Identification of a GTP-binding protein alpha subunit that lacks an apparent ADP-ribosylation site for pertussis toxin.

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Abstract
Recent molecular cloning of cDNA for the .alpha. subunit of bovine transducin (a guanine nucleotide-binding regulatory protein, or G protein) has revealed the presence of two retinal-specific transducins, called Tr and Tc, which are expressed in rod or cone photoreceptor cells. In a further study of G-protein diversity and signal transduction in the retina, we have identified a G-protein .alpha. subunit, which we refer to as Gz.alpha., by isolating a human retinal cDNA clone that cross-hybridizes at reduced stringency with bovine Tr .alpha.-subunit cDNA. The deduced amino acid sequence of Gz.alpha. is 41-67% identical with those of other known G-protein .alpha. subunits. However, the 355-residue Gz.alpha. lacks a consensus site for ADP-ribosylation by pertussis toxin, and its amino acid sequence varies within a number of regions that are strongly conserved among all of the other G-protein .alpha. subunits. We suggest that Gz.alpha., which appears to be highly expressed in neural tissues, represents a member of a subfamily of G proteins that mediate signal transduction in pertussis toxin-insensitive systems.