C-Terminal regions of topoisomerase II alpha and II{szligbeta} determine isoform-specific functioning of the enzymes in vivo

Topoisomerase II removes supercoils and catenanes generated during DNA metabolic processes such as transcription and replication. Vertebrate cells express two genetically distinct isoforms ( alpha and {szligbeta}) with similar structures and biochemical activities but different biological roles. Topoisomerase II alpha is essential for cell proliferation, whereas topoisomerase II{szligbeta} is required only for aspects of nerve growth and brain development. To identify the structural features responsible for these differences, we exchanged the divergent C-terminal regions (CTRs) of the two human isoforms ( alpha 1173-1531 and {szligbeta} 1186-1621) and tested the resulting hybrids for complementation of a conditional topoisomerase II alpha knockout in human cells. Proliferation was fully supported by all enzymes bearing the alpha CTR. The alpha CTR also promoted chromosome binding of both enzyme cores, and was by itself chromosome-bound, suggesting a role in enzyme targeting during mitosis. In contrast, enzymes bearing the {szligbeta} CTR supported proliferation only rarely and when expressed at unusually high levels. A similar analysis of the divergent N-terminal regions ( alpha 1-27 and {szligbeta} 1-43) revealed no role in isoform-specific functions. Our results show that it is the CTRs of human topoisomerase II that determine their isoform-specific functions in proliferating cells. They also indicate persistence of some functional redundancy between the two isoforms.