Conformational studies on a synthetic C-terminal fragment of the alpha subunit of G(S) proteins.

It has recently been reported that synthetic peptides corresponding to the C-terminal sequence of G, can be used to study the molecular mechanisms of interaction between this protein and G protein coupled receptors (Hamm et al., Science, 1988, Vol. 241, pp. 832-835). A conformational analysis on a 11 amino acids peptide from the GS C-terminus, GS(384-394) (H-QRMHLRQYELL-OH), was performed by nmr spectroscopy and molecular modeling methods. Two-dimensional nmr spectra, recorded in hexafluoroacetone/water, a mixture with structure stabilizing properties, showed an unusually high number of nuclear Overhauser effects, forming significative pattern to the drawing of a secondary structure. Conformations consistent with experimental NOE distances were obtained through molecular dynamics and energy minimization methods. These calculations yielded two stable conformers corresponding to an -turn and a type III -turn involving the last five C-terminal residues. Interestingly, the -turn conformation was found to overlap with good agreement the crystallographic structure of the same fragment in the GS protein.