Tài liệu Báo cáo khoa học: Mutational analysis of plasminogen activator inhibitor-1 Interactions of a-helix F and its neighbouring structural elements regulates the activity and the rate of latency transition pdf

Mutational analysis of plasminogen activator inhibitor-1 Interactions of a-helix F and its neighbouring structural elements regulates the activity and the rate of latency transition Troels Wind*, Jan K. Jensen, Daniel M. Dupont, Paulina Kulig and Peter A. Andreasen Laboratory of Cellular Protein Science, Department of Molecular Biology, Aarhus University, Denmark The serpin plasminogen activator inhibitor-1 (PAI-1) is a fast and specific inhibitor of the plasminogen activating serine proteases tissue-type and urokinase-type plasminogen activator and, as such, an important regulator in turnover of extracellular matrix and in fibrinolysis. PAI-1 spontaneously loses its antiproteolytic activity by inserting its reactive centre loop (RCL) as strand 4 in b-sheet A, thereby converting to the so-called latent state. We have investigated the import- ance of the amino acid sequence of a-helix F (hF) and the connecting loop to s3A (hF/s3A-loop) for the rate of latency transition. We grafted regions of the hF/s3A-loop from antithrombin III and a 1 -protease inhibitor onto PAI-1, creating eight variants, and found that one of these rever- sions towards the serpin consensus decreased the rate of latency transition. We prepared 28 PAI-1 variants with individual residues in hF and b-sheet A replaced by an alanine. We found that mutating serpin consensus residues always had functional consequences whereas mutating nonconserved residues only had so in one case. Two variants had low but stable inhibitory activity and a pronounced tendency towards substrate behaviour, suggesting that insertion of the RCL is held back during latency transition as well as during complex formation with target proteases. The data presented identify new determinants of PAI-1 latency transition and provide general insight into the characteristic loop–sheet interactions in serpins. Keywords: alignment; conformation; mutational analysis; PAI-1; proteases; serpin. Plasminogen activator inhibitor-1 (PAI-1) is the primary inhibitor of both urokinase-type and tissue-type plasmino- gen activator (uPA and tPA, respectively) and as such is an important regulator of physiological events in which plasmin-catalysed extracellular proteolysis is involved. PAI-1 belongs to the serine protease inhibitor (serpin) family whose antiproteolytic activity is governed by their structural flexibility. In the active serpin conformation, the reactive centre loop (RCL) with the P 1 –P 1 ¢ bait peptide bond is surface exposed. Formation of the covalent serpin– protease complex involves a Michaelis docking complex, cleavage of the P 1 –P 1 ¢ peptide bond, linkage of the active site Ser of the protease to the carboxyl group of P 1 by an ester bond and insertion of the N-terminal part of the RCL as strand 4 in b-sheet A (s4A) of the serpin. Consequently, the protease is trapped in a covalent acyl-enzyme complex in which its reactive site is distorted, as illustrated by the crystal structure of the complex between a 1 -protease inhibitor (a 1 PI,alsoreferredtoasa 1 -antitrypsin) and trypsin [1]. Under some conditions, however, RCL insertion is delayed, resulting in hydrolysis of the ester bond, release of free protease and insertion of the cleaved RCL as s4A. This pathway is referred to as substrate behaviour of the serpin. Complex formation between serpins and their cognate proteases is fuelled by the thermodynamic properties of the serpin. Accordingly, insertion of the RCL as s4A and the ensuing structural rearrangements of the serpin stabilizes the molecule in a so-called ÔrelaxedÕ conformation, as opposed to the metastable ÔstressedÕ conformation with the RCL exposed on the surface (reviewed in [2–4]). PAI-1 spontaneously converts into a relaxed conforma- tion at a significant rate without cleavage of the RCL (for a review see [5]). During this structural transformation, referred to as latency transition, the N-terminal part of the intact RCL is inserted as s4A [6] (Fig. 1). Latent versions of the serpins antithrombin III (ATIII) [7], a 1 -protease inhibitor (a 1 PI) [8], and a 1 -antichymotrypsin (a 1 ACT) [9] have also been isolated, but none of these undergo this transition as readily as PAI-1. The physiological role of PAI-1 latency transition, if any, remains elusive [5]. Some PAI-1 variants with single mutations and modest decreases in the rate of latency transition have been obtained through heuristic protein engineering [10,11] while others have been identified by chance [12–15]. The variants with the slowest latency transition carry multiple mutations Correspondence to J. K. Jensen, Laboratory of Cellular Protein Science, Department of Molecular Biology, Aarhus University, Gustav Wieds Vej 10C, 8000 A ˚ rhus C, Denmark. Fax: + 45 86123178, Tel.: + 45 89425074, E-mail: jkj@mb.au.dk Abbreviations: PAI-1, plasminogen activator inhibitor-1; RCL, reactive centre loop; a 1 PI, a 1 -protease inhibitor (a 1 -antitrypsin); a 1 ACT, a 1 -antichymotrypsin; ATIII, antithrombin III; hF, a-helix F; HMK, heart muscle kinase. Enzyme: uPA, urokinase-type plasminogen activator (EC 3.4.21.73). *Present address: Centre for Vascular Research, School of Medical Sciences, The University of New South Wales, Sydney NSW 2052, Australia. (Received 4 December 2002, revised 7 February 2003, accepted 13 February 2003) Eur. J. Biochem. 270, 1680–1688 (2003) Ó FEBS 2003 doi:10.1046/j.1432-1033.2003.03524.x and have been obtained by random mutagenesis followed by screening or selection procedures. Berkenpas et al. isolated several PAI-1 variants with increased stability of whichthemoststable,referredtointhefollowingasPAI- 1 stab , carried four amino acid substitutions (N152H, K156T, Q321L and M356I) [16]. The three-dimensional structure of PAI-1 stab has been determined and reveals that the stabil- izing amino acid substitutions N152H and K156T induces a 3 10 -helix spanning residues 155–157 in the loop connecting a-helix F to b-strand 3A (the hF/s3A-loop, Fig. 1) [17–19]. Likewise, Stoop et al. isolated a panel of stable PAI-1 variants of which the most stable carried 10 amino acid substitutions [20]. The structure of this variant, however, has not been determined. A prevalent theme among the stabilizing amino acid substitutions is a reversion towards the consensus sequence of inhibitory serpins, suggesting that part of the molecular basis for latency transition can be found in regions of PAI-1 that deviate from this consensus [16,20]. Based on the following observations, we speculated that the amino acid composition of the hF/s3A-loop and hF plays a role in the stressed-to-relaxed transition of PAI-1: The hF/s3A-loop must fold away from b-sheet A during the structural transition to allow insertion of the RCL as s4A [6] (Fig. 1). Stabilizing mutations have been identified in the hF/s3A-loop [16,20] and the aforementioned 3 10 -helix in PA1–1 stab has been suggested to, at least in part, be responsible for the increased stability of this variant [17–19]. Some monoclonal antibodies towards hF and the hF/s3A- loop [21–24] as well as deletion of this region [25] induce substrate behaviour of PAI-1. In addition, Gettins recently hypothesized that a thermodynamically unfavourable dis- location of hF, resulting from insertion of the RCL, provides an energy-reservoir that subsequently fuels the crushing of the protease concomitantly with the return of hF to its normal position [26]. In the present study, we used structural alignments to define residues in the hF/s3A-loop in PAI-1 that deviate from the serpin consensus and replaced them with the corresponding residues from the representative inhibitory serpins a 1 PI and ATIII. As there are no discrepancies between the PAI-1 sequence and the serpin consensus in hF, we chose alanine-scanning mutagenesis as a means of identifying hF-residues with importance for RCL insertion. Likewise, residues from b-sheet A with putative contacts to hF or the hF/s3A-loop were individually replaced by alanine. In total, 38 PAI-1 variants were characterized in terms of latency transition and functional behaviour upon interaction with uPA and several side chains involved in the retardation of latency transition, and hence of importance for the functional stability of stressed PAI-1, were identified. Finally, we describe PAI-1 variants that can adapt remarkably stable conformations and predom- inantly behave as substrates for uPA. As will be discussed, the data presented can probably be extrapolated to other serpins and thus provide further insight into the molecular details of the stressed-to-relaxed transition of these proteins [27]. Fig. 1. Ribbon diagrams of relaxed, latent PAI-1 (right) and stressed, active PAI-1 stab (left) [18]. Insertion of the reactive centre loop (red) as strand 4 in b-sheet A (pink) requires mobility of hF and the hF/s3A-loop (orange). Ó FEBS 2003 Mutational analysis of PAI-1 (Eur. J. Biochem. 270) 1681 Materials and methods Cloning, mutagenensis and purification of PAI-1 The cDNA for human PAI-1 was modified to include a N-terminal His 6 -tag plus a recognition motif for heart muscle kinase and cloned into the Escherichia coli expres- sion vector pT7-PL [28]. Mutations in PAI-1 were intro- duced with the QuickChange Site-Directed Mutagenesis Kit (Stratagene) as described by the manufacturer, except that the final product was electroporated into E. coli DH5a cells, and confirmed by sequencing using the Thermo Sequenase II Dye Terminator Cycle Sequencing Kit (Amersham Pharmacia Biotech) and a 373A ABI sequencer (Applied Biosystems). Numbering of residues in PAI-1 was S 1 -A 2 -V 3 - H 4 -H 5 … [29]. For gene expression, individual colonies of transformed E. coli cells BL21(DE3)pLysS (Novagen) were inoculated into 2 · TY broth (16 gÆL )1 tryptone, 10 gÆL )1 yeast extract, 5 gÆL )1 NaCl) supplemented with 100 lgÆmL )1 ampicillin and 34 lgÆmL )1 chloramphenicol and incubated overnight at room temperature. The cultures were diluted 1 : 20 and incubated at 37 °CuntilaD 600 between 0.7 and 0.9. A final concentration of 0.5 m M isopropyl thio-b- D - galactoside was added to induce gene expression and the incubation was continued for 2 h. From this point, protein purification was performed at 4 °C. The cells were harvested by centrifugation (7000 g, 20 min), resuspended in 35 mL phosphate-buffered saline (137 m M NaCl, 2.7 m M KCl, 1.4 m M KH 2 PO 4 ,4.3m M Na 2 HPO 4 ) and opened with sonication. The bacterial lysates were cleared by centrifu- gation (15 000 g, 30 min) and filtration (0.22 lm), supple- mented with 2 M NaCl, 10 m M imidazole and 5% glycerol, and applied to a 5-mL Ni-nitrilotriacetic acid column (Qiagen) equilibrated in the same buffer. After extensive washing with the equilibration-buffer, PAI-1 was eluted by increasing the concentration of imidazole to 200 m M .The eluted protein was subjected to gel filtration on a Superdex 75 column (1.6 · 60 cm, Amersham Pharmacia Biotech) that had been equilibrated in Hepes-buffered saline (10 m M Hepes, 0.14 M NaCl, pH 7.4 at 37 °C) supplemented with 5% glycerol and a final concentration of 1 M NaCl. Fractions containing PAI-1 were pooled, the concentration determined from A 280 using the calculated extinction coefficient 0.77 mLÆmg )1 Æcm )1 [30], and stored at )80 °C until used. This procedure routinely gave 2–15 mg PAI-1 per litre of culture and N-terminal sequencing revealed that the recombinant protein had the expected N terminus, i.e. (M)GSMGSHHHHHHGS RRASV 3 …, where the initi- ating M in parentheses is missing. The phosphorylation site for heart muscle kinase (HMK, underlined) allows radio- active labelling of the molecule, a feature not used in the present study. The reactivity of His- and HMK-tagged PAI-1 in terms of uPA inhibition and vitronectin binding was not affected by this modification [31]. SDS/PAGE analysis of functional behaviour Reactions between recombinant PAI-1 (100 lgÆmL )1 )and uPA (200 lgÆmL )1 , Wakamoto Pharmaceutical Company, Tokyo, Japan) were performed in HBS at 37 °Cfor30min and quenched by boiling in SDS sample buffer. The reaction products were subjected to nonreducing SDS/PAGE in 11% acrylamide gels followed by staining with Coomassie brilliant blue. For time-course experiments, PAI-1 (200 lgÆmL )1 ) was incubated in HBS at 37 °C for up to 24 h before reaction with uPA, followed by SDS/PAGE. Band intensities were determined by scanning densitometry. Determination of functional half-lives The general buffer for the assay described was HBS (pH 7.4 at 37 °C) supplemented with 0.25% gelatine and unless stated otherwise, all incubations were at 37 °C. PAI-1 was incubated at a concentration of 20 lgÆmL )1 and at various time-points, aliquots were taken for preparation of a twofold dilution series in a 96-well plate with 100 lL PAI-1 per well in concentrations ranging from 20 to 0.0098 lgÆmL )1 . Immediately thereafter, 100 lL 0.5 lgÆmL )1 uPA (0.25 or 0.125 lgÆmL )1 uPA for PAI-1 variants with activity below 20%) was added to each well followed by incubation for at least 5 min to allow complex formation between uPA and PAI-1. The remaining uPA activity in each well was determined as the absorbance at 405 nm after addition of 25 lL0.3mgÆmL )1 S-2444 (Chromogenix, Sweden) and further incubation for 40 min. The specific inhibitory activity of PAI-1 at the various time- points, i.e. the fraction of the total amount of PAI-1 forming a stable complex with uPA, was calculated from the amount of PAI-1 required to inhibit half the uPA. The half-life of PAI-1 was finally calculated from an exponential decay plot of the data obtained. Generally, only one preparation of each PAI-1 variant was investigated, but the following were investigated with two independent preparations, giving indistinguishable results: wild-type, PAI-1(T96A), PAI- 1(F100A), PAI-1(V126A), PAI-1(F128A), PAI-1(I137A), PAI-1(I138A), PAI-1(N139A), PAI-1(W141A), PAI- 1(T146A) and PAI-1(M149K). Structural analysis Structural analysis was based on the following depositions in the Protein Data Bank ([32]; PDB-ID is given in parenthesis): active (1DVM) and latent (1DVN) PAI-1 [18], active a 1 -PI (1QLP) [33], cleaved a 1 -PI (7API) [34], active and latent antithrombin (1E05) [35]. SWISS PDB - VIEWER v3.51 (http://www.expasy.ch/spdbv/) was used for visu- alization and structural alignments. Statistical analysis Rates of latency transition were compared using an unpaired t-test. Results Reversions to the serpin consensus in the hF/s3A-loop of PAI-1 To determine if the hF/s3A-loop governs latency transition of PAI-1 by more readily allowing insertion of the intact RCL than the corresponding loop from other inhibitory serpins, we prepared PAI-1 variants with hF/s3A-loops that mimic those found in the inhibitory serpins a 1 PI and ATIII, 1682 T. Wind et al. (Eur. J. Biochem. 270) Ó FEBS 2003 respectively. Table 1 shows a structure-based sequence alignment of hF/s3A-loops from the relaxed serpin structures cleaved a 1 PI [34], latent PAI-1 [18], and latent ATIII [35]. Also, the consensus serpin sequence of the hF/s3A-loop is included in Table 1 (adapted from [2]). Alignment of the three structures was performed accord- ing to the C a atoms in the rigid serpin fragment 2c [36], encompassing residues 129–155 in PAI-1. The hF/s3A- loop of PAI-1 deviates from the consensus serpin sequence at position 149 (M instead of a basic residue, e.g. K168 in a 1 PI) and position 152 (Asn instead of an acidic residue, e.g. D171 in a 1 PI). Also, the stretch GKGA(155–158) appears more flexible in PAI-1 than the corresponding stretch in most other serpins, either because of its length (e.g. compared to a 1 PI) or the lack of Pro residues (e.g. compared to ATIII) (Table 1 and [2]). We prepared the PAI-1 variants PAI-1(M149K), PAI- 1(N152D), PAI-1(G155K, D(156–157), A158E) and PAI- 1(G155P, K156S, G157E) where the latter two have the stretch between position 155 and 158 replaced by the corresponding stretch from a 1 PI and ATIII, respectively (Table 1). These variants were all found to be not significantly different from the wild-type in terms of specific inhibitory activity (Table 2). Thus, the introduced mutations did not compromise the correct folding of PAI-1 in its active conformation. Compared with the wild-type protein, PAI-1(M149K), and to a lesser extent PAI-1(G155K, D(156–157), A158E), had a decreased rate of latency transition; PAI-1(N152D) had a similar rate; and a slightly increased rate for PAI- 1(G155P, K156S, G157E) was counteracted by introducing the N152D mutation (Table 2). The two mutations M149K and N152D were introduced individually or together in the PAI-1(G155K, D(156–157), A158E) background and the resulting variants were found to behave as wild-type PAI-1 towards uPA in terms of inhibitory activity (Table 2). Combining the M149K and [G155K, D(156–157), A158E] mutations did not decrease the rate of latency transition compared to M149K alone (T ½ ¼ 156 ± 13 min vs. 136 ± 24 min, P ¼ 0.16) (Table 2). Alanine scanning mutagenesis The s5A residues K325 and K327 have been suggested to coordinate a chloride ion between b-sheet A and the hF/s3A-loop [18]. The s6A residues E283 and E285 are potential partners for electrostatic interactions with K325 and K327, and E285 makes contact with the hF/s3A-loop in PAI-1 stab [18]. Finally, T96 (s2A) forms a hydrogen bond to the hF-residues H145 in latent PAI-1 and W141 in PAI-1 stab [18] and F100 (s2A), V126 (s1A) and F128 (s1A/ hF-loop) form part of the hydrophobic interface between hF and b-sheet A. These eight side chains (E283, E285, K325, K327, T96, F100, V126 and F128), each of the residues in hF (i.e. S129 to K147), and the hF/s3A-loop residues M149 and N152 (see above) were substituted with A and the resulting variants were characterized in terms of Table 2. Reversions to the serpin-consensus in the hF/s3A-loop. For each PAI-1 variant, the specific inhibitory activity towards uPA was determined in a peptidolytic assay and expressed as percentage of the theoretical maximum. The activity was monitored over time and the rate of latency transition expressed as the functional half-life, t ½ . The averages and standard deviations for at least three independent experiments are given. PAI-1 variant Activity (%) t ½ (min) Wild-type 74 ± 13 63 ± 6 M149K 83 ± 11 136 ± 24* N152D 99 ± 8 67 ± 3 G155P, K156S, G157E 75 ± 16 50 ± 2* N152D, G155P, K156S, G157E 83 ± 12 67 ± 6 G155K, D(156–157), A158E 66 ± 12 73 ± 6* M149K, G155K, D(156–157), A158E 58 ± 5 156 ± 13* N152D, G155K, D(156–157), A158E 62 ± 8 76 ± 7* M149K, N152D, G155K, D(156–157), A158E 93 ± 16 148 ± 17* * Significantly different from the corresponding value for wild-type (P < 0.005). Table 1. Structure-based sequence alignment of the hF/s3A-loops from the three inhibitory serpins PAI-1, a 1 PI and ATIII. Residue numbering is according to PAI-1 [29]. The alignment is based on the three-dimensional structures of the relaxed conformations of the serpins (see text for details). Also shown is the serpin consensus sequence for this region, adapted from [2]. Residue 147 148 149 150 151 152 153 154 155 156 157 158 159 160 170 171 172 Consensus – G K I – D E LL V ––––V L I D––T PAI-1 K G M I S N L L G K G A V D Q L T a 1 PI QGKI VDLVK– – ELDRDT ATIII E G R I T D V I P S E A I N E L T Ó FEBS 2003 Mutational analysis of PAI-1 (Eur. J. Biochem. 270) 1683 specific inhibitory activity, functional behaviour and the rate of latency transition. Among the variants tested, the following had a more than threefold reduced specific inhibitory activity: F100A, V126A, F128A, I137A, N139A, W141A, T146A, M149A, N152A, and K327A (Table 3). Their functional behaviour was analysed by treatment with uPA followed by SDS/ PAGE and scanning densitometry (Fig. 2). In that analysis, inhibitory active PAI-1 will migrate as a complex with uPA while PAI-1 exhibiting substrate behaviour will migrate slightly faster than native PAI-1 due to cleavage of the C-terminal 33 amino acids. Latent PAI-1 or PAI-1 other- wise inert to uPA will comigrate with native PAI-1. This analysis showed that the substitutions F100A, V126A, F128A, I137A, N139A, W141A, T146A and N152A increased the fraction of PAI-1 molecules behaving as a substrate for uPA to between 40 and 60%, compared to  15% for the wild-type protein (Fig. 2). They thus showed a readily distinguishable substrate behaviour. To assay the stability of this substrate behaviour, PAI-1 variants were incubated for up to 24 h at 37 °C prior to reaction with uPA and SDS/PAGE analysis. After 24 h, the fraction of molecules behaving as a substrate for uPA decreased approximately twofold for PAI-1(V126A), PAI-1(F100A), PAI-1(F128A) and PAI-1(W141A) with a concomitant increase in the fraction being inert to uPA. Substrate behaviour remained almost constant for PAI-1(I137A), PAI-1(N139A), PAI-1(T146A) and PAI-1(N152A) for 24 h (Fig. 3 and not shown). The lower specific inhibitory activity of M149A and K327A was associated solely with an increased fraction in a form comigrating with native PAI-1, and thus in an inert, probably latent conformation (Fig. 2). The rate of latency transition was determined for all variants. Typical assays are depicted in Fig. 4, and the data for all the variants are summarized in Table 3. Replacing any of the residues E132, R135, D140, K147, M149, E283 and K327, respectively, with an A increased the rate of latency transition more than twofold. Three variants, I137A, V142A, and N152A, had a biphasic loss of activity, one component with a significantly faster latency transition rate and another component with a significantly slower latency transition rate. The activity of the three variants PAI-1 (N139A), PAI-1(W141A) and PAI-1(T146A) remained almost invariant for several hours at 37 °C(Table3). The K325A substitution slightly delays latency transition in PAI-1, which is in agreement with our previous obser- vations [37,38]. Less pronounced, but still significantly slower latency transition were observed with the substitu- tions T96A and I138A. The E285A substitution also slightly delays latency transition of wild-type PAI-1 whereas in the PAI-1 stab background, it accelerates latency transition. Discussion The only noteworthy stabilizing effect resulting from reversions to the serpin consensus in the hF/s3A-loop was seen for PAI-1(M149K) (Table 2). In the relaxed serpin conformation, M149 in PAI-1 and the corresponding K168 Table 3. Alanine-scanning. For each PAI-1 variant, the specific inhibitory activity towards uPA was determined in a peptidolytic assay and expressed as percentage of the theoretical maximum. The activity was monitored over time and the rate of latency transition expressed as the functional half-life, t ½ . The averages and standard deviations for at least three independent measurements are given for each variant. PAI-1 variant Activity (%) t ½ (min) Wild-type 74 ± 13 63 ± 6 T96A 91 ± 6 99 ± 23* F100A 4 ± 1 54 ± 13 V126A 9 ± 3 42 ± 7* F128A 2 ± 1 68 ± 2 S129A 80 ± 17 59 ± 8 E130A 59 ± 4 53 ± 3 V131A 74 ± 13 70 ± 4 E132A 51 ± 9 30 ± 1* R133A 90 ± 10 57 ± 2 R135A 57 ± 6 26 ± 2* F136A 88 ± 12 67 ± 5 I137A 18 ± 2 <10 and 170 ± 10* a I138A 52 ± 4 78 ± 4* N139A 7 ± 1 335 ± 214* D140A 63 ± 2 17 ± 0* W141A 9 ± 2 431 ± 158* V142A 32 ± 4 7 ± 2 and 120 ± 5* a K143A 88 ± 6 48 ± 3* T144A 77 ± 13 47 ± 6* H145A 75 ± 3 61 ± 7 T146A 7 ± 1 217 ± 33* K147A 87 ± 12 26 ± 6* M149A 10 ± 6 18 ± 2* N152A 13 ± 2 17 ± 3 and 110 ± 21* a E283A 33 ± 3 14 ± 3* E285A 97 ± 0 79 ± 3* K325A 82 ± 19 106 ± 8* K327A 6 ± 0 11 ± 1* PAI-1 stab 69 ± 3 3340 ± 661* PAI-1 stab (E285A) 76 ± 3 680 ± 35* , ** a A biphasic loss of activity was observed, suggesting a hetero- geneity in the active fraction. Note that the wild-type residue at position 134 is an A. * Significantly different from the corres- ponding value for wild-type (P < 0.005); ** Significantly different from the corresponding value for PAI-1 stab (P < 0.005). Fig. 2. Reaction products following reaction of PAI-1 variants with uPA. PAI-1 variants (100 lgÆmL )1 ), indicated by their amino acid substitution, were reacted with uPA (200 lgÆmL )1 )at37°CinHBS, pH 7.4 for 30 min and the products were separated by nonreducing SDS/PAGE (11% acrylamide) followed by staining with Coomassie brilliant blue. The migration of the uPA–PAI-1 complex, uPA, intact (inert) PAI-1 and cleaved PAI-1 is indicated on the right. 1684 T. Wind et al. (Eur. J. Biochem. 270) Ó FEBS 2003 in a 1 PI are located at a critical position at the top of the hF/ s3A-loop right above the inserted s4A while in the stressed conformation of PAI-1 stab and a 1 PI, they stack against the aromatic moiety of the s3A residues Y172 and F189, respectively [18,33,34] (Fig. 5). Accordingly, we assume that the aliphatic moiety of the introduced K in PAI-1(M149K) allows the side chain to adapt the same orientation as the original M and therefore the stabilizing effect of the M149K substitution is likely to be governed by the introduced positive charge. In stressed ATIII, the RCL is partially inserted as the top of s4A and the equivalent of M149 from PAI-1, i.e. R197, is located right above the bifurcation between s3A and s5A [35]. In light of this, we suggest that a positively charged side chain close to the point of initial insertion of the RCL [36] represents an obstacle for the local structural rearrangements required for the movements of the RCL during latency transition. N152 is often replaced by D in PAI-1 variants carrying several mutations that lead to a decreased rate of latency transition [20,39]. However, the N152D mutation does not per se delay latency transition in PAI-1 (Table 2). Substitution of the stretch GKGA(155– 158) in PAI-1 with the corresponding stretch from a 1 PI or ATIII had only modest effects on the rate of latency transition (Table 2). Therefore, besides the M at position 149, deviations from the serpin consensus in the hF/s3A- loop of PAI-1 does not contribute to the rate of latency transition. Alanine-scanning mutagenesis identified side chains that contribute to the functional stability of PAI-1 as their removal increased the rate of latency transition more than twofold (Table 3). In principle, this observation can imply two things: (a) the side chain in question contributes to the thermodynamic stability of the stressed serpin conforma- tion, which is why its removal makes the latency transition energetically more favourable; (b) alternatively, the side chain in question is instrumental in obstructing the conformational changes occurring during the latency transition. Accordingly, we propose that the hF side chains of D140, K147 (forming a salt-bridge to s2A), M149 (packing against Y172 in s3A, Fig. 5), and the salt-bridge between E132 and R135 [18] contribute to the thermo- dynamic stability of the stressed conformation and/or are important for the positioning of hF in a way delaying the proper movements of the intact RCL during latency transition. PAI-1(I137A), PAI-1(V142A) and Fig. 3. Time-course experiment showing the substrate behaviour of selected variants. PAI-1 (200 lgÆmL )1 )wasincubatedat37°C in HBS pH 7.4, and at the indicated time points, aliquots were reacted with a twofold molar excess of uPA for 30 min. Reaction products were analysed by nonreducing SDS/PAGE followed by staining with Coomassie brilliant blue. The migration of the uPA–PAI-1 complex, uPA, intact (inert) PAI-1 and cleaved PAI-1 is indicated on the right. Fig. 4. Time-course experiment showing the inhibitory activity towards uPA of representative PAI-1 variants as measured in a peptidolytic assay. PAI-1 (20 lgÆmL )1 ) was incubated in HBS supplemented with 0.25% gelatine at 37 °C and at the indicated time-points, samples were taken and the inhibitory activity determined. Activity was plotted semilogarithmically against time. The experiment shown is a typical one out of a total of at least three. Ó FEBS 2003 Mutational analysis of PAI-1 (Eur. J. Biochem. 270) 1685 PAI-1 (N152A) all showed a biphasic loss of activity suggesting a heterogeneity in the active fraction of these variants. That substituting either of the juxtaposed residues E283 (s6A) or K327 (s5A) with an A increases the rate of latency transition may be related to the proposed role for K327 in the coordination of a stabilizing chloride ion [18] or suggest the existence of a stabilizing salt-bridge between the two side chains (Fig. 5). In contrast, substituting T96 in s2A, I138 in hF, K325 in s5A or E285 in s6A with A increased the half-life of latency transition by 24–68% (Table 3). The T96A substitution is a reversion to the serpin consensus A/G (G115 in a 1 PI) [2], suggesting that the absence of a side chain beyond the C b atom at this position increases the functional stability of the stressed serpin. I138 is highly conserved among serpins (I157 in a 1 PI) [2], buried between hF and b-sheet A (Fig. 5) and may be instrumental in promoting the translocation of hF during RCL insertion. K325 is also conserved among serpins (K335 in a 1 PI) [2] and its substitution for A in a 1 PI, a 1 ACT and ATIII has been suggested to stabilize the stressed conformation of these serpins by relieving the strain of side chain overpacking between the K325 side chain and residues in the hF/s3A-loop, i.e. the conserved I150 and L153 in PAI-1 ([40,41], see Table 1). This is in good agreement with our observation of a decreased rate of latency transition for PAI-1(K325A) (Table 3 and [37,38]). The side chains of K325 and E285 are juxtaposed, which is why the E285A substitution may provide a spatial relief mimicking the effect of the K325A substitution (Fig. 5). Of note is that the side chain of E285 forms a hydrogen bond to the backbone of the hF/s3A-loop in PAI-1 stab [18] (Fig. 5) and in contrast with the wild-type protein, the functional stability of this variant is decreased by the E285A substi- tution (Table 3). This advocates that the contact between the E285 side chain and the hF/s3A-loop contributes to the functional stability of PAI-1 stab and that a similar contact is not present in the stressed conformation of the wild-type protein. Inhibitory activity and substrate behaviour of PAI- 1(N139A) and PAI-1(T146A) were found to be invariant for several hours (Fig. 3 and Table 3). Both events require an exposed RCL, and it therefore seems that insertion of the intact RCL during latency transition as well as insertion of the cleaved RCL during complex formation with uPA is retarded in these variants [42]. Both N139 and T146 are highly conserved among serpins (N158 and T165, respectively, in a 1 PI) [2] and form hydrogen bonds to the hF/s3A-loop [18] (Fig. 5). Considering the almost identical phenotypes of PAI-1(N139A), PAI-1(T146A), and the close spatial proximity and similar structural role of N139 and T146, we find it likely that the structures of these variants, with the RCL exposed, are similar and contains a distorted hF that delays insertion of the RCL. Substituting W141 with an A leads to substrate behaviour and a low, stable inhibitory activity (Fig. 2 and Table 3). This W is located in the cleft between hF and s2A (Fig. 5), and the presence of an aromatic side chain at this position is common in serpins [2]. Mutation of the equivalent Y160 in a 1 PI to A or W resulted in decreased or increased thermodynamic stability, respect- ively, and in line with our observation for PAI-1(W141A), a 1 PI(Y160A) displayed a marked increase in substrate behaviour [43]. Alanine substitution of the residues F100, V126, F128 and I137, respectively, led to increased substrate beha- viour and a low unstable inhibitory activity, and for I137A a biphasic loss of activity (Fig. 2 and Table 3). The phenotype of these substitutions is therefore different from that of N139A, T146A and W141A. The F residues at positions 100 and 128 are highly conserved among serpins (F119 and F147, respectively, in a 1 PI) [2] and buried in the hydrophobic interface between hF and b-sheet A (Fig. 5). The substrate behaviour and low activity, which could not be increased by refolding in vitro (not shown), of these variants suggest that these residues are pivotal for Fig. 5. Selected residues important for the interactions between hF, the hF/s3A-loop and b-sheet A, as seen in the structure of PAI-1 stab [18] (for an overview, see Fig. 1). H-bonds are indicated in green, a-helix F in orange and parts of b-sheet A in pink. Backbone atoms of the s1A/hF- loop (residues D127 to E130) and of the top of hF and the hF/s3A- loop (residues T146 to L154) are shown in CPK colours. The following side chains are shown in CPK colours and numbered (the secondary structure element is given in parenthesis): 1, T96 (s2A); 2, F100 (s2A); 3, V126 (s1A); 4, F128 (s1A/hF-loop); 5, I137 (hF); 6, I138 (hF); 7, N139 (hF); 8, W141 (hF); 9, V142 (hF); 10, T146 (hF); 11, M149 (hF/ s3A-loop); 12, Y172 (s3A); 13, E283 (s6A); 14, E285 (s6A); 15, K325 (s5A); 16, K327 (s5A). Note that in the structure of latent PAI-1, T96 forms a hydrogen bond with H145 from hF [18] (data not shown). 1686 T. Wind et al. (Eur. J. Biochem. 270) Ó FEBS 2003 the correct folding of the stressed serpin conformation. V126 (conserved among serpins; V145 in a 1 PI [2]) and I137 (not conserved) are partially exposed in the cleft between hF and s1A (Fig. 5). The substrate behaviour of the corresponding A-substituted variants suggests that these residues are instrumental for the movements of hF during complex formation. As detailed above, several of the residues investigated in this study are conserved among serpins and substituting any of these with A changed the characteristics of PAI-1. In addition, our observations for the M149A, M149K, N152A and N152D variants of PAI-1 implicate the conserved side chains K168 and D171 in a 1 PI (corresponding to M149 and N152, respectively, in PAI-1) as contributors to the stability of the stressed serpin conformation. The K335A substitu- tion in a 1 PI, corresponding to the K325A substitution in PAI-1, stabilizes a 1 PI by 6.5 kcalÆmol )1 [40], suggesting that the modest decrease in the rate of latency transition resulting from the K325A substitution (Table 3) could reflect a substantial stabilization of the stressed PAI-1 conformation. We cannot, however, exclude the possibility that the observed delay of latency transition resulting from amino acid substitutions reflects features of PAI-1 not shared by other serpins. In contrast, none of the residues in hF that were replaceable by A without functional consequences (i.e. S129, E130, V131, R133, F136, K143, T144 and H145) are conserved among serpins [2]. This supports the general notion that conservation of a residue indicates its import- ance for protein function. Furthermore, with the exception of H145, these residues are pointing away from the hF/s3A- loop and b-sheet A, suggesting that functionally important residues should be sought in the interfaces between secon- dary structural elements. Conclusively, through mutagenesis we have now provi- ded further evidence that the positioning of hF and its movements relative to b-sheet A helps regulate the stressed- to-relaxed transition of serpins in general and latency transition of PAI-1 in particular. 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(2002) Probing the role of the F-helix in serpin stability through a single trypto- phan substitution. Biochemistry. 41, 4575–4581. 1688 T. Wind et al. (Eur. J. Biochem. 270) Ó FEBS 2003 . Mutational analysis of plasminogen activator inhibitor-1 Interactions of a-helix F and its neighbouring structural elements regulates the activity and. investigated the import- ance of the amino acid sequence of a-helix F (hF) and the connecting loop to s3A (hF/s3A-loop) for the rate of latency transition. We grafted