Cell News | Issue 01, 2017 - page 23

Cell News 01/2017
Unraveling the interplay of MISP, IQGAP1 and
Cdc42 in spindle orientation
Barbara Vodicska, Berati Cerikan, Elmar Schiebel, Ingrid Hoffmann
Presenting author: Barbara Vodicska
German Cancer Research Center, Heidelberg,
Mammalian Cell Cycle Control and Carcinogenesis
Spindle positioning and orientation during mitosis play a cru-
cial role in determining both cell fate and tissue organization.
Proper alignment of chromosomes is a result of many process-
es that have to be orchestrated in a precise manner. Proper
orientation and positioning of the mitotic spindle is mediated
through a cortical machinery by capturing astral microtu-
bules. The actin-binding protein MISP (mitotic interactor and
substrate of Plk1, C19orf21) was found in our lab to play a role
in spindle orientation and mitotic progression (Zhu et al. 2013).
However, the exact mechanism on how MISP links astral mi-
crotubules to the actin cytoskeleton remains an open question.
We identified IQGAP1, a scaffolding protein that is believed to
link the microtubule network with the actin cytoskeleton, as an
interaction partner of MISP in HeLa cell lysates. This inter-
action seems to be important for the localization of IQGAP1,
since MISP knockdown (KD) leads to cortical accumulation of
IQGAP1 in mitosis and at cell-cell contacts in interphase.
This aberrant localization can be rescued with the overexpres-
sion (OE) of Cdc42, a small signalling molecule and Rho family
member of GTPases that can be stabilized in its active state by
IQGAP1. MISP forms a ternary complex with IQGAP and Cdc42.
Interestingly, MISP KD also leads to a decrease in active Cdc42
levels in the cell.
Our results show that IQGAP1 OE can rescue spindle misorien-
tation, prolonged mitosis and the loss of astral microtubules
upon MISP KD, probably because IQGAP1 OE can restore the
active levels of Cdc42 in these cells. Unlike WT, an IQGAP1 mu-
tant lacking the Cdc42 binding site cannot rescue the spindle
orientation defects and also does not accumulate at the cell-
cell contacts upon MISP KD indicating that the Cdc42-IQGAP1
interaction is necessary for these processes. Cdc42 OE itself
leads to mitotic disorders and therefore is unable to rescue the
phenotypes of MISP KD on its own.
We propose that MISP KD leads to a conformational change in
IQGAP1 thereby making it unable to stabilize active Cdc42 and
it gets recruited to the cell cortex. Our future goal is to unravel
the mechanisms on how MISP KD leads to the deactivation of
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