Migratory localization of cyclin D2-Cdk4 complex suggests a spatial regulation of the G1-S transition

The association of the cyclin D-Cdk (DC) complex with retinoblastoma protein (pRb) is required for the G1-S transition of the cell cycle. Cyclin synthesis, nuclear localization and degradation are control mechanisms for the transition, but regulation of the DC complex nuclear import also contributes to the transition. Analysis of the timing of the G1-S transition in mammalian cell lines revealed acceleration with overexpression of cyclin D2 and Cdk4. Immunolocalization assays revealed that cyclin D2 and Cdk4 formed a complex in the cytoplasm and approached the nucleus. They accumulated on the cytosolic surfaces of the nuclear pores and then were arrested at the nuclear membrane before the nucleus reached a critical size. Finally, the complex was released into the nucleus and colocalized with pRb there, which led to pRb phosphorylation and DNA synthesis. The translocalization depended on the G1-S transition. In contrast, a truncated cyclin D2 that was not able to fully associate with Cdk4 lost the ability for release into the nucleus. This pattern of translocalization suggests a spatial separation of the cyclin D-Cdk complex from pRb and DNA in the nucleus to regulate the G1-S transition.