Antisera against the five major polypeptides of the mouse mammary tumor virus (MMTV), gp52, gp36, p28, p14, and p10, were characterized. Anti-gp52, anti-gp36, anti-p28, and anti-p10 reacted exclusively with their respectve antigens. Anti-p14 reacted with p28 and p10 but not with p14. Virions, bald particles, cores, and A particles all reacted with anti-p28 and anti-p10. SDS-PAGE revealed that p28 and p10 were the major polypeptides in virions and bald particles but that p10 was reduced in the cores. No mature MMTV polypeptides were identified in the A particles by SDS-PAGE. A salt-detergent extraction for hydrophobic proteins revealed that p10 was distinctly hydrophobic. These results are consistent with the hypothesis that A particles are precursors to MMTV cores. They also suggest that p28, p14, and p10 come from a common precursor particle but that during maturation p10 is released from the MMTV core to associated with the envelope. A model of MMTV assembly and maturation is offered which integrates these observations.
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