Suppression of c-ras transformation by GTPase-activating protein

Abstract

THE ras genes are required for normal cell growth and mediate transformation by oncogenes encoding protein tyrosine kinases^1,2. Normal ras can transform cells in vitro and in vivo, but mutationally activated ras does so much more efficiently, and highly transforming mutant versions of ras have been isolated from a variety of human and animal tumours (reviewed in refs 3–5). The ras genes encode membrane-associated, guanine nucleotide-binding proteins that are active when GTP is bound and inactive when GDP is bound^6–8. The slow intrinsic GTPase activity of normal mammalian Ras proteins can be greatly accelerated by the GTPase-activating protein (GAP), which is predominantly cytoplasmic. This activity of GAP, which can increase with cell density in contact-inhibited cells⁹, suggests that it functions as a negative, upstream regulator of ras. Other studies^10,11, however, show that GAP interacts with a region of ras-encoded protein implicated in ras effector function^12,13, which raises the possibility that GAP might also be a downstream target of ras (reviewed in ref. 14). Mutationally activated ras-encoded proteins also interact with GAP¹⁵, although they are resistant to its catalytic activity⁷. In an attempt to define the role of GAP in ras-mediated transformation, we examined the effects on transformation of normal or mutant ras when cells overexpress GAP. We found that GAP suppresses transformation of NIH 3T3 cells by normal Ha-ras (c-ras) but does not inhibit transformation by activated Ha-ras (v-ras). These results support the hypothesis that GAP functions as a negative regulator of normal ras and make it unlikely that GAP alone is the ras target.