Endogenous membrane phosphorylation increases in serum‐stimulated 3T3 cells

Autophosphorylation of 3T3 cells, utilizing endogenous membrane protein kinase, can be detected by incubating the cells with μgM³²P‐ATP. The phosphorylation activity of growing cells is two to four‐fold greater than quiescent ones. In this study, the increased phosphorylation activity of serum‐stimulated cells was examined. Phosphorylation, measured at times after serum stimulation of quiescent cultures, was found to increase in early G₁ and to reach a maximum prior to DNA synthesis. This increase in stimulated cells was dependent on RNA and protein synthesis but not on DNA synthesis. The increased activity decayed quickly (half‐life approximately 2–3 hours) in the presence of cycloheximide, while the basal activity in quiescent cells was relatively unchanged. Insulin, prostaglandin E₁ or prostaglandin F2α were also found to bring about the same increase in phosphorylation as serum, although in contrast with serum they caused only a small percentage of the culture to synthesize DNA. The results suggest that enhanced phosphorylation activity is a G₁ event. It does not depend on subsequent DNA synthesis. Phosphorylation may be one of the biochemical steps in G₁, necessary but not sufficient for cells to move into S phase.