Two mitotic cyclins, cyclin A and cyclin B, are believed to promote mitotic events. However, previous studies have implicated both specific requirement and redundancy of these cyclins in human cells in mitosis. Using degron-tagging approaches, the roles of cyclin B were analysed in RPE-1 and in HeLa cells. Striking differences were uncovered between the two cell lines. RPE-1 cells require cyclin B for specific substrate phosphorylations only in metaphase, but HeLa cells without cyclin B stall in an intermediate prophase-like stage which was characterized by a specific morphology and a unique pattern of CDK-substrate phosphorylation. In the absence of cyclin B, endogenous levels of cyclin A alone are responsible for establishing this prophase-like stage found in HeLa cells; but overexpre...[ Read more ]

Two mitotic cyclins, cyclin A and cyclin B, are believed to promote mitotic events. However, previous studies have implicated both specific requirement and redundancy of these cyclins in human cells in mitosis. Using degron-tagging approaches, the roles of cyclin B were analysed in RPE-1 and in HeLa cells. Striking differences were uncovered between the two cell lines. RPE-1 cells require cyclin B for specific substrate phosphorylations only in metaphase, but HeLa cells without cyclin B stall in an intermediate prophase-like stage which was characterized by a specific morphology and a unique pattern of CDK-substrate phosphorylation. In the absence of cyclin B, endogenous levels of cyclin A alone are responsible for establishing this prophase-like stage found in HeLa cells; but overexpressing cyclin A promoted separate mitotic events in a dose-dependent manner. On the other hand, RPE-1 cells rely on a combination of cyclin A-mediated CDK activity, as well as phosphatase inactivation, to promote separate mitotic events. These results implied that HeLa cells depend on the quantity of, rather than the specific, CDK-cyclin activity during mitosis, whereas RPE-1 cells require specific CDK phosphorylations conferred by different cyclins. As the expression of mitotic cyclins is altered in many cancer cells, a comprehensive understanding of the regulation of these cyclins in healthy and cancer cells will allow the design of targeted treatments and a better prediction of patient outcome after therapies.