Sos7, an Essential Component of the Conserved Schizosaccharomyces pombe Ndc80-MIND-Spc7 Complex, Identifies a New Family of Fungal Kinetochore Proteins

Abstract

Chromosome segregation is powered by the kinetochore, a large macromolecular structure assembled on centromeric chromatin. Attachment of sister chromatids to microtubules is mediated by the highly conserved tripartite KMN (acronym for KNL-1-Mis12-Ndc80) kinetochore network. In the fission yeast Schizosaccharomyces pombe, the equivalent complex is called NMS (Ndc80-MIND-Spc7). Here, we show that not all components of the NMS complex had been identified previously. A 10th NMS component exists, the essential Sos7 protein, which is a genetic and physical interaction partner of Spc7. The analysis of sos7 kinetochore-null mutant yeast strains demonstrated that Sos7 is central to NMS function. In particular, Sos7 is required for kinetochore targeting of Spc7 as well as components of the MIND complex. sos7 mutant strains show severe chromosome missegregation phenotypes and have compromised microtubule-kinetochore interactions. Sos7 is the founding member of a functionally conserved fungal kinetochore family not present in the point centromere carrying Saccharomycotina clusters, suggesting that the new Sos7 family might be a signature motif of fungi with regional centromeres.T he faithful transmission of eukaryotic chromosomes to the progeny is a complex process that requires coordinated protein interactions at the kinetochore-microtubule interface. The erosion of chromosome segregation fidelity leading to the loss/gain of chromosomes is of great medical relevance, as aneuploidy has been implicated to have a causative role in a number of diseases (1,38,68). The proper partitioning of sister chromatids in mitotis is mediated via direct contact of spindle microtubules with components of the outer kinetochore. Kinetochores are macromolecular structures assembled on the chromosomal centromere region. Apart from providing mechanical attachment sites, these organelles generate the spindle checkpoint signaling that ascertains proper sister chromatid biorientation before anaphase onset (44). Although kinetochores are very complex structures and the number of kinetochore proteins identified is still increasing, the direct interaction with spindle fibers is mediated largely by a highly conserved protein network called the KMN complex in higher eucaryotes and the NMS complex in the fission yeast Schizosaccharomyces pombe (2, 37, 47). The KMN-NMS network is composed of 3 subcomplexes: the Ndc80 complex and the MINDMis12 complex, each comprising 4 components, and a protein called KNL-1 in Caenorhabditis elegans, Blinkin-hSpc105 in vertebrates, Spc105 in Saccharomyces cerevisiae and Drosophila, and Spc7 in S. pombe (59,67,74). In addition, budding yeast Spc105 forms a subcomplex with the Kre28 protein, Blinkin binds metazoan Zwint via its C-terminal end, and KNL-1 interacts with 29,30,[46][47][48]. The mitotic functions of Kre28 and KBP-5 have yet to be analyzed, whereas the Zwint protein is required for kinetochore residency of the mitotic checkpoint relevant RZZ (Rod-Zw10-Zwilch) complex (10,65,70). It has been suggested that Kre28 ...

Inositol Pyrophosphate Kinase Asp1 Modulates Chromosome Segregation Fidelity and Spindle Function in Schizosaccharomyces pombe

“…For example, it has been reported that strains expressing nmt81::GFP-Atb2 at the atb2 ϩ locus have altered MT properties (24,(27)(28)(29). The reason for this is unclear.…”

The KMN protein network – chief conductors of the kinetochore orchestra

“…It has been proposed that this phosphorylation is required for recruitment of the RZZ complex and the dynein motor to kinetochores, which in turn is important for chromosome motility and SAC signaling (Famulski and Chan, 2007;Kasuboski et al, 2011). Very recently, a potential functional homologue of ZWINT has been identified in fission yeast as a binding partner for the yeast KNL1 homologue Spc105 (Jakopec et al, 2012).…”

The process of kinetochore assembly in yeasts

“…The double‐headed arrows indicate mutual dependency between proteins and protein complexes. The interdependence of various KT proteins shown here is based on the data assembled from the literature – for the S. cerevisiae KT (Meluh & Koshland, ; Ortiz et al ., ; Russell et al ., ; Goshima & Yanagida, ; He et al ., ; Janke et al ., , ; Li et al ., ; De Wulf et al ., ; Nekrasov et al ., ; Pinsky et al ., ; Scharfenberger et al ., ; Westermann et al ., ; Collins et al ., ; Camahort et al ., ; Mizuguchi et al ., ; Pagliuca et al ., ; Schleiffer et al ., ; Bock et al ., ; Nishino et al ., ), for the S. pombe KT (Takahashi et al ., ; Chen et al ., ; Hayashi et al ., ; Liu et al ., ; Saitoh et al ., ; Sanchez‐Perez et al ., ; Kerres et al ., , ; Dunleavy et al ., ; Takayama et al ., ; Pidoux et al ., ; Tanaka et al ., ; Williams et al ., ; Jakopec et al ., ) and for the C. albicans KT (Burrack et al ., ; Roy et al ., ; Thakur & Sanyal, , ).…”

“…a). Clustered KTs are found in S. pombe as well except at metaphase where multiple foci of KT proteins were observed (Goshima et al ., ; Tanaka et al ., ; Jakopec et al ., ). Although the exact nature of KT architecture in yeasts is uncertain, various genetic and biochemical studies indicate the presence of functional homologs of several KT proteins at distinct layers of a human KT in these yeasts (Table ).…”

“…Ndc10 is essential for the KT localization of each of the constituents of the MIND complex in S. cerevisiae (Goshima & Yanagida, 2000;Nekrasov et al, 2003;Pinsky et al, 2003). KT localization of the Mis12 complex is independent of Spc105 in S. cerevisiae (Pagliuca et al, 2009) but Mis12, Mis13/Dsn1 and Mis14/Nsl1 require Spc7 and Sos7 for their KT localization in S. pombe (Kerres et al, 2007;Pagliuca et al, 2009;Jakopec et al, 2012). Depletion of a subunit of the Dam1 complex affects Mis12 localization in C. albicans (Thakur & Sanyal, 2012).…”