The role of calmodulin in the proliferation of transformed and phenotypically normal ts ASV‐infected rat cells

NRK cells infected with a temperature‐sensitive, transformation‐defective mutant of avian sarcoma virus (ASV), ts LA23, behaved as if nontransformed at a nonpermissive 40°C and were rendered quiescent by serum deprivation. These serum‐deprived cells were stimulated to start entering S phase about 7 hours after serum addition at 40°C or about 9 hours after shifting the cultures to 36°C, a temperature allowing the production of active viral pp60 src and expression of the transformed phenotype. The transit of both serum‐and temperature‐stimulated ts LA23‐NRK cells through later G 1 was inhibited by the unrelated calmodulin antagonists W7 and R24571. The former drug was found to block the cells at a point in the cell cycle no more than 2 hours from the G 1 /S transition. The weaker calmodulin antagonist, W5, was less effective in impairing progression. Thus, calmodulin is likely required for the transit of both transformed and phenotypically normal ts LA23‐NRK cells through the later stages of their G 1 phases. Cells neoplastically transformed by ASV contain more calmodulin than uninfected, non‐neoplastic cells. At the nonpermissive 40°C, the calmodulin content of the ts LA23‐NRK cells dropped to the non‐neoplastic level. When these phenotypically non‐transformed cells were enabled to reenter the cell cycle while still in low‐serum medium by a 40 to 36°C shift, they passed through the G 1 and S phases and divided without a concomitant rise in the total calmodulin content. Thus, a calmodulin rise does not appear to be required for the expression of one characteristic of transformed cells, i.e., reduced requirement for exogenous growth factors.