THE SECONDARY STRUCTURE OF PEPTIDES DERIVED
FROM THE THIRD INTRACELLULAR LOOP OF THE SERPENTINE TYPE RECEPTORS
AND ITS INTERRELATION WITH THEIR BIOLOGICAL ACTIVITY
E.A. Shpakova,1 E.A. Skvortsova,2 I.I. Tarasenko,1 A.O. Shpakov 3,*
1 Institute of Macromolecular Compounds RAS, 2 Institute of Cytology RAS,
and 3 I.M. Sechenov Institute of Evolutionary Physiology and Biochemistry RAS, St. Petersburg;
* e-mail: alex_shpakov@list.ru
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 b-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 b-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.
Key words: adenylyl cyclase, G-protein, secondary structure, circular dichroism, peptide, luteinizing hormone
receptor, serotonin, 5-hydroxytryptamine receptor of the type 6, the third intracellular loop
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