M-Ras, a widely expressed 29-kD homologue of p21 Ras: expression of a constitutively active mutant results in factor-independent growth of an interleukin-3-dependent cell line.

M-Ras, a recently identified homologue of p21 Ras, is widely expressed, with levels of the 29-kD protein in spleen, thymus, and NIH 3T3 fibroblasts equaling or exceeding those of p21 Ras. A G22V mutant of M-Ras was constitutively active and its expression in an interleukin-3 (IL-3)-dependent mast cell/megakaryocyte cell line resulted in increased survival in the absence of IL-3, increased growth in IL-4, and, at high expression levels, in factor-independent growth. Expression of M-Ras G22V, however, had a negative effect on growth in the presence of IL-3, suggesting that M-Ras has both positive and negative effects on growth. Expression of M-Ras G22V in NIH-3T3 fibroblasts resulted in morphological transformation and growth to higher cell densities. M-Ras G22V induced activation of the c-fos promoter, and bound weakly to the Ras-binding domains of Raf-1 and RalGDS. Expression of a mutant of M-Ras G22V that was no longer membrane-bound partially inhibited (40%) activation of the c-fos promoter by N-Ras Q61K, suggesting that M-Ras shared some, but not all, of the effectors of N-Ras. An S27N mutant of M-Ras, like the analogous H-Ras S17N mutant, was a dominant inhibitor of activation of the c-fos promoter by constitutively active Src Y527F, suggesting that M-Ras and p21 Ras shared guanine nucleotide exchange factors and are likely to be activated in parallel. Moreover, M-Ras was recognized by the monoclonal anti-Ras antibody Y13-259, commonly used to study the function and activity of p21 Ras. Mammalian M-Ras and a Caenorhabditis elegans orthologue exhibit conserved structural features, and these are likely to mediate activation of distinctive signaling paths that function in parallel to those downstream of p21 Ras.