Iml3p, a component of the Ctf19 complex of the budding yeast kinetochore is required to maintain kinetochore integrity under conditions of spindle stress

The Ctf19 multi-protein complex of the central kinetochore in Saccharomyces cerevisiae is required for precise chromosome segregation during mitosis. Of 11 proteins of this complex, at least six are required for cell survival when microtubules are defective. To find individual roles of these proteins in kinetochore stability, double deletion mutants of the corresponding genes were constructed in several combinations. The growth phenotype of all the mutants was in accordance with the current model of hierarchical assembly of kinetochore proteins except one that lacked CHL4 and IML3 genes in a tubulin-defective background (tub1-1 chl4 iml3). tub1-1 chl4 iml3 showed synergistic growth defect, decrease in minichromosome stability compared with its single mutants, and a greater accumulation of cells at the G2/M checkpoint of the cell cycle. Furthermore, in the absence of Iml3p, the two-hybrid interaction between Ctf19p (a member of the Ctf19 complex) and Dam1p (a member of the outer kinetochore DASH complex) was disrupted and the localization of Dam1p at the kinetochore was also compromised. These results indicate a role for Iml3p distinct from Chl4p at the kinetochore. Iml3p may be acting as a link between the central and the outer complexes thus contributing to a functional kinetochore.