This report presents evidence which supports a relationship between intracytoplasmic A particles (CAP) and mouse mammary tumour virus (MMTV). Three MMTV-specific antigenic determinants in CAP (MMTV p27, p14 and p10) were detected by immunodiffusion. No structural proteins of comparable mol. wt. were found in CAP; however, exposure of CAP to trypsin resulted in the cleavage of the CAP structural proteins to MMTV-like polypeptides. This process was accompanied by the preservation of MMTV-specific antigenic determinants. Disulphide bonds were necessary for the structural maintenance of CAP. Reducing agents destroyed the organized structure of CAP, whereupon processing of CAP proteins to MMTV-like polypeptides by trypsin was prevented. CAP p82, possessed only MMTV p27 antigenic determinants, while CAP polypeptides p20–18 possessed p10 antigenic determinants. Following processing of CAP structural proteins by trypsin, MMTV-specific p27 antigenic determinants were shifted from CAP p82 to CAP p27; MMTV-p10 antigenic determinants were found with CAP p15-10. These results suggest a model wherein CAP structural proteins are modified by protease during maturation, resulting in the shift of their proteins to sizes consistent with those which have been currently identified as the major internal components of the virion and that this phenomenon is largely predicated on the folding of CAP proteins into the morphologically intact A particle.


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