Mutation of valine residue unique to alpha subunit of Gs abolishes activation

We recently characterized a decapeptide sequence (residues 367-376) that is important for the membrane association of the activated alpha subunit of Gs. We report here that when this sequence is replaced by the cognate sequence of Gi1 alpha subunit, the chimeric protein (Gsis alpha) still interacts with the membrane but cannot be activated, regardless of the mode of activation. Construction of various chimeras demonstrates that the single replacement of valine 367 by threonine, the cognate residue of Gi1 alpha subunit, fully reproduces the loss of activation. Analysis of nucleotide interaction reveals that the mutant V367T Gs alpha protein poorly binds GDP or GTP. On the other hand, the conservative change of valine to isoleucine does not alter activation. Interestingly, members of the Gs and G12 classes have a valine and an isoleucine, respectively, at this position, whereas members of the Gi or Gq class contain a threonine residue. The evolutionary relationship between the different classes suggests that the presence of a hydrophobic or a hydrophilic residue is not fortuitous in these alpha subunits and might provide distinctive structural and/or functional properties.