ARF-Like (ARL) Proteins - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 219

dissociation inhibitor (GDI) on members of the RHO family (Sasaki and Takai
1998 ; DerMardirossian and Bokoch 2005 ; Cherfils and Zeghouf 2013 ). Both ARL2
and cofactor D are also found at the centrosome (Cunningham and Kahn 2008 ).
Although it is not known whether they interact at that site, this seems likely based
on functional overlap between these two proteins (Bhamidipati et al. 2000 ; Zhou
et al. 2006 ).
ARL2 and cofactor D are key regulators of microtubule dynamics (for a recent
review of cofactor D, see Tian and Cowan 2013 ). However, while cofactor D was
first identified and consistently found to be one of five cofactors required for in vitro
folding of functional tubulin heterodimers (Tian et al. 1996 ), no clear role for ARL2
in tubulin folding has been documented. Rather, it appears to be involved in the
regulation of microtubule dynamics through its binding to, and regulation of the
activities of, cofactor D and its ability to bind native tubulin dimers and likely
microtubules and drive them to an inactive state (Bhamidipati et al. 2000 ; Tian
et al. 2010 ). Expression of a constitutively active mutant of ARL2, ARL2[Q71L],
resulted in an overall loss in microtubule density in Chinese Hamster Ovary cells,
with ~35 % of cells exhibiting a complete loss of microtubules. These effects were
prevented by the treatment of cells with the microtubule stabilizing drug taxol.
ARL2[Q71L] expression resulted in the loss of the ability to polymerize new
microtubules (aster formation) after washout of the microtubule depolymerizing
drug nocodazole (Zhou et al. 2006 ). Thus, ARL2[Q71L] appears to be preventing
the polymerization of microtubules, perhaps instead of stimulating catastrophe.
Expression of ARL2[Q71L] also caused fragmentation of the centrosome and cell
cycle arrest during M phase (Zhou et al. 2006 ). These data suggest that ARL2
[Q71L] binds and sequesters a component(s) of the centrosome, e.g., the ʳ -tubulin

ring complex, that is essential for the polymerization of microtubules.
In MCF-7 cells stably transfected with ARL2 siRNA or expression vectors to
modify cellular levels of ARL2, there was a correlation between ARL2 content and
the rates of both microtubule growth and catastrophe (Beghin et al. 2007 ). The
proposed mechanism of ARL2 in regulating these processes is by regulating the
amount of polymerizable αʲ -tubulin heterodimers present in cells as there was also
a correlation between these levels and ARL2 content in cells. This role for ARL2 is
further supported by the finding that overexpression of ARL2 prevents the loss of
microtubules resulting from overexpression of cofactor D (Bhamidipati et al. 2000 ;
Tian et al. 2010 ). ARL2 and cofactor D have also been implicated in the dissoci-
ation of the apical junctional complex through an unknown mechanism that appears
to be independent of their roles in the regulation of microtubule dynamics (Shultz
et al. 2008 ).
Examination of ARL2 in the unicellular parasite Trypanosoma brucei revealed
roles in cytokinesis consistent with its roles in regulating microtubule dynamics in
mammalian cells (Price et al. 2010 ). Depletion or overexpression of TbARL2
inhibited cleavage furrow formation resulting in cell cycle arrest and
multinucleated cells. Taken together, studies of the roles of ARL2 and cofactor D
in microtubule dynamics provide the most detailed models of a central function of
ARL2 in cells that is certainly highly conserved. However, this is also unlikely to