Fibroblast growth factor causes an early increase in phosphorylation of a membrane protein in quiescent 3T3 cells.

THE regulatory systems controlling cell division have not been identified, but it has been shown that growth factors such as epidermal growth factor, fibroblast growth factor (FGF), and serum initiate rapid changes in cellular metabolism, probably involving post-transcriptional control mechanisms(1-5). As phosphorylation has been shown to be an important regulatory mechanism in several metabolic pathways, we initiated experiments to determine whether factors which stimulate DNA synthesis also stimulate endogenous phosphorylation. We find mat, within 5 min of addition of FGF or serum to (32)P-labelled Swiss 3T3 cells, there is a specific increase in the phosphorylation of a membrane protein with an apparent molecular weight of 33,000. Experiments with isolated cell fractions demonstrate that the phosphorylation of this protein is stimulated by cyclic AMP. This rapid and specific response to mitogens raises the possibility that this phosphorylation might be part of the initial, cellular signal for DNA synthesis.