Purification and characterization of a transformation-dependent protein secreted by cultured murine fibroblasts

The major excreted protein (MEP) of transformed mouse fibroblasts has been purified, and monospecific antisera against it have been prepared. Synthesis and secretion of this protein have previously been shown to be stimulated by transformation or treatment with tumor-promoting phorbol esters, but its function is still not known [Gottesman, M. M. (1978) Proc, Natl. Acad. Sci. U.S.A. 75, 2767--2771; Gottesman, M. M., & Sobel, M. E. (1980) Cell (Cambridge, Mass.) 19, 449--455]. The purified protein shows charge heterogeneity by two-dimensional gel electrophoresis; the major intracellular and extracellular species have a molecular weight of 35 000 and a pI of 6.8--7.3. The purified secreted protein contains approximately 5--10% neutral sugar by weight and binds specifically to a concanavalin A--Sepharose affinity column. Translation of messenger ribonucleic acid (mRNA) from cells actively synthesizing MEP in cell-free reticulocyte or wheat germ systems, which are reported to be unable to glycosylate translated proteins, results in a product of Mr 33 000 which is presumably devoid of neutral sugar. However, on two-dimensional electrophoresis, the MEP mRNA translation products continue to show charge heterogeneity similar to that seen in intact cells, suggesting that there may be multiple coordinately controlled mRNAs for MEP or a single mRNA species which can be translated in a variety of ways.