Ras(wt)•Mg
2+
•GppNHp or Ras(wt)•Mg
2+
•GppCH
2
p.
Whereas DG
|
of the exchange in Ras(T35S)•Mg
2+
•GTPcS is equal to that obtained for the other com-
plexes, both DH
|
, and DS
|
are somewhat lower. For the
other nucleotides studied, relaxation times T
2
at 278 K
for the a- and c-phosphate group were quite different
for the two conformational states 1 and 2. We did not
find such large differences between the corresponding
T
2
relaxation times for theconformational states 1a and
1b of Ras(T35S)•Mg
2+
•GTPcS.
Complex of Ras•Mg
2+
•GTPcS with the
Ras-binding domain of Raf-kinase
Addition ofthe Ras-binding domain of Raf-kinase
(Raf-RBD) to Ras(wt)•Mg
2+
•GTPcS leads to line
broadening ofthe resonances (Fig. 5, Table 2), but
only to very small changes in the chemical shifts
(|Dd| £ 0.16 p.p.m). This is in line withthe assumption
that the wild-type protein occurs mainly in conforma-
tional state 2 when theGTPanalogue GTPcSis
bound. Correspondingly, in Ras(T35S)•Mg
2+
•GTPcS,
lines preliminary assigned to states 1a and 1b decrease
in intensity when Raf-RBD is bound, whereas the
intensity of lines located close to those assigned in
wild-type Ras to state 2 increases (Fig. 5, Table 2).
The changes in chemical shift induced by Raf binding
are rather large in the mutant, suggesting that
none ofthe states visible in the spectrum of
Ras(T35S)•Mg
2+
•GTPcS corresponds to state 2 found
in the wild-type protein. Complex formation between
Raf-RBD and Ras(T35A)•Mg
2+
•GTPcS (Fig. 5,
Table 2) leads only to a line broadening ofthe two
lines ofthe c-phosphate group, and not to significant
changes in chemical shift or the relative populations of
the resonances. In particular, the relative intensity of
the downfield-shifted c-phosphorus resonance is not
increased in the presence ofthe effector as would be
expected if it corresponded to effector binding state 2.
Influence oftheGTPanalogue on the affinity
between Raf-RBD and Ras
The affinities of wild-type and (T35S)Ras complexed
with the different GTP analogues GppNHp, GppCH
2
p
and GTPcS to Raf-RBD were determined using isother-
mal titration calorimetry (ITC) at 298 K in a buffer
identical to that used in the NMR spectroscopy
experiments. Within the limits of error, the effective
Table 2.
31
P chemical shifts and conformational states ofRascomplexedwith different GTP analogues. Data were recorded at various tem-
peratures. Shifts were taken from spectra recorded at 278 K. The equilibrium constant K
12
between state 1 and 2 is calculated from inte-
grals ofthe c-thiophosphate resonances defined by K
12
¼ k
12
⁄ k
21
¼ [2]] ⁄ ([1a] + [1b]). State 2 is assigned to the conformation close to the
effector binding state. The error is < 0.03 p.p.m. for the chemical shifts and < 0.1 for the equilibrium constants. ND, not detected.
Ras-complex
a-phosphate b-phosphate c-phosphate
K
12
K
1a1b
d
1
(p.p.m.)
d
2
(p.p.m.)
d
1
(p.p.m.)
d
2
(p.p.m.)
d
1
(p.p.m.)
d
2
(p.p.m.)
Ras(wt)•Mg
2+
•GTPcS )11.30 )16.67 37.01 > 10 ND
b
Ras(T35S)•Mg
2+
•GTPcS )10.70 )17.96
a
)17.22
a
32.73
a
37.89
a
36.87 0.06 0.5
Ras(T35A)•Mg
2+
•GTPcS )10.80 )17.92
a
32.79
a
< 0.05 0.5
)17.19
a
37.91
a
Ras(wt)•Mg
2+
•GTPcS )11.19 )16.55 36.85 > 10 ND
b
+ Raf-RBD
Ras(T35S)•Mg
2+
•GTPcS )11.22 )16.52 36.54 > 10 ND
b
+ Raf-RBD
Ras(T35A)•Mg
2+
•GTPcS )10.50 )17.55 32.48
a
< 0.05 0.5
+ Raf-RBD 37.91
a
a
Chemical shifts in state 1a (lower) and 1b (upper).
b
Values could not be determined since signal cannot be detected.
Conformational dynamicsofRas bound to GTPcS M. Spoerner et al.
1424 FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS
association constant K
A
between wild-type Ras and
Raf-RBD is not influenced by the type of bound ana-
logue (Table 4). However, in all cases, the contributions
of enthalpy and entropy to DG° differ between nucleo-
tide analogues. Although for the Thr35 mutant the error
ranges for the three nucleotide analogues overlap, a dif-
ference in affinities between Ras(T35S) bound to the
analogue GTPcS, where the oxygen between b- and c-
phosphate is still available, and GppCH
2
p may exist. A
significant decrease in K
A
, by a factor of $ 20, is seen,
independent oftheanalogue used when the wild-type
protein is compared with Ras(T35S). The decrease in
affinity is due to changes in DH° and DS°, which partly
compensate.
Discussion
The environment ofthe nucleotide bound to the
protein
NMR spectroscopy very sensitively reports changes in
the environment of a given atom by measuring a
change in its resonance frequency. Whenever chemical
shift changes are visible they indicate that there is a
change in the environment ofthe observed nucleus.
For phosphorus resonance spectroscopy on nucleo-
tides, it is known that two factors mainly determine
chemical shift changes, a conformational strain and
electric field effects polarizing the oxygen atoms of the
phosphate groups. In addition to these direct effects,
long-range effects may occur that are caused by a
structure-dependent change in the anisotropy of the
magnetic susceptibility. Here, ring current effects may
be the most dominant contribution.
We have previously studied the complexes of Ras
using theGTP analogues GppCH
2
p and GppNHp [2],
which differ in the position ofthe b–c-bridging oxygen
by replacing the naturally occurring oxygen either with
an apolar group or a hydrogen-bond donator. We
have now completed the picture using the slowly
hydrolysing GTPanalogue GTPcS, in which the b–c-
bridging oxygen is not affected, but the physicochemi-
cal properties ofthe c-phosphate group are modified.
For a quantitative analysis ofthe chemical shift chan-
ges induced by protein binding it was necessary to
have reliable data for the system not perturbed by
Table 3. Exchange rates and thermodynamic parameters in different Ras–nucleotide complexes. The rate constants k
12
and k
21
(k
1a1b
and
k
1b1a
) were calculated by a line-shape analysis based on the density matrix formalism as described in Experimental procedures. The free acti-
vation energy DG
|
, the activation enthalpies DH
|
, and the activation entropies DS
|
, were calculated from the temperature dependence of the
exchange rates on the basis ofthe Eyring equation. The values for the transition between state 1 and state 2 k
12 and
k
21
are given. The
states are defined as in Table 1. DG
12
or DG
1a1b
is the difference in free enthalpy between state 2 (1b) and 1 (1a). T
2
times given are without
exchange contribution and were obtained from the line shape analysis. The estimated error is ± 0.3 ms.
Protein complex
Temp.
(K)
Exchange rate
constant (s
)1
)_
DG
j
1a1b
DH
j
1a1b
TDS
j
1a1b
DG
1a1b
k
1a1b
k
1b1a
(kJÆmol
)1
) (kJÆmol
)1
) (kJÆmol
)1
) (kJÆmol
)1
)
Ras(T35S)•Mg
2+
•GTPcS 278 70 137 41 ± 2 61 ± 1 18 ± 1 1.56 ± 0.15
a
288 170 330
298 430 810
k
12
k
21
DG
j
12
DH
j
12
TDS
j
12
DG
12
Ras(wt)•Mg
2+
•GppNHp
a
278 80 42 42 ± 5 70 ± 3 28 ± 2 ) 1.48 ± 0.15
288 250 135
298 700 387
Ras(wt)•Mg
2+
•GppCH
2
p
a
278 80 39 41 ± 5 63 ± 3 29 ± 2 ) 1.65 ± 0.15
288 260 131
298 740 391
Relaxation times T
2
(ms) ofthe resonances of
Protein-complex a-phosphate b-phosphate c-phosphate
(1) or (1a) (2) or (1b) (1) or (1a) (2) or (1b) (1) or (1a) (2) or (1b)
Ras(T35S)•Mg
2+
•GTPcS 278 4.0 4.0 4.2 4.9 5.3 5.3
Ras(wt)•Mg
2+
•GppNHp
a
278 5.8 3.9 4.8 4.8 4.1 7.1
Ras(wt) •Mg
2+
•GppCH
2
p
a
278 4.2 4.0 6.4 6.4 3.8 5.2
a
Data from Spoerner et al. [2]. Note that the values given differ somewhat from those given by Geyer et al. [1] because absolute tempera-
ture was controlled independently and the new assignment ofthe signals were considered.
M. Spoerner et al. ConformationaldynamicsofRas bound to GTPcS
FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS 1425
protein binding that we provide here. Although data
had been published previously for free GTPcS [16],
they were measured under different experimental
conditions and the referencing system (external stand-
ard) in particular is not sufficiently reliable for precise
comparisons.
When one compares the chemical shift changes Dd
in the free Mg
2+
–nucleotide complexes (Table 1) with
those induced by protein binding (Table 2) one may
obtain information on the change ofthe environment
of the phosphate groups in the different complexes. In
wild-type Ras in state 2, one finds Dd values of )0.26,
6.39 and 3.10 p.p.m., respectively for the a-, b- and
c-phosphate of GTPcS. The corresponding shift chan-
ges are )1.15, 7.51 and )2.41 p.p.m. for GppNHp and
)2.44, 6.32 and )3.03 p.p.m. for GppCH
2
p. The
a-phosphate groups in the three GTP analogues should
be least influenced by the modifications. In accordance
with this observation, in the absence of protein, their
response to a change in pH (acidity) is very small, only
an upfield shift of < 0.26 p.p.m. is observed when the
c-phosphate group is protonated by a decrease in pH.
After binding to the protein, for all three analogues an
upfield shift between of 0.26 and 2.44 p.p.m. is
observed, indicating that the environmental changes
are qualitatively similar but differ in detail.
Potential phosphate group interactions can be
derived from the published X-ray structures, although
one should be aware that they show differences in
effector loop details that may reflect the occurrence of
different conformational states in solution. Because
NMR data indicate that the interaction ofRas with
Raf-RBD stabilizes the effector loop in a well-
defined, state 2-like conformation, the X-ray structure
of the Ras-like mutant of Rap1A, called Raps
[Rap(E30D,K31E)], complexedwith Mg
2+
•GppNHp
and Raf-RBD [7] can serve as a model.
The most important interactions derived from the
X-ray structure are depicted in Fig. 6. It is assumed to
represent state 2 ofthe protein. Interactions assumed
to be absent in state 2 and ⁄ or weakened (or abolished)
by the replacement of an oxygen atom with a sulfur
Fig. 5. 31p NMR spectra of wild-type Ras and Ras mutants bound
to Mg
2+
•GTPcS in complex with Raf-RBD. Initially the samples con-
tained 1.0 m
M Ras•Mg
2+
•GTPcS (lower), 1.2 mM Ras(T35S)•
Mg
2+
•GTPcS (middle) or 1.0 mM Ras(T35A)•Mg
2+
•GTPcS (upper) in
40 m
M Hepes ⁄ NaOH pH 7.4, 10 mM MgCl
2
, 150 mM NaCl, 2 mM
1,4-dithioerythritol and 0.1 mM 2,2-dimethyl-2-silapentane-5-sulfo-
nate in 5% D
2
O, 95% H
2
O, respectively. A solution of 9.8 mM
Raf-RBD dissolved in the same buffer was added in increasing
amounts. The molar ratios of Raf-RBD ⁄ Ras are 1.5 for Ras(wt) and
2 in the mutant samples. Data were recorded at 278 K.
31
P reso-
nances assigned to Ras–nucleotide complex in conformation of
state 1a or state 1b are coloured red, the resonances assigned to
state 2 are coloured green.
Table 4. Affinities of Raf-RBD to Rascomplexedwith different GTP analogues. The association constant K
A
between Raf-RBD and Ras com-
plexed with different GTP analogues was determined using ITC. Measurements were performed at 298 K in 40 m
M Hepes ⁄ NaOH pH 7.4,
10 m
M MgCl
2
, 150 mM NaCl, 2 mM 1,4-dithioerythritol. Data were analysed using ORIGIN FOR ITC 2.9 assuming a 1 : 1 complex formation [28]
and DG° ¼ G
complex
) G
free
¼ -RTlnK
A
.
Raf-RBD complexed
with
K
A
(lM
)1
)
DG°
(kJÆmol
)1
)
DH°
(kJÆmol
)1
)
TDS°
(kJÆmol
)1
)
Ras(wt)•Mg
2+
•GppNHp 2.50 ± 0.4 )36.5 ± 0.6 )13.4 ± 1.5 23 ± 2.1
Ras(wt)•Mg
2+
•GppCH
2
p 2.50 ± 0.4 )36.5 ± 0.6 )18.4 ± 2.0 18 ± 2.6
Ras(wt)•Mg
2+
•GTPcS 2.44 ± 0.6 )36.4 ± 0.9 )7.5 ± 1.5 29 ± 2.4
Ras(T35S)•Mg
2+
•GppNHp 0.12 ± 0.04 )29.0 ± 0.06 )9.7 ± 1.0 19 ± 1.1
Ras(T35S)•Mg
2+
•GppCH
2
p 0.09 ± 0.04 )28.2 ± 0.06 )15.3 ± 1.5 13 ± 1.6
Ras(T35S)•Mg
2+
•GTPcS 0.18 ± 0.04 )30.0 ± 0.06 )13.6 ± 1.5 16 ± 1.6
Conformational dynamicsofRas bound to GTPcS M. Spoerner et al.
1426 FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS
atom in the c-phosphate group are represented by bro-
ken lines.
Influence ofthe nucleotide bound on the Ras
conformational states
31
P NMR spectroscopy allows us to probe the con-
formational states ofnucleotide-binding proteins, such
as Ras-related proteins, which lead to structural rear-
rangement in the active centre. In principle, whenever
chemical shift changes are visible they indicate that
there is a change ofthe environment ofthe phospho-
rus nuclei, although small changes in structure can
lead to large differences in chemical shifts and vice
versa. The main mechanisms leading to changes in
chemical shifts are conformational strain and electric
field effects polarizing the oxygen atoms of the
phosphate groups. In addition to these direct effects,
long-range effects may occur, caused by a structure-
dependent change in the anisotropy ofthe magnetic
susceptibility, with ring current effects making the
most dominant contribution.
Binding ofthe different GTP analogues to Ras leads
to large changes in chemical shift, namely a strong
upfield shift in the a-phosphate resonance and a strong
downfield shift in the b-phosphate resonance compared
with data from free Mg
2+
–nucleotide complexes
(Table 2). In complexes with GTPcS, a relatively small
upfield shift of 0.63 p.p.m. is observed for the a-phos-
phate resonance and a strong downfield shift of
3.84 p.p.m. is observed for b-phosphate resonance.
c-Phosphorus resonances do not show the typical shift
changes common to all analogues. Thus, qualitatively
the phosphorus ofthe a-phosphate group in the mag-
nesium complexes ofGTP and its analogues is less
shielded when bound to the protein, whereas the
strong downfield shift in the resonance most probably
results from strong polarization ofthe phosphorus–
oxygen bonds in the b-phosphate group. Such bond
polarization in Ras•Mg
2+
•GppNHp has been dis-
cussed by Allin et al. [17], as an explanation of strong
infrared shifts seen in the P–O vibrational bands after
complexation. It should be mentioned that the degree
of shift differences in the chemical shift values cannot
be related in a simple way to the degree of conforma-
tional change causing this change.
Whereas wild-type Ras complexes withthe GTP
analogues GppNHp or GppCH
2
p exist in a conform-
ational equilibrium between two main conformational
states 1 and 2, with a K
12
value of $ 2, the complex
with theanalogueGTPcS obviously exists in predom-
inantly only one conformation. It shows the spectral
characteristics of state 2 as the effector binding state.
(a) The interaction with Ras-binding domains leads
AB
C
Fig. 6. Schematic representation ofthe coordination sphere ofthe phosphate groups and the thiophosphate ofGTPcS in wild-type and
mutant Ras nucleotide complexes. G, guanosine. (A) Coordination that predominantly exists in wild-type protein containing Thr35. (B,C)
Other possible complexes with Ras(T35S) or Ras(T35A). Note, that not all contacts between the nucleotide and theprotein are included.
Bonds that probably exist only in state 1 or are weakened or abolished in the thiophosphate group are represented by broken lines. The sul-
fur atom was assumed to be negatively charged as shown previously for free ATPcS [32]. However, in theprotein bound nucleotide the
charge distribution is probably also influenced by theprotein environment and could be thus different in different conformations.
M. Spoerner et al. ConformationaldynamicsofRas bound to GTPcS
FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS 1427
only to small chemical shift changes. (b) Weakening
or destruction ofthe naturally occurring hydrogen-
bond interaction ofthe side-chain hydroxyl group of
Thr35 withthe metal ion, and ofthe main-chain
amide withthe c-phosphate by mutations to serine or
alanine leads to large changes in chemical shift. (c)
These chemical shift changes can usually be reversed
in Ras(T35S) by Raf-binding because serine still con-
tains a side-chain hydroxyl, however this is not the
case in Ras(T35A). Geyer et al. [1] suggested that in
the GTP-bound form, Ras(wt) also exists predomin-
antly in one conformation. In terms ofthe conforma-
tional equilibria of Ras, GTPcS seems to be the
analogue which is more similar to physiological GTP
than both other commonly used analogues GppNHp
or GppCH
2
p.
Structural states of Ras(T35S) and Ras(T35A)
Mutation of Thr35 to serine or alanine leads to two
new phosphorus lines ofthe c-thiophosphate group
and the b-phosphate group, which both show charac-
teristics of state 1. The two states are in a dynamic
equilibrium as evident from their temperature depend-
ence. They are therefore assumed to represent sub-
states of state 1 and are called states 1a and 1b. The
alanine mutation makes coordination ofthe side chain
with the divalent ion typical for state 2 impossible and
can therefore only exist in state 1. In the serine
mutant, metal ion coordination is perturbed but still
possible. It shows, in addition to lines assigned to sub-
states 1a and 1b, a very weak line at the position of
the c-phosphate resonance in wild-type Ras, suggesting
that Ras(T35S) shows in equilibrium a sparse popula-
tion of state 2. As in the case ofthe complexes of
Ras(T35A) or Ras(T35S) withthe two analogues
GppNHp and GppCH
2
p, the resonance ofthe a-phos-
phate is shifted downfield relatively to state 2, whereas
the b-phosphate resonance is shifted upfield and is split
into two. The c-phosphate resonance is also split into
two well-separated lines, but one is shifted downfield
and one upfield from the resonance positions obtained
with the wild-type protein.
As observed earlier for GppNHp and GppCH
2
p
complexes of Ras, and now for GTPcS, not only is the
hydroxyl group of Thr35 that interacts in the X-ray
structures withthe metal ion important for stabiliza-
tion of state 2, but so too is its methyl group. This is
evident because in Ras(T35S) an hydroxyl group
remains available but state 2 is destabilized. Stabiliza-
tion of state 2 by the side-chain methyl group of
Thr35 does not seem to be due to a simple hydropho-
bic interaction, but rather to sterical restraints, because
it is located in a cavity formed by the side chains of
Ile36 and the charged ⁄ polar side chains of Asp38,
Asp57 and Thr58.
In GTPcS bound to Ras three different stereoiso-
mers ofthe thiophosphate group are possible (Fig. 6).
In principle, they can occur in state 1 and state 2 of
the protein, but the corresponding populations may
differ greatly. However, they are not equivalent ener-
getically because sulfur is coordinated more weakly to
magnesium ions than oxygen and is a weaker acceptor
of hydrogen bonds than oxygen. As a consequence,
GTPcS binds more weakly to Ras than does GTP
itself [18]. In state 2, the amide group of Thr35 is
probably involved in a hydrogen bond with one of the
nonbridging c-phosphate oxygen atoms and the diva-
lent ion withthe other oxygen; the third oxygen is
probably involved in a hydrogen bond withthe amide
of Gly60 and the interaction withthe positively
charged side chain of Lys16. Energetically, a sterical
position such as that shown in Fig. 6A is strongly
favoured, in agreement withthe experimental observa-
tion of a single phosphorus resonance for the c-phos-
phate (Fig. 6A). In the mutant proteins, state 1 is
strongly preferred because the side-chain interaction of
Thr35 withthe Mg
2+
ion is perturbed (T35S) or
impossible (T35A). It has been suggested previously [2]
that weakening of metal ion coordination most prob-
ably leads to a concerted breaking ofthe hydrogen
bond between the amide group of amino acid 35 and
the c-phosphate group.
Indeed, M-Ras [11] and H-Ras(G60A) [12] in the
GppNHp form show
31
P NMR spectra typical of
state 1 and recently published X-ray structures show
that the amide group of Thr35 is distant from the
c-phosphate group. Ford et al. [12] proposed a third
conformational state for human wild-type H-Ras
because their spectrum contained three
31
P resonances
corresponding to the a- and c-phosphate (note that a
new resonance assignment published by Spoerner et al .
[2] was not known to Ford et al. [12]). However,
because the third state could not be observed in our
experiments, and the chemical shifts are very close to
those observed for H-Ras•Mg
2+
•GDP, they should
most probably be assigned to the a- and b-resonances
of Ras-bound GDP.
When a hydrogen bond exists between the amide
group of amino acid 35 and the c-phosphate in the
mutant proteins, in the GTPcS-complex the free
energy differences DG° and thus the equilibrium popu-
lations ofthe three stereoisomers are changed (Fig. 6).
In the stereoisomer that most probably dominates in
wild-type Ras (Fig. 6A), coordination ofthe c-phos-
phate group withthe metal ion and the interaction
Conformational dynamicsofRas bound to GTPcS M. Spoerner et al.
1428 FEBS Journal 274 (2007) 1419–1433 ª 2007 The Authors Journal compilation ª 2007 FEBS
[...]... to the increased line width seen in some resonances in theGTPcS complex Affinity of Ras- binding domains of Raf-kinase to Rascomplexedwith different GTP analogues ITC measurements show that under our experimental conditions the affinities between Ras( wt) and the tightly binding Raf-RBD are not influenced much by the type of bound GTPanalogueThe association constant is identical within the limits of. .. qualitatively explain theConformationaldynamicsofRas bound to GTPcS excessive line broadening ofthe resonances of residues located in these regions Additional local conformational changes may strengthen this effect GTP and GTPcS exist predominantly in state 2 meaning that the line broadening associated withthe transition will be smaller The equilibrium between different stereoisomers around the thiophosphate... by using different GTP analogues or by specific mutations ofRas A hydrogen bond ofthe amide group of Gly13 and ⁄ or the amino group of Lys16 withthe b–c-phosphorus-bridging oxygen may be one factor responsible for stabilization of state 2 in theGTP complex Thus, Ras( wt)•Mg2+ GTPcS exists predominantly in state 2 Other factors stabilizing state 2 are clearly the interactions ofthe amide and side-chain... group of threonine which is missing in Ras( T35S) From the NMR point of view, Ras( wt) and the serine mutant seem to exist in the same conformation when bound to effectors This is not true for the complexes of Raf-RBD with Ras( T35A) where the interaction withthe RBD cannot restore the correct conformation [9] For the T35S-mutant the dissociation constant increases by about one order of magnitude with the. .. probably because of exchange broadening In the complex between Ras( 1–171) and GppNHp the amide resonances of 22 nonproline residues are not visible, whereas in the complex withGTPcS or GTP $ 20 additional resonances can be detected Some of these resonances are broader in theGTPcS complex than in theGTP complex [4] It is clear that any protein exists in multiple conformational states (now often called... structural hypothesis However, the chemical shifts ofthe c-phosphate resonance in state 1b are close to those observed in metal-free GTPcS (Tables 1 and 2) suggesting that it represents the arrangement seen in Fig 6B, with coordination withthe metal ion abolished Dynamics and energetics oftheconformational transitions The DG| values for the transition between state 1 and 2 in complexes of wild-type protein. .. given macroscopic dissociation state i is then pi ¼ M X ð3Þ pji j¼i Withthe macroscopic equilibrium constants Ki 1=sex ¼ k1 þ kÀ1 ð1Þ For the fit ofthe data the two-bond phosphorus–phosphorus coupling constants were taken from proton decoupled spectra ofGTPcS measured at 278 K in the same buffer used for the experiments withRas For free GTPcSthe absolute value of 2Jab and 2Jbc are 19.7 Hz and 29.1... obtained within this range of MgCl2 concentration, at 3 mm MgCl2 a plateau in the magnesium induced chemical shifts was observed for theGTPcS at pH 7 and pH 9 Therefore this concentration was used for the study ofthe nucleotide–metal complexes The pH ofthe solutions was adjusted by adding HCl or NaOH and was determined with a calibrated glass electrode The dependence ofthe chemical shift d on the pH... values are seen in the three nucleotide complexes Qualitatively, the differences may be rationalized withthe help ofthe NMR results as follows In a dynamic equilibrium Ras in complex with GppNHp or GppCH2p has a mobile effector loop which is fixed upon RBD binding Therefore, the change in the configurational entropy (as part ofthe total entropy) is smaller than in theGTPcS complex, where the effector loop... 1b The transition velocity between these two states and thus the energy ofthe transition state is similar to that found for transition between states 1 and 2 ofRas bound to the analogues GppCH2p or GppHNp The activation barrier may reflect a transient breakage ofthe bond between the metal ion and the c-phosphate Experimental procedures Protein purification Wild-type and Thr35 mutants of human H -Ras( 1–189) . between the corresponding T 2 relaxation times for the conformational states 1a and 1b of Ras( T35S)•Mg 2+ GTPcS. Complex of Ras Mg 2+ GTPcS with the Ras- binding domain of Raf-kinase Addition of the. Slow conformational dynamics of the guanine nucleotide-binding protein Ras complexed with the GTP analogue GTPcS Michael Spoerner 1 , Andrea Nuehs 1 ,. on Ras( wt)•Mg 2+ GTPcS (data not shown). Binding of GTPcS to the Ras protein leads to rather large chemical shift changes. In contrast to the observations made for the GTP analogues GppNHp and