Microsomal localization of cyclin A and cdk2 in proliferating rat liver cells.

The expression and intracellular localization of cyclin A and cdk2 have been analyzed in rat liver cells proliferatively activated in vivo by a partial hepatectomy. Western blot analysis revealed that cyclin A started to increase during G1 (at 6 h after hepatectomy) reaching maximal levels during S phase (at 18 h). Cdk2 began to increase during late G1 (at 12 h) peaking also at 18 h. At the latter time cyclin A was mainly localized in the microsomal fraction, although it was also present in cytosol, plasma membrane and nucleus. Active cyclin/cdks complexes containing cyclin A and cdk2 were obtained by precipitation with p13-Sepharose after solubilization of microsomes with triton X-100. The presence of active cyclin A/cdk2 complexes in microsomes was confirmed by immunoprecipitation experiments with anti-cdk2 antibodies. These results suggest a putative role of cyclin A/cdk2 during S phase which would be related with microsomal function.