Immunochemical characterization of two major polypeptides from murine mammary tumor virus

A major murine mammary tumor viral (MMTV) antigen, sl, originally described by Nowinski et al. (1967, 1968, 1971), has been purified from RIII mouse milk MMTV by sequential ion-exchange and gel chromatography. The purified protein with sl antigenic reactivity contains carbohydrate, and has an apparent minimal molecular weight of 52,000. It can be designated as gp52 (sl). Another major MMTV viral protein with a molecular weight of 27,000 has also been isolated, and antisera have been prepared against it. Both MMTV gp52 (sl) and p27 viral polypeptides have been iodinated with (125)I and used in immunoprecipitation and competition assays. The two MMTV proteins differ absolutely from each other and from major mouse type C viral polypeptides in molecular weight, immunological reactivity, and amino acid composition. Purified gp52 (sl) in radioimmunoprecipitation inhibition assays reacted in two distinct patterns. One pattern showed partial displacement of antibody which could be converted to the second, a complete displacement, by heating the antigen, presumably by exposing additional reactive determinants. Biologically, the patterns of major MMTV polypeptide expression in milk correlated with spontaneous mammary tumor incidence in different strains of mice, indicating that the sl antigen is group specific for MMTV or that several mouse strains contain the same virus type.