The γ-subunit of the principal G-protein from squid ( Loligo forbesi) photoreceptors contains a novel N-terminal sequence

Abstract

The squid ( Loligo forbesi) visual system presents as accessible a system for study of G-protein mediated signal transduction as the vertebrate rod outer segment with the added advantage that the major G-protein is a member of the Gq-class. Here the cDNA clone encoding the γ-subunit of this G-protein is reported, thereby completing the molecular cloning of the heterotrimeric G-protein. The deduced protein structure of G-γ has relatively little sequence identity with known mammalian counterparts particularly in comparison with the relatively high degree found for both the α- and β-subunits of this protein. In particular, the N-terminus of the squid visual G-γ contains a repetitive, highly charged region, rich in lysine and glutamate, that has no parallel in other G-proteins. The amino acid sequence of a number of peptides derived by chemical cleavage of G-γ accounted for much of the protein sequence predicted from the cDNA, including the unusual N-terminal region.