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VOL. 73 (4), 901-925, 2007 ROLES OF PROTEIN PHOSPHATASE TYPE 1...

extracts, which display low microtubule-binding activity of
kinetochore complexes in vitro and exhibit a high frequency of
chromosome loss in vivo (49) Thus, a role for Glc7 in the regulation
of kinetochore-microtubule attachment by phosphorylation can be
inferred. However, Ndc10 has not yet been shown to be a substrate
of Glc7. Moreover, even when Biggins and colleagues demonstra-
ted that Ndc10 was phosphorylated by Ipl1 in vitro (44), its
phosphorylation status in vivo has not been studied yet.

Recent studies in S. cerevisiae have generated insight into the
molecular mechanisms of kinetochore bipolar microtubule
attachment (chromosome bi-orientation). One of the key players in
establishing and monitoring bi-orientation is the chromosome
passenger complex Aurora B/survivin (Ipl1/Sli15) (50, 51). The
complex Ipl1-Sli15 has been proposed to facilitate the re-orientation
of kinetochore-spindle pole connections until tension is exerted on
sister kinetochores by bipolar microtubule attachment (51). Sli15
associates with Ipl1 and stimulates its kinase activity (52, 53).
Interestingly, in S. cerevisiae the temperature-sensitive mitotic defects
of sli15 mutants are attenuated by overexpression of a dominant-
negative truncated version of Glc7 (53) Moreover, it seems that Ipl1
is also required to activate the spindle checkpoint when tension is
not exerted on kinetochores (44).

Furthermore, it has been found that Ipl1 phosphorylates Dam1 in
vitro and that Dam1 phosphorylation is dependent on Ipl1 in vivo
(52). Dam1 is a component of the multimeric spindle-associated
DASH complex that is required for bi-orientation of sister
kinetochores and for mitotic spindle integrity (54, 55). Interestingly,
dam1 mutants that might mimic constitutive phosphorylation of
Dam1 are able to partially suppress the defects of ipl1.2 but are
synthetically lethal with glc7.10. Furthermore, overexpression of Glc7
exacerbates the temperature-sensitive growth defect of dam1 mutant
cells, indicating that Dam1 may also be a substrate of PP1 (52).
Recently, Cheeseman and co-workers have shown that Dam1
phosphorylation does not reduce DASH complex binding to
microtubules in vitro or change the composition of the complex (52).
Instead, they suggest that Dam1 phosphorylation weakens/abolishes
the association between the DASH complex and the rest of the
kinetochore (56, 57). Interestingly, this hypothesis has been recently

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