The von Hippel-Lindau (VHL) tumor suppressor gene is mutated in most human kidney cancers. The VHL protein is part of a complex that includes Elongin B, Elongin C, and Cullin-2, proteins associated with transcriptional elongation and ubiquitination. Here it is shown that the endogenous VHL complex in rat liver also includes Rbx1, an evolutionarily conserved protein that contains a RING-H2 fingerlike motif and that interacts with Cullins. The yeast homolog of Rbx1 is a subunit and potent activator of the Cdc53-containing SCFCdc4 ubiquitin ligase required for ubiquitination of the cyclin-dependent kinase inhibitor Sic1 and for the G1 to S cell cycle transition. These findings provide a further link between VHL and the cellular ubiquitination machinery.

Haploidization of the genome in meiosis requires that chromosomes be sorted exclusively into pairs stabilized by synaptonemal complexes (SCs) and crossovers. This sorting and pairing is accompanied by active chromosome positioning in meiotic prophase in which telomeres cluster near the spindle pole to form the bouquet before dispersing around the nuclear envelope. We now describe telomere-led rapid prophase movements (RPMs) that frequently exceed 1 microm/s and persist throughout meiotic prophase. Bouquet formation and RPMs depend on NDJ1, MPS3, and a new member of this pathway, CSM4, which encodes a meiosis-specific nuclear envelope protein required specifically for telomere mobility. RPMs initiate independently of recombination but differ quantitatively in mutants that fail to complete recombination, suggesting that RPMs respond to recombination status. Together with recombination defects described for ndj1, our observations suggest that RPMs and SCs balance the disruption and stabilization of recombinational interactions, respectively, to regulate crossing over.

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Control of cyclin levels is critical for proper cell cycle regulation. In yeast, the stability of the G1 cyclin Cln1 is controlled by phosphorylation-dependent ubiquitination. Here it is shown that this reaction can be reconstituted in vitro with an SCF E3 ubiquitin ligase complex. Phosphorylated Cln1 was ubiquitinated by SCF (Skp1-Cdc53-F-box protein) complexes containing the F-box protein Grr1, Rbx1, and the E2 Cdc34. Rbx1 promotes association of Cdc34 with Cdc53 and stimulates Cdc34 auto-ubiquitination in the context of Cdc53 or SCF complexes. Rbx1, which is also a component of the von Hippel-Lindau tumor suppressor complex, may define a previously unrecognized class of E3-associated proteins.

In meiotic prophase, telomeres associate with the nuclear envelope and accumulate adjacent to the centrosome/spindle pole to form the chromosome bouquet, a well conserved event that in Saccharomyces cerevisiae requires the meiotic telomere protein Ndj1p. Ndj1p interacts with Mps3p, a nuclear envelope SUN domain protein that is required for spindle pole body duplication and for sister chromatid cohesion. Removal of the Ndj1p-interaction domain from MPS3 creates an ndj1⌬-like separation-of-function allele, and Ndj1p and Mps3p are codependent for stable association with the telomeres. SUN domain proteins are found in the nuclear envelope across phyla and are implicated in mediating interactions between the interior of the nucleus and the cytoskeleton. Our observations indicate a general mechanism for meiotic telomere movements.meiosis ͉ SUN ͉ telomere ͉ nondisjunction H omologous chromosome pairing and recombination during meiotic prophase are required to orient chromosomes for disjunction and haploidization during the meiotic divisions. Early in meiotic prophase, telomeres attach to and move along the nuclear envelope to concentrate transiently in one sector of the nucleus, generally adjacent to the centrosome, forming the chromosome bouquet, a widely conserved arrangement (1-7). Bouquet formation may promote homologous chromosome pairing and also may help to untangle chromosomes, to regulate recombination at telomeres, to regulate crossover distribution, to coordinate or synchronize chromosomal events by propagating signals from the telomeres, and/or to facilitate synaptonemal complex formation (see refs. 1, 5, and 7-10). Despite the wide conservation of bouquet formation, the molecular mechanisms responsible for telomere attachments and movements in meiosis are poorly understood.In Saccharomyces cerevisiae, the meiotic bouquet resembles that of multicellular organisms (11)(12)(13)(14). Ndj1p is a meiosisspecific protein that accumulates at telomeres in S. cerevisiae (15,16) and is required for bouquet formation (17). Deletion of NDJ1 delays axial element formation, homolog pairing, synapsis and onset of the first meiotic division, causes an elevated frequency of nondisjunction and of ectopic recombination, and reduces spore formation and viability (15-21). Early recombination intermediates form between homologs with wild-type kinetics in ndj1⌬, suggesting that some aspect of bouquet formation is required for the normal coupling of the molecular and cytological events of pairing (22).A large-scale two-hybrid screen (23) identified an interaction between NDJ1 and MPS3/NEP98 (24, 25). Mps3p is an essential, integral membrane protein that is concentrated at the spindle pole body (SPB), the S. cerevisiae centrosome equivalent, and is present at lower levels throughout the nuclear membrane. Mps3p is required for duplication of the SPB (24, 25) and also functions in karyogamy and in sister chromatid cohesion (25,26). The present study demonstrates a critical requirement for the Ndj1p-Mps3p interaction for bouquet forma...