A novel form of the G protein beta subunit G beta sub(5) is specifically expressed in the vertebrate retina

• Published in: The Journal of biological chemistry 1996-11, Vol.271 (45), p.28154-28160
• Main Authors: Watson, A J, Aragay, A M, Slepak, V Z, Simon, MI
• Format: Article
• Language: English
• ISSN: 0021-9258

The G protein beta subunit, G beta sub(5), is predominantly expressed in the central nervous system. In rodent brain, G beta sub(5) is expressed as a protein with an apparent molecular mass of 39,000 daltons (39 kDa). We have identified an additional G beta sub(5) 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 G beta sub(5) (G beta sub(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 G beta sub(5) 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 G beta sub(5L) protein co-purified with rod outer segment membranes. Incubation of rod outer segment membranes with the nonhydrolyzable guanine nucleotide, GTP gamma S (guanosine 5'-3-O-(thio)triphosphate), which released the G beta subunit of transducin (G beta sub(1)), failed to remove G beta sub(5L). The 39-kDa G beta sub(5) protein displayed differential association with retinal and brain membranes. In the retina, G beta sub(5) was present as a soluble protein and was undetectable in the membrane fraction, whereas in the brain approximately 70% of G beta sub(5) was associated with cellular membranes. In transient COS-7 cell expression experiments, G beta sub(5L) formed functional G beta gamma dimers and G alpha beta gamma heterotrimers, and activated phosphoinositide-specific phospholipase C beta sub(2) in a manner indistinguishable from the 39-kDa G beta sub(5) protein. The clowning of the retinal-specific G beta sub(5L) cDNA suggests the existence of potentially novel G protein-mediated signaling cascades in photoreception.