Abstract
Subcellular fractions have been isolated from synchronized fibroblasts in S phase and from parallel cultures in which effective entry into S is prevented by either hydroxyurea or cholera toxin, for analysis of changes in endogenous protein phosphorylation with ATP-γ- 32P. Soluble fractions from cells exposed to cholera toxin or hydroxyurea revealed a similar pattern which differed from that of control cells in which a 90K phosphoprotein was evident. Ribosomal and nuclear fractions from hydroxyureatreated cells revealed different phosphoprotein patterns from those seen in cholera toxin-treated cells, supporting a different mode of action of both drugs in their inhibition of DNA synthesis.
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