The cyclin-dependent kinase (Cdk) 1–cyclin B (CycB) complex plays critical roles in cell cycle regulation. Before Drosophila male meiosis, CycB is exported from the nucleus to the cytoplasm via the nuclear porin 62kD (Nup62) subcomplex in the nuclear pore complex. When this is inhibited, Cdk1 is not activated, and meiosis does not initiate. We investigated the mechanism that controls cellular localization and activation of Cdk1. Cdk1–CycB continuously shuttled into and out of the nucleus in the prolonged G2 phase before meiosis. Overexpression of CycB, but not that with nuclear localization signal sequences, rescued reduced cytoplasmic CycB and inhibition of meiosis in Nup62-silenced cells. Full-scale Cdk1 activation occurred in the nucleus shortly after its rapid nuclear entry. Cdk1-dependent centrosome separation did not occur in Nup62-silenced cells, while Cdk1 interacted with Cdk-activating kinase and Twine/Cdc25C in the nucleus of Nup62-silenced cells, suggesting the involvement of another suppression mechanism. Silencing of roughex rescued Cdk1 inhibition and initiated meiosis. Nuclear exports of Cdk1 ensured its escape from inhibition by cyclin-dependent kinase inhibitor. The complex reentered the nucleus via importin β at the onset of meiosis. We propose a model regarding the dynamics and activation mechanism of Cdk1–CycB to initiate male meiosis.

Background: The central channel of the nuclear pore complex (NPC) plays an important role in the selective transport of proteins between the nucleus and cytoplasm. Previous studies have demonstrated that depletion of the Nup62 complex, constructing the nuclear pore channel in premeiotic Drosophila cells, resulted in absence of meiotic cells. We attempted understanding the mechanism underlying the cell cycle arrest before meiosis. Methods: We induced dsRNAs against the nucleoporin mRNAs using the Gal4/UAS system in Drosophila. Results: The cell cycle of the Nup62-depleted cells was arrested before meiosis without CDK1 activation. Ectopic over-expression of CycB, but not constitutively active CDK1, resulted in partial rescue from the arrest. CycB accumulated precociously in the nuclei of Nup62-depleted cells and cells depleted of exportin (encoded by emb). Protein complexes containing CycB, Emb, and Nup62 complex components were observed in premeiotic spermatocytes. CycB, which had precociously migrated into the nucleus, was associated with Emb, and the complex was transported back to the cytoplasm through the central channel, interacting with the Nup62 complex. Conclusion: We proposed that CycB is exported with Emb through the channel interacting with the Nup62 complex before initiation of meiosis. The nuclear export ensures production of active CycB-CDK1 in the cytoplasm.