G Proteins participate in the transduction of cellular signals by cycling between an active conformation (complexed with GTP) D and an inactive conformation (complexed with GDP). The ras proteins are guanine nucleotide binding proteins believed to participate in the transduction of proliferative signals, although their specific role is still unknown. Using a variety of experimental approaches, we showed that they are not regulatory elements of either of the two main signaling pathways in mammalian cells, namely the adenylate cyclase and phosphoinostide pathways; however, using Xenopus oocytes, we were able to show that they exert a regulatory role (even in the absence of protein synthesis) in cascades of phosphorylation associated with the induction of proliferation and cell division. The HIV nef gene product has also been proposed to act as a regulatory G protein based on reports attributing GTP binding and GTPase activity to these proteins. We expressed different cDNA clones of nef using the same bacterial and mammalian vectors used previously for ras. Unlike ras, nef from the different HIV isolates lacked GTP binding activity but showed autophosphorylation using either GTP or ATP as the phosphate donor. In addition, unlike ras, HIV nef did not exhibit oncogenic potential in focus-forming assays with NIH 3T3 cells, nor did it cause meiotic maturation of Xenopus oocytes. It therefore appears that the biological function of nef does not follow the pattern of G proteins; however, its phosphorylation, either by protein kinase C or through its autokinase activity, may be functionally significant in vivo.
