RESUMO
To better understand cytoplasmic loop 3/G protein coupling, variations in a bioactive synthetic peptide probe (P1) were constructed according to the published sequences of the human 5HT1a receptor. These probes were tested in a model system of human 5HT1a receptor stably expressed in Chinese hamster ovary cells. In agonist inhibition studies, peptides with amino acid substitutions of residues 6-9 from the amino terminus of loop 3 were less active than P1. Truncated peptide P4, conserving the residue 6-9 region, was also less active than P1. Truncates P5 and P6, deleting the residue 6-9 region, were inactive. When cAMP levels were measured, both substituted peptides were more active than P1 in this negatively coupled system. In contrast, the truncated peptides were without activity in the cAMP assays. Thus, P1 and its derivatives (P2-P6) constitute a small group of peptides with differential uncoupling (agonist inhibition) and signal transduction (cAMP) activities in this G-protein-linked system. It is proposed that these peptides will be useful in future studies detailing the molecular determinants at the receptor/G protein interface.