The Pr65 proteolytic factor obtained from Moloney (MoLV) or Rauscher (RLV) leukaemia virus has been characterized. We found that it was present in small amounts in virions and was extremely unstable. Although it eluted at the trailing edge of p12 on Sephadex G-75 columns, it could clearly be separated from p12 on DEAE-Sephadex A-50M columns, making it unlikely that the factor is p12 or any other major murine leukaemia virus (MuLV) protein. This fact also distinguishes the murine factor from the avian p15-associated protease which is present in large amounts in avian tumour viruses and is stable to column purification methods (von der Helm, 1977; Dittmar & Moelling, 1978). We further observed that: (i) the murine proteolytic factor had an estimated mol. wt. of 20000 to 22000, relative to MuLV p12, which eluted as a dimer on Sephadex G-75 columns in the presence of 0.1% NP-40; and (ii) cleavage of an iodinated Pr65-rich, p30-deficient substrate yielded a clear increase in both p30 and p12, which suggests that the cleavage of Pr65 is similar to its processing .


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