Characteristics of the structural components of the mouse mammary tumor virus: II. Viral proteins and antigens

Abstract Mild Tween 80-ether treatment of mouse mammary tumor virus (MTV) results in disruption of the viral membrane and release of subviral components. Prolonged lipid extraction, however, results in virtually complete solubilization of the virion. Soluble proteins have been separated by Sephadex G-200 chromatography and used for immunization of rats These antisera identify five viral antigens (designated MTV - s 1, s 2, s 3, s 4, and s 5). The s 1 and s 2 antigens are group-specific, and are the antigens of the viral nucleoid. MTV - s 3 antigen is a structural protein of the viral membrane. Poly-acrylamide gel electrophoresis of MTV shows five major polypeptides in the virion; the molecular weights of these are p1 (90,000), p2 (70,000), p3 (52,000), p4 (33,000), and p5 (23,000). The major protein of the virus is p3; it is found in the nucleoid and carries the s 1 antigen. Protein p5 is the structural protein of the membrane, and it carries the s 3 antigen. Antisera prepared against isolated MTV proteins were used also in immunofluorescence tests to determine the intracellular deposition of MTV antigens. These studies show that all MTV structural proteins are synthesized cytoplasmically. Screening of a variety of neoplasms by immunofluorescence and immunodiffusion tests demonstrates MTV infection in only three types of neoplastic diseases, mammary tumors, ML+ leukemias, and Leydig cell tumors of MTV-infected mice. Mammary tumors synthesize all viral structural proteins, while the synthesis of MTV proteins in the ML+ leukemias and Leydig cell tumor is limited to the s 1 and s 2 antigens. Electron microscopic examination of these tissues confirms the synthesis and accumulation of viral nucleocapsids (A particles) without the production of budding virions.