Cyclin D1Promotes Cell Cycle Progression Through Enhancing The Kinase Activity Of NDR1/2Independent Of Cdk4

Cyclin/Cdks are critical protein kinases in regulating cell cycle progression. Among them, cyclin D1/Cdk4mainly exerts its function in the G1phase. By taking the TAP-tag purification approach, we identified a set of proteins interacting with Cdk4including NDR1/2(nuclear-Dbf2-related protein1/2). Interestingly, as confirming the interactions between NDR1/2and cyclin D1/Cdk4, we observed that NDR1/2could interact with cyclin D1independent of Cdk4. But, NDR1/2and cyclin D1/Cdk4could not phosphorylate each other. In addition, we found that NDR1/2did not affect the kinase activity of cyclin D1/Cdk4on phosphorylation of GST-Rb. However, cyclin Dl but not Cdk4can promote the kinase activity of NDR1/2. We also demonstrated that cyclin D1K112E, which can not bind with Cdk4, can enhance the kinase activity of NDR1/2. To test whether cyclin D1could promote G1/S transition though enhancing NDR1/2kinase activity, we performed flow cytometry analysis using cyclin D1and cyclin Dl Kl12E tet-on inducible cell lines. The data showed that both cyclin D1and cyclin Dl K112E could promote G1/S transition. Importantly, knockdown of NDR1/2almost completely abolished the function of cyclin D1K112E in promoting G1/S transition. Consistently, we found the protein level of p21was reduced in cells overexpressing cyclin D1K112E but not as NDR1/2were knocked down. Taken together, these results revealed a novel function of cyclin D1in promoting cell cycle progression by enhancing NDR kinase activity independent of Cdk4.