Studies on the conformation and functions of ras-related low Mr GTP-binding proteins.

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It is well known that a variety of ras-related low Mr GTP-binding proteins are present from yeast to mammalian. We have purified and identified several kinds of low Mr GTP-binding proteins such as rhoA, rac,ram and c25KG proteins in mammalian tissues. rhoA was identified a sole substrate of botulinum C3 enzyme in human platelet and suggested to be involved in cytoskeletal regulation. We observed that the C3 enzyme-like enzyme is present in bovine brain and partially purified it. rho protein was also observed to take part in contraction of smooth muscle although the mechanism has not been totally elucidated. rac protein was reported in phagocytes to control the superoxide-generating NADPH oxidase system which consists of at least one membrane and three cytosolic components. In yeast, SAR1 plays an essential role in intracellular protein transport and RSR1 has similar function as that of mammalian homolog, rap1.Recently, several kinds of regulatory proteins for low Mr GTP-binding proteins have been reported. GTPase activating protein for ras p21 (ras-GAP) was first purified, identified and cloned. Thereafter, we purified GAPs for smg p21 and rho p20. GDP dissociation stimulator (GDS) and GDP dissociation inhibitor (GDI) have also been purified and identified for some low Mr GTP-bining proteins such as ras p21, smg proteins, rho p20, and smg p25A. The modes of the action of these regulatory proteins on the GTP-binding proteins were investigated and it was shown that the C-terminal prenylation such as farnesylation and geranylgeranylation is necessary for their functions.