Identification of a critical aspartate residue in transmembrane domain three necessary for the binding of somatostatin to the somatostatin receptor SSTR2.

To determine which residues within the rat somatostatin receptor subtype SSTR2 may be interacting with the lys9 of somatostatin-14 (S-14), mutant SSTR2 receptors were created by mutating asp89 or asp122. [125I Tyr11]S-14 binding to D89A and D89E mutants suggests that asp89 is not directly involved in S-14 binding. Binding studies with the charge switch mutants, asp9S-14, and D122K, suggest that asp122 may be interacting with the lys9 of S-14. [125I Tyr11]asp9S-14 displayed saturable binding to D122K with an affinity comparable to that seen with [125I Tyr11]S-14 and WT SSTR2. These data suggest that the interaction between lys9 of S-14 and the TM3 asp122 of SSTR2 represents one contact site between S-14 and SSTR2.