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Cell cycle and cyclic AMP‐dependent phosphorylation of plasma membrane proteins p14 and p24: Defects in smooth surface transformed cells

Two proteins which are localized to the cytoplasmic surface of the plasma membrane, p14 and p24, undergo cyclic AMP‐dependent phosphorylation in rapidly growing nontransformed murine embryo cells. In this cell system, growth arrest in the G1 phase of the cell cycle induced by growth factor deprivation is associated with the reversible loss in ability to phosphorylate these substrates. By contrast Simian virus 40 and methylcholanthrene transformed cells show both defective G1 growth control and defects in their ability to phosphorylate p14 and p24 under all tested growth conditions. These data suggested a correlation between defects in the physophorylation of p14 and p24 and defects in the ability of transformed cells to G1 growth arrest. The results of the current studies by contrast show that 3T3 T proadipocytcs which have been transformed by the smooth surface tumorigenesis method show different characteristics. They retain the ability to G1 growth arrest in serum‐deficient medium. They show cyclic AMP‐dependent phosphorylation of p14 and p24 during exponential growth. They do not, however, down regulate p14 and p24 phosphorylation in association with G1 growth arrest. These observations suggest that neoplastic transformation is not necessarily associated with absolute defects in the ability to phosphorylate p14 and p24. Rather, the results of the current study suggest that the inability to modulate the cyclic AMP‐dependent phosphorylation of plasma membrane p14 and p24 proteins during the G1 phase of the cell cycle may be more tightly associated with neoplastic transformation.