ViralP21 Ki-RAS protein: A potent intracellular mitogen that stimulates adenylate cyclase activity in early G1 phase of cultured rat cells

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Abstract
Rat kidney (NRK) cells infected with a temperature‐sensitive mutant of the Kirsten sarcoma virus were arrested in the G0/G1phase 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 g1 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 G1 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 G1regulatory protein.