Transforming NRK cells with avian sarcoma virus reduces the extracellular Ca2+ 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 degrees C) that inactivated the viral pp60v-src-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 degrees C or by reducing the temperature to 36 degrees C which restored the transformed phenotype by rapidly reactivating pp60v-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 calcicalmodulin-dependent mechanism also needed for the G1/S transition.