Purification of immature cores of mouse mammary tumor virus and immunolocalization of protein domains.

The immature capsids of the mouse mammary tumor virus (MMTV), known as intracytoplasmic A particles, have been isolated from murine L1210 leukemia cells. The diameter of the isolated particles was 80 nm as determined by negative staining. Two polypeptides of 77 and 110 kDa were found to be their major polypeptide components, in agreement with the expected sizes of the Gag and Gag-Pro precursor polypeptides of the mature MMTV proteins. Both polypeptides were recognized by antibodies directed toward the matrix (p10) and capsid (p27) proteins of MMTV. Immunogold labeling of p10 on isolated A particles, visualized by negative staining, showed that this protein is located at the surface of the immature capsids, whereas p27 can be detected only in broken or disrupted particles, suggesting that it has an internal location. These observations were confirmed by immunolabeling of both proteins on thin sections of A particle-producing cells. In addition, the viral protease had a more internal position than p27. Since the sequential order of the viral proteins in the Gag precursor is p10-pp21-p27-p14 and that in Gag-Pro is p10-pp21-p27-p30-protease, our results demonstrate the radial organization of the polypeptide precursors forming the intracytoplasmic A particles.