GTP-binding proteins as oncogenes in human tumors.

Ras oncogenes are the most commonly known oncogenes in human tumors; their prevalence in different types of tumors and their role in the development of these tumors have been discussed in recent reviews (1,2). We have investigated the biochemical mechanism that accounts for the activation of ras proto-oncogenes to become oncogenes. In normal cells, the ras p21 cycles between the active guanosine triphosphate (GTP)bound form and the inactive form, bound to guanosine diphosphate (GDP). The conversion of the GTPto the GDP-bound form is catalyzed by a cellular protein referred to as GAP (GTPase activating protein) (3). The mechanism by which the GTP-bound state is generated is not known; however, this mechanism must be much slower than the GAP-catalyzed reaction, so that most of the ras p21 in normal cells is in the GDP-bound state. In ras-transformed cells, the oncogenic mutant ras protein exists in its GTP-bound state constitutively. This is because GAP fails to catalyze hydrolysis of GTP bound to these proteins. Oncogenic mutants can therefore be thought of as mutants that escape from this aspect of GAP function. Two types of GAP cDNA have been cloned from human placenta (4). Figure 1 shows the structure of the proteins encoded by these cDNAs. Type I GAP is a 116,000 Da protein, with a hydrophobic amino terminus, two SH2 regions (src-homology region 2), one SH3 region, and a catalytic domain comprising the C-terminal one-third of the protein (5). All cells express type I GAP, whereas type II, which lacks the first 180 amino acids of type I GAP and is generated by an alternative splicing mechanism, has only been detected in human placenta and in certain human cell lines, such as MRC5 diploid fibroblasts. Interaction of GAP with ras p21 may have consequences additional to conversion of the GTP-bound to the GDP-bound form. This possibility was raised by the