G protein diversity: a distinct class of alpha subunits is present in vertebrates and invertebrates.

Abstract

Heterotrimereic guanine nucleotide-binding proteins (G proteins) are integral to the signal transduction pathways that mediate the cell''s response to many hormones, neuromodulators, and a variety of other ligands. While many signaling processes are guanine nucleotide dependent, the precise coupling between a variety of receptors, G proteins, and effectors remains obscure. We found that the family of genes that encode the .alpha. subunits of heterotrimeric G proteins is much larger than had previously been supposed. These novel alpha subunits could account for some of the diverse activities attributed to G proteins. We have now obtained cDNA clones encoding two murine .alpha. subunits, G.alpha.q and G.alpha.11, that are 88% identical. They lack the site that is ordinarily modified by pertussis toxin and their sequences vary from the canonical Gly-Ala-Gly-Glu-Ser (GAGES) amino acid sequence found in most other G protein .alpha. subunits. Multiple mRNAs as large as 7.5 kilobases hybridize to G.alpha.q specific probes and are expressed at various levels in many different tissues. G.alpha.11 is encoded by a single 4.0-kilobase message which is expressed ubiquitously. Amino acid sequence comparisons suggest that G.alpha.q and G.alpha.11 represent a third class of .alpha. subunits. A member of this class was found in Drosophila melanogaster. This .alpha. subunit, DG.alpha.q, is 76% identical to G.alpha.q. The presence of the Gq class in both vertebrates and invertebrates points to a role that is central to signal transduction in multicellular organisms. We suggest that these .alpha. subunits may be involved in pertussis toxin-insensitive pathways coupled to phospholipase C.