Control of Cyclin B1 localization through regulated binding of the nuclear export factor CRM1

Abstract

Activation of the Cyclin B/Cdc2 kinase complex triggers entry into mitosis in all eukaryotic cells. Cyclin B1 localization changes dramatically during the cell cycle, precipitously transiting from the cytoplasm to the nucleus at the beginning of mitosis. Presumably, this relocalization promotes the phosphorylation of nuclear targets critical for chromatin condensation and nuclear envelope breakdown. We show here that the previously characterized cytoplasmic retention sequence of Cyclin B1, responsible for its interphase cytoplasmic localization, is actually an autonomous nuclear export sequence, capable of directing nuclear export of a heterologous protein, and able to bind specifically to the recently identified export mediator, CRM1. We propose that the observed cytoplasmic localization of Cyclin B1 during interphase reflects the equilibrium between ongoing nuclear import and rapid CRM1-mediated export. In support of this hypothesis, we found that treatment of cells with leptomycin B, which disrupted Cyclin B1-CRM1 interactions, led to a marked nuclear accumulation of Cyclin B1. In mitosis, Cyclin B1 undergoes phosphorylation at several sites, a subset of which have been proposed to play a role in Cyclin B1 accumulation in the nucleus. Both CRM1 binding and the ability to direct nuclear export were affected by mutation of these phosphorylation sites; thus, we propose that Cyclin B1 phosphorylation at the G 2 /M transition prevents its interaction with CRM1, thereby reducing nuclear export and facilitating nuclear accumulation.

Generation of an indestructible cyclin B1 by caspase-6-dependent cleavage during mitotic catastrophe

“…As expected, full-length cyclin B1 was mainly confined to the cytoplasm during interphase (Figure 6a). Addition of leptomycin B, a CRM1 inhibitor, promoted nuclear accumulation of cyclin B1 (Yang et al, 1998). We found that ND122 displayed similar localization patterns as cyclin B1.…”

p21-Mediated Nuclear Retention of Cyclin B1-Cdk1 in Response to Genotoxic Stress

“…Based on these observations, it has been assumed that cytoplasmic cyclin B1-Cdk1 is not active. However, both earlier and most recent work suggest that cyclin B1-Cdk1 might already be activated in the cytoplasm and most probably at the centrosome (Heald et al, 1993;De Souza et al, 2000;Hirota et al, 2003;Jackman et al, 2003).According to the current model, signals elicited by DNA damage in S and G2 phases prevent mitotic entry by inhibiting both activation (via inactivation of Cdc25) and nuclear import of cyclin B1-Cdk1 (Jin et al, 1998;Yang et al, 1998Yang et al, , 2001Deming et al, 2001). G2 arrest is initiated via phosphorylation of Cdc25 by Chk1/2 kinases, a process that does not require active p53.…”

“…In cycling cells, cyclin B1 synthesis increases during late S-early G2 phase (Pines and Hunter, 1989), but the newly formed cyclin B1-Cdk1 complexes are kept inactive by inhibitory phosphorylations on Thr 14 and Tyr 15 (Norbury et al, 1991) and by active and constant nuclear exclusion via Crm1-mediated nuclear export (Hagting et al, 1998;Yang et al, 1998). The rapid nuclear accumulation of cyclin B1-Cdk1 that occurs in prophase coincides with phosphorylation of cyclin B1 at the cytoplasmic retention sequence, which also contains its nuclear export sequence (Hagting et al, 1999; for review, see Porter and Donoghue, 2003).…”

“…According to the current model, signals elicited by DNA damage in S and G2 phases prevent mitotic entry by inhibiting both activation (via inactivation of Cdc25) and nuclear import of cyclin B1-Cdk1 (Jin et al, 1998;Yang et al, 1998Yang et al, , 2001Deming et al, 2001). G2 arrest is initiated via phosphorylation of Cdc25 by Chk1/2 kinases, a process that does not require active p53.…”

Cdk2 associates with MAP Kinase in vivo and its nuclear translocation is dependent on MAP Kinase activation in IL-2-dependent Kit 225 T lymphocytes

“…The mechanisms responsible for the transport of cdk and cyclins from cytoplasm to nucleus are still poorly understood. Besides the G2/M regulatory cyclin B which contains a`nuclear export sequence' (Yang et al, 1998) the other cyclins and cdks which control G1 and S phases are mostly localized in the nucleus but do not contain typical`nuclear localization signal' sequences (NLS) required for the transport of many proteins (Gorlich and Mattaj, 1996;Nigg, 1997;Pines and Hunter, 1994). Therefore their translocation to the nucleus probably depends on their association with partner or carrier proteins containing an NLS sequence or not.…”