ABERRANT FUNCTION OF THE RAS-RELATED PROTEIN TC21 R-RAS2 TRIGGERS MALIGNANT TRANSFORMATION/

Although the human Ras proteins are members of a large superfamily of Ras-related proteins, to date, only the proteins encoded by the three mammalian ras genes have been found to possess oncogenic potential. Am ong the known Ras-related proteins, TC21/R-Ras2 exhibits the most sign ificant amino acid identity (55%) to Ras proteins. We have generated m utant forms of TC21 that possess amino acid substitutions analogous to those that activate Ras oncogenic potential [designated TC21(22V) and TC21(71L)] and compared the biological properties of TC21 with those of Ras proteins in NIH 3T3 and Rat-1 transformation assays. Whereas wi ld-type TC21 did not show any transforming potential in vitro, both TC 21(22V) and TC21(71L) displayed surprisingly potent transforming activ ities that were comparable to the strong transforming activity of onco genic Ras proteins. Like Ras-transformed cells, NIH 3T3 cells expressi ng mutant TC21 proteins formed foci of morphologically transformed cel ls in monolayer cultures, proliferated in low serum, formed colonies i n soft agar, and developed progressive tumors in nude mice. Thus, TC21 is the first Ras-related protein to exhibit potent transforming activ ity equivalent to that of Ras. Furthermore, mutant TC21 proteins also stimulated constitutive activation of mitogen-activated protein kinase s as well as transcriptional activation from Ras-responsive promoter e lements (Ets/AP-1 and NF-kappa B). We conclude that aberrant TC21 func tion may trigger cellular transformation via a signal transduction pat hway similar to that of oncogenic Ras and suggest that deregulated TC2 1 activity may contribute significantly to human oncogenesis.