G-alfa s protein C-terminal alfa-helix at the interface: Does the plasma membrane play a critical role in the Gα s protein functionality?

The heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins, Gαβγ ) mediate the signalling process of a large number of receptors, known as G protein-coupled receptors. The C-terminal domain of the heterotrimeric G protein α-subunit plays a key role in the selective activation of G proteins by their cognate receptors. The interaction of this domain can take place at the end of a cascade including several successive conformational modifications. Gαs(350–394) is the 45-mer peptide corresponding to the C-terminal region of the Gαs subunit. In the crystal structure of the Gαs subunit it encompasses the α4/β6 loop, the β6 β-sheet segment and the α5 helix region. Following a previous study based on the synthesis, biological activity and conformational analysis of shorter peptides belonging to the same Gαs region, Gαs(350–394) was synthesized and investigated. The present study outlines the central role played by the residues involved in the α4/β6 loop and β6/α5 loops in the stabilization of the C-terminal Gαsα-helix. H2O/2H2O exchange experiments, and NMR diffusion experiments show interesting evidence concerning the interaction between the SDS micelles and the polypeptide. These data prompt intriguing speculations on the role of the intracellular environment/cellular membrane interface in the stabilization and functionality of the C-terminal Gαs region.