A novel form of the G protein β subunit Gβ5 is specifically expressed in the vertebrate retina

Abstract

The G protein β subunit, Gβ5, is predominantly expressed in the central nervous system. In rodent brain, Gβ5 is expressed as a protein with an apparent molecular mass of 39,000 daltons (39 kDa). We have identified an additional Gβ5 immunoreactive protein of apparent size 44 kDa in the vertebrate retina. Molecular cloning and sequencing of polymerase chain reaction products revealed that the cDNA encoding the larger species of Gβ5 (Gβ(5L)) was identical to the shorter form with the addition of 126 base pairs of 5' DNA sequence potentially encoding an in-frame 42-amino acid extension. Sequencing of mouse Gβ5 genomic clones demonstrated that the 126-base pair of retinal-specific coding material is derived from a hitherto undetected 5' exon. During sucrose density gradient fractionation of bovine retinas, the 44-kDa Gβ(5L) protein co-purified with rod outer segment membranes. Incubation of rod outer segment membranes with the nonhydrolyzable guanine nucleotide, GTPγS (guanosine 5'-3-O-(thio)triphosphate), which released the Gβ subunit of transducin (Gβ1), failed to remove Gβ(5L). The 39-kDa Gβ5 protein displayed differential association with retinal and brain membranes. In the retina, Gβ5 was present as a soluble protein and was undetectable in the membrane fraction, whereas in the brain approximately 70% of Gβ5 was associated with cellular membranes. In transient COS-7 cell expression experiments, Gβ(5L) formed functional Gβγ dimers and Gαβγ heterotrimers, and activated phosphoinositide-specific phospholipase Cβ2 in a manner indistinguishable from the 39-kDa Gβ5 protein. The cloning of the retinal-specific Gβ(5L) cDNA suggests the existence of potentially novel G protein-mediated signaling cascades in photoreception.