Separation of a murine leukaemia virus protein kinase activity from its Pr65gag polyprotein substrate after DNA--cellulose chromatography.

We have recently found that, in vitro, the murine leukaemia virus (MuLV)-associated protein kinase activity predominantly phosphorylates Pr65gag, a virus protein present in relatively small amounts in partially purified virus preparations. Other virus proteins, such as p10, Pr27gag and Pr40gag, are also phosphorylated in vitro, but to a lesser degree. Furthermore, when immature core subparticles which are enriched in Pr65gag are prepared from virions by Sepharose 6B exclusion column chromatography, about 50% of the kinase activity (as assayed with the exogenous substrate phosvitin) remains associated with the cores. We report here that this core-associated activity is distinct from Pr65gag since it can be separated from Pr65gag by chromatography on denatured DNA--cellulose columns followed by centrifugation of the 0.2 M-NaCl-eluted fraction. Under these conditions, Pr65gag is pelleted while the kinase activity, which can phosphorylate both endogenous (MuLV Pr65gag and p10) as well as exogenous (phosvitin) substrates, remains in the supernatant. Interestingly, when the amount of Pr65gag is reduced, as in such preparations, p10 then becomes more heavily phosphorylated.