A number of different conditions were evaluated to obtain a quantitatively efficient isolation procedure for the production of mouse mammary tumor virus (MMTV) cores. The efficiency of each procedure was determined by the recovery of radiolabeled MMTV-RNA in particles with a density of 1.24 - 1.26 g/cm3. Maximum recoveries of 40-60% were obtained with pretreatment of virions with 0.5 - 1% nonionic detergent at 37° for 15 min followed by isopycnic centrifugation. The resulting particles were morphologically identical to the internal core structure of MMTV virions and contained 60-70 S RNA and reverse transcriptase. The isolated cores were analyzed by immunodiffusion and by sodium dodecyl sulfate (SDS)-polyacrylamide-gel electrophoresis. The major polypeptides of the cores were p30, p28, and p14. Two polypeptides, p28 and p14, were also major virion polypeptides. The emergence of p30 as a major core polypeptide was considered to be from interconversion and selective retention of other MMTV polypeptides. The envelope glycoproteins could not be identified among the many minor polypeptides.
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