Characterization of activated and normal mouse Mos gene in murine 3T3 cells.

We have characterized the mouse Mos proto-oncogene product, pp39Mos, in murine fibroblasts. When expressed in NIH3T3 cells under the influence of the long terminal repeat regulatory element from Moloney murine sarcoma virus [NIH(pTS-1) cells], the Mos protein was present in low levels and had a half-life of about 30 min. In extracts from NIH(pTS-1) cells, we detected additional forms of Mos protein that apparently arose from internal initiation codons (p24Mos and p29Mos) or from upstream non-AUG initiation codons (p42Mos and p44Mos). The Mos protein was found to exist in these cells as a phosphoprotein, pp39Mos, and, when immunoprecipitated with an antiserum specific for the Mos N-terminus [anti-Mos(6-24)], had autophosphorylating kinase activity. We found that anti-Mos(6-24) also detected non-Mos protein kinase activity and non-Mos phosphoproteins in addition to p39Mos. We present evidence, on both the RNA and protein levels, that non-transformed mouse 3T3 cells do not express endogenous Mos.