p21 of Kirsten murine sarcoma virus is thermolabile in a viral mutant temperature sensitive for the maintenance of transformation.

We have recently described an intracellular protein, p21, in nonproducer cells transformed by either the Kirsten (Ki-MSV) or Harvey (Ha-MSV) strain of murine sarcoma virus (Shih et al., Virology, in press). The p21 is phosphorylated and has been shown to be coded for by either Ki-MSV or Ha-MSV. In this report, we compare the thermal stability of the newly synthesized [35S]methionine-labeled p21 in cells transformed by the wild-type Ki-MSV or by a mutant of Ki-MSV (ts 371) which is temperature sensitive in a viral function required for the maintenance of several properties of the transformed phenotype. The immunoprecipitability of the p21 coded for by the ts 371 Ki-MSV was markedly more thermolabile than the p21 of the wild-type Ki-MSV when the cell extracts are heated in vitro. The present finding suggests that the p21 is required for the maintenance of transformation induced by Ki-MSV.