Somatic cell expression of the c-mos protein

The c-mos protein has been found to be enriched in germ cells of male mice, as described in a recent report from this laboratory (Herzog et al., Oncogene 3, 225, 1988). We report on further studies which indicate that the c-mos protein (a 41 to 43 kDa protein termed p43(c-mos)) is expressed in somatic tissues of mice and in cells grown in culture. In testes of mice, germ cell fractions have increased levels of p43(c-mos) relative to other cells of the testes. However, non-germ cells harbor significant levels of p43(c-mos), as judged by comparison of testes from normal mice to those with mutations that affect the germ cell content of the testes. Thus, homozygous S1, at, and the W/W(v) mutant mice are sterile due to severe deficiencies of germ cells. Such mice had only an estimated 50%-60% reduction in p43(c-mos) as judged by western immunoblotting using two different site-directed anti-mos antibodies. Similary, X/X-sex reversed mice in which germ cells die after 10 days of age had only an 85% reduction of p43(c-mos) in mice 35 days of age. Thus, the germ cell content of testes did not correlate with p43(c-mos) levels in this tissue. Direct analyses of non-germ cells derived from mouse testes confirmed these findings, since Sertoli and Leydig cell lines grown in culture expressed p43(c-mos). In addition, tissues such as kidney, liver, spleen and brain were found to contain p43(c-mos). Surprisingly, mouse NIH3T3 cells were found to express significant levels of the c-mos protein based upon immunoblotting and one-dimensional peptide mapping experiments performed with both anti-mos antibodies. The concentration of the c-mos protein was not affected by expression of viral mos proteins. We conclude that the c-mos protein is enriched in male germ cells, but p43(c-mos) is also expressed in significant amounts in somatic tissues and in fibroblastic cells grown in culture.