A basic cytoplasmic protein (p27) induced by serum in growth-arrested 3T3 cells but constitutively expressed in primary fibroblasts.

Serum stimulation of quiescent 3T3 cells immediately induces the synthesis of a set of basic proteins that are absent in growing cells. The induction of some of these polypeptides p27 (27 kd), p35 (35 kd), p38 (38 kd) and p69 (69 kd) can be 'superinduced' in the presence of cycloheximide and completely blocked by actinomycin D. In vitro translation experiments show that the levels of mRNA coding for these proteins in serum-stimulated cells are several fold higher than in non-stimulated cells. Induction of p35 and p38 is transient (4 h); in contrast, p27 and p69 are induced for a longer period (8 h). Platelet-derived growth factor and fibroblast growth factor strongly induce p35 and p69 but weakly induce p27 and p38. Cultures of primary mouse fibroblasts express p27 but not the other polypeptides at levels similar to those found in serum-stimulated quiescent 3T3 cells. Enucleation and Triton extraction of cells show that p27 is a soluble cytoplasmic protein. The synthesis of this protein in density-arrested or serum-deprived primary cultures is only 20% reduced showing that the expression of p27 in these cells is independent of cell proliferation.