Heterotrimeric guanine nucleotide binding proteins, commonly known as G proteins form a super-family of signal transduction proteins. They are peripherally associated with the plasma membrane and provide signal coupling to seven transmembrane surface receptors. G proteins are composed of monomers of alpha, beta, and gamma subunits. The beta- and gamma-subunits are tightly associated. The receptors activated by the appropriate "signal", interact catalytically with specific G-proteins to mediate guanine nucleotide exchange at the GDP/GTP binding site of the G-protein alpha-subunits, thus displacing the bound GDP for GTP. The GTP bound form of the g-protein alpha-subunit and in some cases the free betagamma-subunits initiate cellular response by altering the activity of specific effector molecules. Recent studies have indicated that the asyncronous activation of these proteins can lead to the oncogenic transformation of different cell types. The mechanism by which G-proteins regulate the various cell functions appear to involve a complex net-working between different signaling pathways. This review summarizes the signaling mechanisms involved in the regulation of cell proliferation by these transforming G proteins.
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