Trafficking between the nucleus and cytoplasm occurs through the nuclear pore complexes (NPCs). During mitosis, metazoan NPCs disassemble into approximately a dozen subunits. Two of these subunits are targeted to mitotic kinetochores: First, the RanBP2 complex associates with kinetochores in a microtubule-dependent manner. This complex consists of RanBP2 (a large nucleoporin that is also known as Nup358), SUMO-1-conjugated RanGAP1 (the activating protein for the Ran GTPase) and Ubc9 (the sole conjugating enzyme for the SUMO family of ubiquitin-like modifiers). Second, the nine protein vertebrate Nup107-160 complex associates to kinetochores throughout mitosis in a microtubule-independent manner. The Nup107-160 complex includes Nup160, Nup133, Nup107, Nup96, Nup85, Nup43, Nup37, Sec13, and Seh1. During telophase, Nup107-160 is targeted to chromosomes and it acts in a critical and early fashion during NPC re-assembly. Nup107-160 is broadly distributed on spindles during prometaphase. It remains kinetochore-bound throughout mitosis, and shows enhanced accumulation on unattached kinetochores. Our earlier findings showed that Nup107-160 promotes spindle assembly in a context that is distinct from intact interphase NPCs. We have found that Nup107-160 interacts with an active form of the gamma-tubulin ring complex (gamma-TuRC), an essential and conserved microtubule nucleator. Like Nup107-160, gamma-TuRC localized to unattached mitotic kinetochores, prompting us to test whether Nup107-160 and gamma-TuRC might function in a coordinated manner to promote nucleation of microtubules near mitotic chromosomes and at kinetochores. Xenopus egg extracts lacking the Nup107-160 complex or gamma-TuRC failed to assemble spindles around sperm chromatin or DNA beads. Moreover, HeLa cells lacking Nup107-160 or gamma-TuRC were profoundly deficient in kinetochore-associated microtubule nucleation. Our findings indicate that Nup107-160 promotes spindle assembly through the regulated nucleation of microtubules by gamma-TuRC at kinetochores and perhaps other sites within mitotic spindles. These observations suggest an important and novel relationship between the NPC and the microtubule cytoskeleton. Our future studies will focus on a number of issues, including the component(s) at kinetochores that is directly involved in Crm1 recruitment, the relationship between the Nup107-160 and RanBP2 complexes during mitosis, and how they together regulate the attachment of microtubules to kinteochores and the spindle assembly checkpoint. We are also examining the mitotic localization and function of other NPC components.
