The cyclin-dependent kinase inhibitor p27(Kip1) is localized to the cytosol in Swiss/3T3 cells

p27(Kip1) plays an important role in cell cycle progression by negatively r egulating the activity of cyclin-Cdk complexes. To understand how p27(Kip1) functions, the level and subcellular location of p27(Kip1) in Swiss/3T3 ce lls following serum stimulation of quiescent cells was examined. Surprising ly, p27(Kip1) was observed exclusively in the cytosol throughout G1 and int o early S phase. However, as expected, p27(Kip1) in the cytosolic fraction was greatly reduced following serum stimulation and reached very low levels hy late G1, The decline in the level of p27(Kip1) corresponded in time to an increase in the nuclear level of both Cdk2 and cyclin E. In quiescent 3T 3 cells Cdk2 was inactive and co-precipitated with p27(Kip1). After serum s timulation, both nuclear and cytosolic Cdk2 was activated and this correspo nded to the decline in p27(Kip1). Overexpression of p27(Kip1) allowed accum ulation of the inhibitor in the nucleus but inhibited entry of Cdk2 into th e nucleus following serum stimulation. The subcellular localization of p27( Kip1) was also examined in a,variety of other mammalian cells. In all the c ell lines examined the preponderance of p27(Kip1) was found in the cytosoli c fraction. However, a substantial level of nuclear p27(Kip1) was observed for several cell lines. In a primary mixed glial cell culture p27(Kip1) loc alized to the nucleus. The results suggest that cytosolic p27(Kip1) has a f unctional role in regulating cell cycle progression, possibly through inhib iting transport of cyclin E-Cdk 2 complexes into the nucleus.