Purification and Molecular Cloning of Bovine GAP
The ras oncogenes encode 3 highly homologous proteins: Harvey (Ha), Kirsten (Ki), and N-ras (Barbacid, 1987). These proteins are 21 kDa (p21) and have the biochemical properties of GTP and GDP binding, GTPase activity, and membrane localization. The ras-encoded proteins are present in normal mammalian cells and in cells of diverse evolutionary origin such as yeast, Drosophila, Dictyostelium, and Xenopus. Up to 40% of human tumors have been identified as having a biologically activated ras gene. Activation can occur by point mutations that inhibit GTPase activity or facilitate GTP for GDP nucleotide exhange. By analogy to the GTP/GDP cycle of known guanine nucleotide-binding proteins, both of these activation mechanisms are predicted to promote formation of the biologically active ras p21-GTP complex. Although the target of ras protein in the yeast S. cerevisiae has been identified as adenylyl cyclase (Toda et al., 1985), the ras p21 target in mammalian and other eucaryotic cells has not yet been discovered.
KeywordsAdenylyl Cyclase GTPase Activity GTPase Activate Protein Nonreceptor Tyrosine Kinase Intrinsic GTPase Activity
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