ABSTRACT
We and other authors have shown that synthetic peptides corresponding to regions of the third intracellular loop (ICL-3) of receptors of the serpentine type are capable of activating G-protein signaling cascades and trigger them in the absence of hormone. To create on the basis of these peptides the selective regulators of hormonal signaling systems the relationship between their biological activity and secondary structure are studied. It is assumed that most suitable is a helical conformation, which allows the peptide effectively interact with signaling proteins. The aim of this study was to test the biological activity and secondary structure of synthesized by us linear peptides and their dimeric and palmitoylated analogs, corresponding to C-terminal region of the ICL-3 of luteinizing hormone receptor (LHR) and 5-hydroxytryptamine receptor of the type 6 (5-HT6R). It is shown that LHR-peptides at the micromolar concentrations stimulate the basal activity of adenylyl cyclase (AC) and the GTP-binding of G-proteins in the plasma membranes of rat testes, while 5-HT6R-peptides activate AC and G-proteins in the synaptosomal membranes of rat brain. The action of peptides is tissue-specific and observed in the tissues where there are homologous receptors. The most effective were palmitoylated peptides. LHR-peptide reduced the AC stimulatory effect of human chorionic gonadotropin, while 5-HT6R-peptides the effect of 5-HT6R-agonist, EMD-386088, and the action of the peptides was not found in the case of non-homologous receptors. Using circular dichroism spectroscopy it is shown that in neutral (pH 7) and acidic (pH 2) medium all the peptides are exist predominantly in the antiparallel beta-sheet (37-42%) and disordered conformations (33-35%). In alkaline medium (pH 10) in the case palmitoylated peptides the increase of the contribution of the helical conformation to 12-27% was observed. In the presence of trifluoroethanol (10-80%), a helix-forming solvent, the contribution of helical conformation for the majority of peptides was slightly increased (for palmitoylated analogs to 14%), however, in this case the antiparallel beta-sheet and disordered conformation prevailed. The conclusion was made that the lack of clearly expressed ability to form helices in peptides derived the ICLs of receptors did not significantly affect their activity. This is consistent with proposed mechanism of peptides action, whereby peptide interacts with the complementary regions of homologous receptor that does not require the helix formation.
