Functional cooperation of Dam1, Ipl1, and the inner centromere protein (INCENP)-related protein Sli15 during chromosome segregation

We have shown previously that lpl1 and Sli15 are required for chromosome segregation in Saccharomyces cerevisiae. Sli15 associates directly with the lpl1 protein kinase and these two proteins colocalize to the mitotic spindle. We show here that Sli15 stimulates the in vitro, and likely in vivo, kinase activity of lpl1, and Slil5 facilitates the association of lpl1 with the mitotic spindle. The lpl1-binding and -stimulating activities of Sli15 both reside within a region containing homology to the metazoan inner centromere protein (INCENP). lpl1 and Sli15 also bind to Dam1, a microtubule-binding protein required for mitotic spindle integrity and kinetochore function. Sli15 and Dam1 are most likely physiological targets of lpl1 since lpl1 can phosphorylate both proteins efficiently in vitro, and the in vivo phosphorylation of both proteins is reduced in ipl1 mutants. Some dam1 mutations exacerbate the phenotype of ipl1 and sli15 mutants, thus providing evidence that Dam1 interactions with lpi1-Sli1 5 are functionally important in vivo. Similar to Dam1, lpl1 and Sli15 each bind to microtubules directly in vitro, and they are associated with yeast centromeric DNA in vivo. Given their dual association with microtubules and kinetochores, lpl1, Sli15, and Dam1 may play crucial roles in regulating chromosome-spindle interactions or in the movement of kinetochores along microtubules.