We have shown previously that lpl1 and Sli15 are required for chromosome se
gregation in Saccharomyces cerevisiae. Sli15 associates directly with the l
pl1 protein kinase and these two proteins colocalize to the mitotic spindle
. We show here that Sli15 stimulates the in vitro, and likely in vivo, kina
se activity of lpl1, and Slil5 facilitates the association of lpl1 with the
mitotic spindle. The lpl1-binding and -stimulating activities of Sli15 bot
h reside within a region containing homology to the metazoan inner centrome
re protein (INCENP). lpl1 and Sli15 also bind to Dam1, a microtubule-bindin
g 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 phosphor
ylation of both proteins is reduced in ipl1 mutants. Some dam1 mutations ex
acerbate the phenotype of ipl1 and sli15 mutants, thus providing evidence t
hat 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 d
ual association with microtubules and kinetochores, lpl1, Sli15, and Dam1 m
ay play crucial roles in regulating chromosome-spindle interactions or in t
he movement of kinetochores along microtubules.