The Conserved Spc7 Protein Is Required for Spindle Integrity and Links Kinetochore Complexes in Fission Yeast

Abstract

Spc7, a member of the conserved Spc105/KNL-1 family of kinetochore proteins, was identified as an interaction partner of the EB1 homologue Mal3. Spc7 associates with the central centromere region of the chromosome but does not affect transcriptional silencing. Here, we show that Spc7 is required for the integrity of the spindle as well as for targeting of MIND but not of Ndc80 complex components to the kinetochore. Spindle defects in spc7 mutants were severe ranging from the inability to form a bipolar spindle in early mitosis to broken spindles in midanaphase B. spc7 mutant phenotypes were partially rescued by extra alpha-tubulin or extra Mal2. Thus, Spc7 interacts genetically with the Mal2-containing Sim4 complex.

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

“…The temperature-sensitive Spc7-23 protein is present in the cell at the nonpermissive temperature but is not kinetochore associated (26). We therefore analyzed whether extra sos7 ϩ rescued the temperature-sensitive phenotype of spc7 mutant strains by assisting kinetochore targeting of the mutant Spc7 protein.…”

“…It has been suggested that Kre28 and Zwint are orthologs, although sequence similarities are very limited (48). In this work, we have identified the missing interaction partner of the S. pombe Spc7 protein.Two synergistic microtubule-binding activities exist for the KMN-NMS network, one mediated by the Ndc80 complex via the Ndc80-Nuf2 homodimer and the other by the Spc7-Spc105-KNL-1-Blinkin family, which associates with microtubules via their N-terminal ends (2,26,48,71,73,75). Kinetochore-microtubule interactions are regulated by Aurora B-mediated phosphorylation on several KMN-NMS targets, including the Spc7-Spc105-KNL-1-Blinkin family (4,5,32,72).…”

“…Depletion of Spc7-Spc105-KNL-1-Blinkin proteins by RNA interference (RNAi) or expression of conditional lethal alleles demonstrates the essential function of this protein family in maintaining genome stability as spindle microtubule-kinetochore interactions are severely impaired or absent under such conditions (6,27,30,46,55) (22). The main phenotype of S. pombe mitotic cells expressing temperature-sensitive, kinetochore-null alleles of spc7 at the restrictive temperature is a failure to segregate the highly condensed chromatin, although spindle elongation had occurred (26). The constitutively kinetochore-localized Spc7 protein is needed for kinetochore targeting of MIND components and provides a link between the S. pombe NMS and the Sim4-Mal2-Mis6 kinetochore complexes while kinetochore linkage of the Ndc80 complex is independent of Spc7 (26).…”

“…The main phenotype of S. pombe mitotic cells expressing temperature-sensitive, kinetochore-null alleles of spc7 at the restrictive temperature is a failure to segregate the highly condensed chromatin, although spindle elongation had occurred (26). The constitutively kinetochore-localized Spc7 protein is needed for kinetochore targeting of MIND components and provides a link between the S. pombe NMS and the Sim4-Mal2-Mis6 kinetochore complexes while kinetochore linkage of the Ndc80 complex is independent of Spc7 (26). In this work, we have extended our analysis of Spc7 function and have isolated a new interaction partner, the Sos7 protein.…”

Overcoming inhibition in the spindle checkpoint: Figure 1.

“…1). Interestingly, Spc105/KNL1 is also a kinetochore docking site for the vertebrate checkpoint components Bub1 and BubR1 (Kiyomitsu et al 2007), and is required for proper kinetochore-microtubule attachments in many model systems (Kerres et al 2007;Wan et al 2009). Thus, Spc105-PP1 complexes would be at the right place on kinetochores to regulate and coordinate both checkpoint silencing and kinetochore-microtubule stabilization.…”