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In support of this idea, previous studies had reported that Cdc42, which is
necessary for triggering actin polymerization, was also needed for MTOC reori-
entation (Stowers et al.
1995
). Based on reports that IQGAP links microtubule plus
ends to the actin cortex (Fukata et al.
2002
; Watanabe et al.
2004
), Stinchcombe
and colleagues looked at IQGAP at the synapse and found that both actin and
IQGAP clear out of the synapse before the MTOC arrives. They proposed that
IQGAP might link microtubules to actin. Then as the actin ring expanded, the
microtubules would spread out with it generating the tension that would pull the
MTOC forward (Fig.
21.5
).
In a study of NK cells, Banerjee and colleagues overexpressed normal or
mutant Cdc42 interacting protein (CIP4) and showed that in both cases, MTOC
translocation was blocked (Banerjee et al.
2007
). CIP4, like IQGAP, is also
thought to link actin to microtubules. Once again this study argued for a role of
Cdc42 in MTOC polarization. On the other hand, when Tskvitaria-Fuller and
colleagues loaded T cells with a dominant negative Cdc42, it had only a small
effect, slowing down MTOC repositioning (Tskvitaria-Fuller et al.
2006
). In a
study by Gomez and colleagues, Cdc42 expression was reduced using shRNA and
this apparently also had no effect on MTOC translocation. They argued instead
that Rac1 was necessary, perhaps though its ability to activate the formin FML1
(Gomez et al.
2007
).
The notion that actin-based motility is involved with MTOC repositioning is
not simple to dissect because actin dynamics are intimately linked to T cell
activation. The idea of microtubules linked to an expanding actin ring is perhaps in
theory, a plausible way to initially drive the MTOC toward the synapse but at the
same time, it is hard to reconcile with the oscillating MTOC seen when two target
cells are in contact with one CTL (Kuhn and Poenie
2002
). Furthermore, the
notion that this is supported by a role for Cdc42 is undermined by the papers
described above. It should also be noted that Sedwick and colleagues demonstrated
that the MTOC could be induced to translocate to the opposite end of the cell from
where actin accumulated (Sedwick et al.
1999
). Finally, we should note that we
introduced an IQGAP mutant construct into Jurkat cells that has a non-functional
actin binding domain (IQGAP G75Q, courtesy of David Sacks). We saw no effect
on MTOC translocation (Fig.
21.6
).
21.4 MTOC Polarization and T Cell Signaling
MTOC translocation is intimately tied a number of signaling events associated
with T cell activation. When the TcR binds to peptide-MHC complexes, a complex
series of events unfold that involve several kinases and scaffold proteins. It is
thought that one of the earliest events triggered by TcR ligation is activation the
Src-family kinase Lck which then phosphorylates regions of the TcR complex
known as ITAMs (Smith-Garvin et al.
2009
). However, this view has been chal-
lenged by other models (Davis and van der Merwe
2011
). Once ITAMs are
