Structural basis for the binding specificity of a SSTR1-selective analog of somatostatin

The availability of subtype-specific agonists and antagonists for somatosta tin (SS) receptors (SSTRs) will be important for elucidation of the functio n of each receptor isoform in vivo. A SS analog, des-AA(1,2,5)-[D-Trp(8), I Amp(9)]SS (CH275), has been shown previously to bind preferentially to SSTR 1. In this report, we identify structural determinants in the ligand and re ceptor responsible for the selective binding of CH275 to SSTR1 by modifying both the ligand and the receptor, We propose that IAmp(9) in CH275, like L ys(9) in SS, interacts with Asp137 in the middle of the third transmembrane domain of SSTR1 to form an ion pair, while other residues unique to SSTR1 conbribute to binding selectivity of CH275 for SSTR1. Replacement of Asp137 with Asn resulted in loss of binding of radiolabeled SS and decreased pote ncies of both SS and CH275 to induce a change in the extracellular acidific ation rate measured by microphysiometry. The structural determinants for sp ecific binding to SSTR1 were mapped in chimeric SSTR1/SSTR2 receptors. One chimera, 2 beta, with the N-terminus to second transmembrane domain (TM2) f rom SSTR2 and the remainder of the receptor from SSTR1, had low affinity fo r CH275, Furthermore, when a single residue, Leu107, in TM2 of SSTR1 was re placed with Phe, the corresponding residue in SSTR2, a 20-fold decrease in affinity for CH275 with no significant change in affinity for SS was observ ed. A reciprocal change from Phe to Leu in the chimeric receptor 2 beta res ulted in a 10-fold increase in affinity for CH275. Thus, Leu107 is an impor tant determinant for CH275 binding to SSTR1, To identify the moiety in CH27 5 which could interact with Leu107, a new analog des-AA(1,2,5)-[D-Trp(8), A mp(9)]SS was prepared. This analog bound to both SSTR1 and SSTR2 with simil ar affinities; thus, subtype selectivity was lost, Collectively, these data support a binding model for CH275 in which the positively charged IAmp int eracts with the negatively charged Asp137 in TM3 of SSTR1 and the isopropyl group of IAmp forms a hydrophobic interaction with Leu107 in TM2. (C) 1999 Academic Press.