IDENTIFICATION OF A G-PROTEIN ALPHA-SUBUNIT FROM NEUROSPORA-CRASSA THAT IS A MEMBER OF THE G(I) FAMILY

Heterotrimeric G proteins, consisting of alpha, beta, and gamma subuni ts, are implicated in major signal transduction pathways controlling a diversity of functions in eukaryotic organisms. In the filamentous fu ngus Neurospora crassa, G proteins are implicated in the regulation of several environmental responses. As a first step in studying the role of G proteins in these processes, we have cloned the genes for two al pha subunits, gna-1 and gna-2, from Neurospora. The genes are located on different chromosomes and are differentially regulated during asexu al development. The encoded proteins (Gna-1 and Gna-2) are the same si ze as members of the G(i)-alpha family (approximately 40 kDa). The Gna -1 protein sequence is 55% identical overall to members of the G(i) fa mily and contains the consensus sequences for ADP-ribosylation by pert ussis toxin and incorporation of myristic acid, which are found in thi s group. These properties make Gna-1 the first identified microbial al pha subunit to be a member of any class. Furthermore, incubation of a N. crassa plasma membrane fraction with pertussis toxin results in ADP -ribosylation of a protein substrate which is the approximate size of Gna-1. The predicted Gna-2 protein sequence does not share a high degr ee of sequence identity with the G(i) class. However, the coding regio n contains at least one intron in a position conserved in the G(i) fam ily. We propose that the G(i) family of alpha subunits is ancient and during evolution may have first appeared in filamentous fungi.