Precursor-product relationship between nonglycosylated polypeptides of A and B particles of mouse mammary tumor virus

Abstract

Polypeptides and antigens of various preparations of intracytoplasmic A particles were analyzed in detail in comparison with those of B particles of mouse mammary tumor virus (MTV). SDS-polyacrylamide-gel electrophoresis (PAGE) revealed that B particles consisted of three major nonglycosylated polypeptides (B-p25, B-P15, and B-P7; numerals indicate calculated molecular weights in 10 3 daltons) and six glycopeptides. All A-particle polypeptides were nonglycosylated. The particles purified by a conventional method contained seven major bands in the 70,000- to 37,000-molecular weight region, but their relative amounts varied from preparation to preparation. In most preparations, one other major band, A-p7 (an A-particle polypeptide with a molecular weight of 7000), was also observed. Thus, the polypeptide composition of A particles was quite different from that of B particles, having in common only one major band, p7, and three other bands of variable amounts, p43, p37, and p13. Those A particles that had been incubated at 37° for 20 hr showed a systematic change in PAGE pattern; major bands disappeared except for A-p43, A-p37, and A-p7, while a strong new band, A-p25, appeared. Consequently, incubated A particles were now similar to B particles in their PAGE pattern except for the absence of p15 in the-A particles. In spite of such a profound change in components, no ultrastructural alteration was observed in the A particles after incubation. Polypeptide conversion induced by incubation was completely inhibited by diisopropylfluorophosphonate (DFP). A particles purified in the presence of phenylmethanesulfonyl fluoride (PMSF) consisted of a single major polypeptide, A-p70. Incubation of these A particles resulted in generation of A-p15 in addition to A-p25 and A-p7 at the sacrifice of A-p70, although the rate of polypeptide conversion was much retarded. Antigenic analysis of individual polypeptides eluted from polyacrylamide gels shows that (i) B-p25, B-p15, and B-p7 carried distinct antigens; (ii) B-p25 and B-p7 were antigenically identical with A-p25 and A-p7, respectively; (iii) A-p70 carried all three of these different antigenicities of B particles; and (iv) other major polypeptides of A particles also carried these antigens in ways characteristic to each antigen. This indicates that three major internal components of B particles are generated from a common precursor, A-p70, through enzymatic cleavage and, hence, that A particles are the real pronucleocapsids of B particles. Present observations are discussed in connection with the precursor-product relationships proposed in other RNA tumor virus systems.