Identification of a G protein alpha subunit from Neurospora crassa that is a member of the Gi family.

Heterotrimeric G proteins, consisting of alpha, beta, and gamma subunits, are implicated in major signal transduction pathways controlling a diversity of functions in eukaryotic organisms. In the filamentous fungus 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 alpha subunits, gna-1 and gna-2, from Neurospora. The genes are located on different chromosomes and are differentially regulated during asexual development. The encoded proteins (Gna-1 and Gna-2) are the same size as members of the Gi-alpha family (approximately 40 kDa). The Gna-1 protein sequence is 55% identical overall to members of the Gi family and contains the consensus sequences for ADP-ribosylation by pertussis toxin and incorporation of myristic acid, which are found in this group. These properties make Gna-1 the first identified microbial alpha 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 degree of sequence identity with the Gi class. However, the coding region contains at least one intron in a position conserved in the Gi family. We propose that the Gi family of alpha subunits is ancient and during evolution may have first appeared in filamentous fungi.