Transforming NRK cells with avian sarcoma virus reduces the extracellular Ca requirement without affecting the calcicalmodulin requirement for the G1/S transition

NRK rat cells infected with a transformation-defective, temperature-sensitive (ts) mutant of the avian sarcoma virus could not proliferate in Ca²⁺-deficient medium at a nonpermissive temperature (40 °C) that inactivated the viral pp60^v-scr-transforming product and rendered the cells phenotypically untransformed. However, these arrested cells were stimulated to initiate DNA replication with little or no delay while still in the Ca²⁺-deficient medium, either by adding Ca²⁺ or calmodulin at 40 °C or by reducing the temperature to 36 °C which restored the transformed phenotype by rapidly reactivating pp60^v-src. The G1/S transition triggered by restoring the transformed phenotype was suppressed by three different anticalmodulin drugs (R24571, trifluoperazine, W7). The suppression by one of these drugs, trifluoperazine, was overcome by adding calmodulin. Thus, neoplastic transformation by the avian sarcoma virus sharply reduces the extracellular Ca²⁺ requirement for the initiation of DNA replication without bypassing a calcical-modulin-dependent mechanism also needed for the G1/S transition.