Role of serine 10 phosphorylation in p27 stabilization revealed by analysis of p27 knock-in mice harboring a serine 10 mutation

The inhibition of cyclin-dependent kinase activity by p27 contributes to regulation of cell cycle progression. Serine 10 is the major phosphorylation site of p27, and its phosphorylation has been shown to affect the stability and nuclear export of p27 at the G₀-G₁ transition in iransfected cultured cells. To investigate the physiological relevance of p27 phosphorylation on Ser¹⁰, we generated p27 "knock-in" mice that harbor an S10A mutation in this protein. Mice homozygous for the mutation (p27^S10A/S10A mice) were normal in body size, but the abundance of p27 was decreased in many organs, including brain, thymus, spleen, and testis. The stability of p27 in G₀ phase was markedly reduced in lymphocytes of p27^S10A/S10A mice compared with that in wild-type cells, whereas p27 stability in S phase was similar in cells of the two genotypes. The degradation of p27 in cells of the mutant mice at G₀ phase was prevented by a proteasome inhibitor. These data indicate that the physiological role of p27 phosphorylation on Ser¹⁰ is to stabilize the protein in G₀ phase. Unexpectedly, the nuclear export of p27 at the G ₀-G₁ transition occurred normally in p27 ^S10A/S10A mouse embryonic fibroblasts, indicating that phosphorylation of Ser¹⁰ is dispensable for this process.