Biology Reference
In-Depth Information
A typical mammalian cell contains two distinct populations of microtubules, which
both originate in the pericentrosomal region. About half of cellular microtubules are
nucleated and anchored at the centrosome, from which they extend radially toward the
plasma membrane (Efimov et al.
2007
). The other half are nucleated at the Golgi, from
which they form an asymmetrical array, with their plus ends extending predominantly
toward the leading edge of the cell (Chabin-Brion et al.
2001
; Efimov et al.
2007
;
Vinogradova et al.
2009
). Golgi-nucleated microtubules associate with CLASP pro-
teins, which are recruited by the peripheral TGN protein GCC185 (Efimov et al.
2007
).
The binding of CLASP proteins to these microtubules increases their overall stability,
which, together with their enhanced acetylation and tyrosination, explains why they
are more difficult to depolymerize (Chabin-Brion et al.
2001
; Efimov et al.
2007
;
Rivero et al.
2009
; Thyberg and Moskalewski
1999
).
Microtubules play a critical role in the organization and positioning of the Golgi
apparatus (Thyberg and Moskalewski
1999
). Their depolymerization by treatment
with the compound nocodazole leads to the loss of the pericentrosomal Golgi ribbon,
which is converted into mini-stacks at ER exit sites (Cole et al.
1996
; Rogalski and
Singer
1984
). These mini-stacks are fully functional for protein processing and
secretion, and are reminiscent of the ER-Golgi units of Drosophila. Upon nocodazole
removal, microtubules repolymerize and promote the reassembly of the Golgi ribbon
through active transport of Golgi mini-stacks toward the cell center. Interestingly,
Golgi reassembly after treatment with nocodaozle occurs in two distinct steps that are
each dependent on a different microtubule population (Miller et al.
2009
). In the first
step (Golgi- or G-phase), Golgi mini-stacks spread along microtubules and fuse into
larger structures in the cell periphery. This step is dependent on Golgi-nucleated
microtubules and does not occur in CLASP-depleted cells, in which this microtubule
subset is absent. In the second step (centrosome- or C-phase), peripheral Golgi
clusters are transferred from the cell periphery to their normal position next to the
centrosome. This translocation of Golgi membranes to the cell center requires cen-
trosome-nucleated microtubules and still takes place in CLASP-depleted cells, in
which the fusion of mini-stacks in the cell periphery is prevented (Miller et al.
2009
).
In these cells, Golgi mini-stacks are positioned normally adjacent to the centrosome,
but they are unable to form an interconnected ribbon. Thus, Golgi-nucleated
microtubules are responsible for the integrity and morphology of the Golgi ribbon,
whereas centrosome-nucleated microtubules determine the localization of Golgi
membranes next to the centrosome (Miller et al.
2009
).
Microtubules serve as tracks for the movement of Golgi membranes, but the
actual locomotive force for the G- and C-stages of Golgi assembly is provided by
the microtubule motor proteins dynein and kinesin (Miller et al.
2009
). Microtu-
bule motors are mechano-chemical enzymes that transport cargo along microtu-
bule tracks, with dyneins moving toward microtubule minus ends, and kinesins
generally moving toward plus ends. Dyneins associate with the Golgi apparatus
and are important for Golgi organization and positioning (Allan et al.
2002
;
Thyberg and Moskalewski
1999
). Disrupting their function by depleting dynein or
ATP from the cytosol blocked the directional movement of the Golgi toward the
centrosome, producing a Golgi fragmentation phenotype (Corthesy-Theulaz et al.
