Polypeptides of the mouse mammary tumor virus: I. Characterization of two group-specific antigens

Abstract

Polyacrylamide-gel electrophoresis of the mouse mammary tumor virus (MuMTV) reproducibly resolves at least 11 polypeptides. The estimated molecular weights of the five major polypeptides are 55,000 (p55), 34,000 (p34), 28,000 (p28), 18,000 (p18) and 12,000 (p12). Two of these polypeptides, p55 and p34, have been characterized as glycoproteins on the basis of staining with periodic acid-Schiff reagent and incorporation of radioactively labeled glucosamine. Intracytoplasmic A particles have been found to contain mainly five polypeptides; the molecular weights of these are 52,000 (Ap52), 43,000 (Ap43), 37,000 (Ap37), 29,000 (Ap29) and 15,000 (Ap15). Antisera prepared against purified p55 have been used for ferritin labeling by indirect immunoelectron microscopy with hybrid antibody (anti-rat-λg/anti-ferritin) and ferritin. Anti-p55 serum resulted in labeling the viral surface with ferritin; it also precipitated intact virus. These results unambiguously established that polypeptide p55 is a component of the viral surface. Antigen p55 is the most prominent viral protein and is common to viruses isolated from RIII, C3H, GR and A mouse strains. Therefore, p55 may be considered as a group-specific, surface antigen (gs-1) of MuMTV. In contrast, polypeptide p28 is not a glycoprotein; antisera against purified p28 does not react with the viral surface. Immunodiffusion results show that p55 and p28 are immunologically unique and that p28 shares a common antigenic determinant with intracytoplasmic A particles, probably with the polypeptide Ap29. Furthermore, p28 is found to be a common antigen in RIII, C3H, GR and A viruses. These data indicate that p28 is most likely a major group-specific internal antigen (gs-2) of MuMTV.