Summary Sexual reproduction requires the unique cell division called meiosis, in which a diploid cell undergoes a reductional division to generate haploid gametes. A hallmark of meiotic prophase is the formation of pairwise linkages between homologous chromosomes, which later enable them to segregate from each other. In most organisms the pairing of homologous chromosomes is reinforced by synapsis, the polymerization of the synaptonemal complex (SC) between paired chromosome axes. The primary questions addressed here are: 1) how pairing is accomplished and 2) how synapsis is regulated so that it occurs selectively between homologs. We provide evidence that a connection between the chromosomes and the microtubule cytoskeleton via a bridge across the nuclear envelope is critical for both of these mechanisms. Our results indicate the existence of a mechanism that uses dynein to assess homology before licensing SC polymerization. The molecular components of this mechanism are conserved from fungi to mammals.

Summary Faithful segregation of homologous chromosomes during meiosis requires pairing, synapsis, and crossing-over. In C. elegans, homolog pairing and synapsis depend on pairing centers (PCs), special regions near one end of each chromosome that interact with the nuclear envelope (NE) and cytoplasmic microtubules. Here we report that PCs are required for nuclear reorganization at the onset of meiosis. We demonstrate that PCs recruit the Polo-like kinase PLK-2 to induce NE remodeling, chromosome pairing, and synapsis. Recruitment of PLK-2 is also required to mediate a cell cycle delay and selective apoptosis of nuclei containing unsynapsed chromosomes, establishing a molecular link between these two quality control mechanisms. This work reveals unexpected functions for the conserved family of Polo-like kinases, and advances our understanding of how meiotic processes are properly coordinated to ensure transmission of genetic information from parents to progeny.

UNC-84 is required to localize UNC-83 to the nuclear envelope where it functions during nuclear migration. A KASH domain in UNC-83 was identified. KASH domains are conserved in the nuclear envelope proteins Syne/nesprins, Klarsicht, MSP-300, and ANC-1. Caenorhabditis elegans UNC-83 was shown to localize to the outer nuclear membrane and UNC-84 to the inner nuclear membrane in transfected mammalian cells, suggesting the KASH and SUN protein targeting mechanisms are conserved. Deletion of the KASH domain of UNC-83 blocked nuclear migration and localization to the C. elegans nuclear envelope. Some point mutations in the UNC-83 KASH domain disrupted nuclear migration, even if they localized normally. At least two separable portions of the C-terminal half of UNC-84 were found to interact with the UNC-83 KASH domain in a membrane-bound, split-ubiquitin yeast two-hybrid system. However, the SUN domain was essential for UNC-84 function and UNC-83 localization in vivo. These data support the model that KASH and SUN proteins bridge the nuclear envelope, connecting the nuclear lamina to cytoskeletal components. This mechanism seems conserved across eukaryotes and is the first proposed mechanism to target proteins specifically to the outer nuclear membrane. INTRODUCTIONA variety of cellular and developmental processes, including fertilization, cell division, cell migration, and establishment of polarity, depend on positioning the nucleus to a specific location within the cell. For example, in budding yeast the nucleus must migrate to the bud neck before the onset of mitosis. Also, nuclei actively follow the leading edge of migratory cells, such as those in the developing cerebral cortex. Nuclear migration defects in these two examples lead to the missegregation of chromosomes or the neurological disease lissencephaly, respectively (reviewed in Morris, 2000). The role of microtubules and associated dynein and kinesin motors in nuclear migration are well established (reviewed in Reinsch and Gonczy, 1998). Although less established, actin also plays an important role in many nuclear positioning events (reviewed in Starr and Han, 2003). Recently, a definitive role for actin networks has been described in nuclear migration during NIH 3T3 cell polarization (Gomes et al., 2005). It remains relatively unknown how the nucleus connects to the cytoplasmic cytoskeleton during nuclear migration. Furthermore, it is not clear how the forces involved in nuclear positioning are transferred across both membranes of the nuclear envelope from the cytoskeleton to the nuclear matrix.We have previously proposed that two C. elegans proteins, UNC-84 and UNC-83, function to control nuclear migration by bridging the nuclear envelope, connecting the cytoskeleton with the nuclear matrix (Starr et al., 2001;Lee et al., 2002;Starr and Han, 2003). Mutations in unc-83 or unc-84 disrupt nuclear migration in at least three cell types: embryonic hypodermal hyp7 precursors, larval hypodermal P-cells, and embryonic intestinal primordial cells (Horvitz and Sulst...