Abstract
In contrast to conventional cyclin-dependent kinases that are important for mitotic cell division, cyclin-dependent kinase 5 (CDK5) is predominantly activated in post-mitotic cells and is involved in various cellular events. The kinase activity of CDK5 is tightly regulated by specific activators including p35, p39, and cyclin I (CCNI). Here we show that cyclin I-like (CCNI2), a homolog of CCNI, interacts with CDK5 and activates the kinase activity of CDK5. Different from CCNI, which colocalizes with CDK5 in the nuclei in transfected cells, CCNI2 mainly retains CDK5 in the cytoplasm as well as on the cell membrane. Furthermore, although the expression level of CCNI2 mRNA and CCNI2 protein do not change significantly during cell cycle, depletion of CCNI2 with siRNA affects cell cycle progression as well as cell proliferation. In conclusion, our data strongly suggest that CCNI2 is a novel CDK5 activator and is involved in cell cycle regulation.
Highlights
Cyclin-dependent kinase 5 (CDK5) is a proline-directed serine/threonine kinase
cyclin I (CCNI) was originally identified based on its similarity to other known cyclins[18], and has been shown to bind and activate CDK517
CCNI2 is considered as a homolog of CCNI, the overall similarity between these two proteins is relatively low except for the cyclin box (Fig. 1A,B)
Summary
Cyclin-dependent kinase 5 (CDK5) is a proline-directed serine/threonine kinase. Despite the high degree of homology between CDK5 and other conventional cyclin-dependent kinases (CDKs) that are important for mitotic cell division, CDK5 usually is not directly involved in cell cycle regulation. The expression pattern of CCNI2 protein in different chicken tissues was examined by western blot with a polyclonal CCNI2 antibody. We examined the subcellular localization of CDK5 and CCNI2 in cultured cells. Human CCNI2 mainly retains CDK5 in the cytoplasm as well as on the plasma membrane (Fig. 3E).
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