Human TopBP1 Participates in Cyclin E/CDK2 Activation and Preinitiation Complex Assembly during G1/S Transition

Human TopBP1 with eight BRCA1 C terminus domains has been mainly reported to be involved in DNA damage response pathways. Here we show that TopBP1 is also required for G1 to S progression in a normal cell cycle. TopBP1 deficiency inhibited cells from entering S phase by up-regulating p21 and p27, resulting in down-regulation of cyclin E/CDK2. Although co-depletion of p21 and p27 with TopBP1 restored the cyclin E/CDK2 kinase activity, however, cells remained arrested at the G1/S boundary, showing defective chromatin-loading of replication components. Based on these results, we suggest a dual role of TopBP1 necessary for the G1/S transition: one for activating cyclin E/CDK2 kinase and the other for loading replication components onto chromatin to initiate DNA synthesis.