Cyclin B1-Cdk1 is the key initiator of mitosis, but when and where activation occurs has not been precisely determined in mammalian cells. Activation may occur in the nucleus or cytoplasm, as just before nuclear envelope breakdown, Polo-like kinase1 (Plk1) is proposed to phosphorylate cyclin B1 in its nuclear export sequence (NES), to trigger rapid nuclear import. We raised phospho-specific antibodies against cyclin B1 that primarily recognise the active form of the complex. We show that cyclin B1 is initially phosphorylated on centrosomes in prophase and that Plk1 phosphorylates cyclin B1, but not in the NES. Furthermore, phosphorylation by Plk1 does not cause cyclin B1 to move into the nucleus. We conclude that cyclin B1-Cdk1 is first activated in the cytoplasm and that centrosomes may function as sites of integration for the proteins that trigger mitosis.

SUMMARY The activity of keratinocytes in the hair follicle is regulated by signals from a specialized mesenchymal niche, the dermal papilla. Here, mice expressing cre recombinase in the dermal papilla were developed to probe the interaction between follicular keratinocyte populations and the dermal papilla in vivo. Inactivation of the β-catenin gene within the dermal papilla of fully developed hair follicles results in dramatically reduced proliferation of the progenitors and their immediate progeny that generate the hair shaft and, subsequently, premature induction of the destructive phase of the hair cycle (catagen). It also prevents regeneration of the cycling follicle from stem cells resident in the permanent portion of the follicle. Gene expression analysis reveals that β-catenin activity in the dermal papilla regulates at least two signaling pathways, FGF and IGF, that can mediate the inductive effects of the DP on keratinocytes. This study reveals a reciprocal signaling loop that employs Wnt/β-catenin signaling in both epithelial progenitor cells and their mesenchymal niche to govern and coordinate the interactions that are essential for the function of these two compartments.

We have found that key mitotic regulators show distinct patterns of degradation during exit from mitosis in human cells. Using a live-cell assay for proteolysis, we show that two of these regulators, polo-like kinase 1 (Plk1) and Aurora A, are degraded at different times after the anaphase-promoting complex/cyclosome (APC/C) switches from binding Cdc20 to Cdh1. Therefore, events in addition to the switch from Cdc20 to Cdh1 control the proteolysis of APC/CCdh1 substrates in vivo. We have identified a putative destruction box in Plk1 that is required for degradation of Plk1 in anaphase, and have examined the effect of nondegradable Plk1 on mitotic exit. Our results show that Plk1 proteolysis contributes to the inactivation of Plk1 in anaphase, and that this is required for the proper control of mitotic exit and cytokinesis. Our experiments reveal a role for APC/C-mediated proteolysis in exit from mitosis in human cells.