Viral P21 Ki‐RAS protein: A potent intracellular mitogen that stimulates adenylate cyclase activity in early G 1 phase of cultured rat cells

Rat kidney (NRK) cells infected with a temperature‐sensitive mutant of the Kirsten sarcoma virus were arrested in the G 0 /G 1 phase of their cell cycle by incubation in serum‐deficient medium at a p21 ‐inactivating temperature of 41°C. These quiescent ts K‐NRK cells were then stimulated to transit g 1 and initiate DNA replication by lowering the temperature to 36°C, which rapidly reactivated p21 Reactivating the viral Ki‐RAS protein by temperature shift led to an increase in adenylate cyclase activity in early G 1 phase. The Ki‐RAS protein increased the sensitivity of adenylate cyclase to guany1 nucleotides by a mechanism that seemed to involve inactivation of the enzyme's inhibitory G 1 regulatory protein.