Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 96

the release of spindle assembly factors (SAFs) from inhibitory interactions
with the transport molecules, Importin a / b , resulting in MT nucleation
and organization around mitotic chromatin ( Fig. 3.4 B). Motor proteins
organize the MTs into an antiparallel array by sliding them away from
the chromatin and focusing their minus-ends into spindle poles. In cell types
with centrosomes, clustering of MT minus-ends may also be facilitated by
enriching MT binding or motor proteins at the centrosomes or by the astral
MTs, which provide tracks on which to focus the antiparallel array
( Compton, 1998 ).
How MTs are nucleated downstream of RanGTP is not yet clear.
Immunodepletion of the MAP and Importin a -cargo TPX2 completely
abrogated RanGTP-dependent MT nucleation in Xenopus egg extracts,
and recombinant TPX2 could induce MT nucleation in solutions of pure
tubulin in vitro ( Gruss et al., 2001; Schatz et al., 2003 ; Fig. 3.4 ). This protein
is predicted to have little tertiary structure and may oligomerize, but it
remains unclear whether TPX2 nucleates MTs directly or if it acts to stabi-
lize small MT assemblies or requires other cellular factors such as the
g -TuRC for activity.
In addition to activating SAFs by liberation from importins, new evi-
dence has revealed a role for Ran in regulating the anaphase promoting
complex/cyclosome (APC/C)-induced degradation of certain SAFs during
mitosis and spindle assembly. The APC/C has been localized to the spindle
poles, and its inhibition resulted in aberrant spindle phenotypes, suggesting a
role for the APC/C in spindle maintenance before the spindle checkpoint is
satisfied and prior to anaphase ( Ban et al., 2007; Goshima et al., 2007; Kraft
et al., 2003; Somma et al., 2008; Tugendreich et al., 1995; Williamson et al.,
2009 ). Two SAFs, NuSAP and Hepatoma upregulated protein (HURP), are
protected fromAPC/C recognition and degradation by binding to Importin
b . One model is that an additional layer of spatial and temporal regulation on
Ran cargoes is generated: RanGTP liberates them from Importin b to per-
form their assembly and MT-stabilizing tasks but limits their lifetime in the
spindle by rendering them susceptible to ubiquitylation by the APC/C
( Song and Rape, 2010 ). This subtle mode of regulation implies that Ran-
regulated SAFs are powerful modulators of spindle architecture that require
precise temporal and spatial regulation to generate a functional spindle.
In addition to the factors regulated by Ran/importin binding, the
chromatin-mediated pathway includes additional signals generated by the
chromosome passenger complex (CPC; Ruchaud et al., 2007 ). In most
organisms, the CPC consists of the mitotic kinase Aurora B and associated