SUMMARY

The Pr65 gag proteolytic factor obtained from Moloney (MoLV) or Rauscher (RLV) leukaemia virus has been characterized. We found that i was present in small amounts in virions and was extremely unstable. Although it eluted at the trailing edge of pt 2 on Sephadex G-75 columns, it could clearly be separated from pI 2 On DEAE-Sephadex A-5oM 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 p~5-associated protease which is present in large amounts in avian tumour viruses and is stable to column purifica-tion methods (yon der Helm, I977; Dittmar & Moelling, I978). We further observed that: (i) the murine proteolytic factor had an estimated mol. wt. of 2o ooo to 22 ooo, relative to MuLV p I 2, which eluted as a dimer on Sephadex G-75 columns o / NP-4o: and (ii) in vitro cleavage of an iodinated Pr65 ga~- in the presence ofo. ~ /o rich, p3o-deficient substrate yielded a clear increase in both p3o and pI2, which suggests that the in vitro cleavage of Pr65 ~g is similar to its processing in vivo