Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 101

they are functionally redundant. However, a molecular understanding of the
factors generating MTs remains important to reveal regulatory mechanisms
and the structure-function relationships of these proteins.
In addition to potential effects on MT nucleation, an increase in MT sta-
bility can stem from at least three mechanisms: the stabilization of tubulin
dimer or protofilament interactions, an increase inMT growth, or suppression
ofMT catastrophes. There are a large number of proteins that appear to operate
through these mechanisms within the spindle, but we will focus on represen-
tative examples from each of the three classes, including HURP, XMAP215/
chTOG, Patronin, and MCRS1. These factors are either required for spindle
assembly or have severe spindle defects when absent, indicating the importance
of MT stabilization in determining spindle architecture in general.
The function of HURP in the spindle seems to be to stabilize dimer or
protofilament interactions ( Fig. 3.5 B). Interestingly, HURP associates
specifically with k-fiber MTs in the spindle and has been studied most exten-
sively in Drosophila , where the HURP ortholog is called Mars. Mars prefer-
entially binds to k-fibers and promotes resistance to MT-destabilizing
conditions ( Yang and Fan, 2008 ). Electron microscopy studies have
suggested that HURP facilitates wrapping of a MT sheet around existing
MTs in a sheath-like structure to provide extra stability, but whether this
occurs in vivo is not known ( Santarella et al., 2007 ). This is an example of
a protein that stabilizes dimer or protofilament interactions, but it may also
lead to increased cross-linking of MTs within the k-fiber bundle if HURP
molecules can bridge MTs or interact with multiple other proteins concur-
rently. Elucidating HURP's mechanism of action in detail may contribute to
our understanding of the architectural requirement for different MT

populations in the spindle.
An alternative mode of MT stabilization is the promotion of MT
growth. This can negate the effects of MT destabilization across a popula-
tion, and XMAP215 is an established MT-polymerizing protein that has
been shown to antagonize the activity of known MT-destabilizing proteins
( Brouhard et al., 2008; Tournebize et al., 2000 ; Fig. 3.5 C). Depletion of
XMAP215 from Xenopus egg extracts decreased MT aster size and led to dis-
organized spindles. Interestingly, XMAP215 can rescue the formation of
MTs under conditions of decreased nucleation/stabilization induced by
depleting TPX2 or g -tubulin ( Groen et al., 2009 ). This clearly shows that
promoting MT growth is an important requirement of spindle assembly,
but it remains to be seen how specific spindle architectural features are mod-
ulated in response to different types of MT stabilization.