Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 102

Finally, some classes of MT-stabilizing proteins seem to protect MT ends
from the activities of depolymerizing proteins. While many of the known
proteins are plus-end binding, historically, it has been unclear what activities
are able to regulate MT minus-end dynamics in cells. Minus-end-binding
proteins, such as Patronin and MCRS1, have now been identified as impor-
tant for regulating MT stability from the minus-end ( Fig. 3.5 C). Cell-based
assays in Drosophila showed that knockdown of Patronin increased depoly-
merization rates from the MT minus-end mediated by kinesin-13s (dis-
cussed below). In vitro , Patronin could compete with kinesin-13s for MT
binding and reduce the depolymerization rate ( Goodwin and Vale, 2010 ).
Similarly, MCRS1 is also thought to counteract the kinesin-13 MCAK
but instead displays preferential binding to MT minus-ends in k-fiber bun-
dles ( Meunier and Vernos, 2011 ; Fig. 3.5 B). Pull-down assays in Xenopus
egg extracts showed indirect interaction between MCRS1 and MCAK,
and MCRS1 decreased in vitro MCAK activity on centrosome-nucleated
MTs in a dose-dependent manner ( Meunier and Vernos, 2011 ). Knock-
down of either Patronin or MCRS1 caused severe spindle defects—a small
spindle phenotype or loss of chromosomally nucleated MTs and constitutive
spindle checkpoint activation, respectively—suggesting that modulating
minus-end stability throughout the spindle is important for spindle architec-
ture and function.
4.2. Microtubule-destabilizing proteins
MT destabilization is necessary to disassemble the interphase MT array at the
transition from interphase to mitosis and thus provides the substrate for
building the spindle. Importantly, within the spindle, MT-destabilization
rates determine the lifetime of MTs and therefore can have a profound effect
on spindle size and organization. MT-destabilization activities have a strong
effect on spindle size when overexpressed or depleted and have been iden-
tified as effectors utilized under different physiological conditions to control
meiotic spindle size between different species and mitotic spindle size
during development.
There are three major classes of MT-destabilizing proteins: destabilizing
kinesin family members (kinesin-13, kinesin-14, and kinesin-8),
MT-severing enzymes of the AAA ATPase family (Katanin, Spastin, Fid-
getin), and tubulin dimer-sequestering proteins (OP18/Stathmin, RB3;
Fig. 3.5 D). Kinesin family molecules have complex regulation through
phosphorylation, protein interaction partners, and localization and are