A fifth member of the mammalian G-protein beta-subunit family. Expression in brain and activation of the beta 2 isotype of phospholipase C

Abstract

The beta-subunits of the heterotrimeric GTP-binding proteins (G-proteins) are important regulators of G-protein alpha-subunits as well as of certain signal transduction receptors and effectors. We have identified a fifth member of the G-protein beta-subunit family, G beta 5, by molecular cloning. Sequence analysis of recombinant clones revealed an open reading frame of 1,059 base pairs, potentially encoding a protein of 353 amino acids having 53% identity to previously identified G beta-subunit proteins. Northern blot analysis of poly(A)-selected mRNA prepared from different mouse tissues demonstrated that G beta 5 is predominantly expressed in adult brain as two transcripts of 2.3 and 1.75 kilobases. A G beta 5-specific antiserum detected a band of 39.3 kilodaltons on immunoblots of crude membrane proteins isolated from normal mouse brain but not from other tissues. Using a transient cotransfection assay, we showed that a plasmid construct expressing the G beta 5 open reading frame, when coexpressed with different G-protein gamma (G gamma)-subunits, can stimulate the activity of the beta 2 isoform of phosphoinositide-specific phospholipase C (PLC beta 2). The stimulation of PLC beta 2 was most pronounced when G beta 5 was co-expressed with G gamma 2, although activation was observed when G beta 5 was cotransfected with either G gamma 1, G gamma 3, G gamma 5, or G gamma cone. These results prove that G beta 5 can functionally associate with G gamma proteins and is therefore a bona fide member of the G beta protein family. The rather low level of identity between G beta 5 and the other mammalian G beta-subunits may shed light on structural/sequence elements necessary for G beta protein function.