Abstract
Members of the poloโlike kinases (Plk1, Plk2, Plk3 and Plk4) are involved in the regulation of various stages of the cell cycle and have been implicated in cancer progression. Unlike its other family members, the expression of Plk3 remains steady during cell cycle progression, suggesting that its activity may be spatiotemporally regulated. However, the mechanism of regulation of Plk3 activity is not well understood. Here, we show that calciumโ and integrinโbinding protein 1 (CIB1), a Plk3 interacting protein, is widely expressed in various cancer cell lines. Expression of CIB1 mRNA as well as protein is increased in breast cancer tissue as compared to normal tissue. CIB1 constitutively interacts with Plk3 as determined by both in vitro and in vivo assays. This interaction of CIB1 with Plk3 is independent of intracellular Ca^2+^. Furthermore, binding of CIB1 results in inhibition of Plk3 kinase activity both in vitro and in vivo. Interestingly, this inhibition of the Plk3 activity by CIB1 is Ca^2+^โdependent. Taken together, our results suggest that CIB1 is a regulatory subunit of Plk3 and it regulates Plk3 activity in a Ca^2+^โdependent manner. Furthermore, upregulation of CIB1 in cancer cells could thus inhibit Plk3 activity leading to abnormal cell cycle regulation in breast cancer cells. Thus, in addition to Plk3, CIB1 may be a potential biomarker and target for therapeutic intervention of breast cancer.