... (D) Interaction sites for cAMP in the structure of the cyaB2 GAF A domain. (E) Interaction sites for cAMP in the structure of the cyaB2 GAF B domain. Amino acids interacting with the purine moiety ... B bind cAMP (Fig. 1A,B,D,E). Again, the b1–b3region is the major interaction site for the adeninemoiety. In b1, an arginine residue (Ile417 in rPDE2)binds to N1 and a threonine (Ser419 in ... (Phe433 ⁄ Asp434 in rPDE2) that interactswith the pyrimidine ring of the purine, either individu-ally or in various combinations in 12 different con-structs, enhanced cAMP binding but had little...