+TIPs
and cortical signaling
Microtubule
plus end-binding proteins (or +TIPs) are a class of
microtubule-associated proteins that selectively localize
to the plus end. Work from many labs has demonstrated that
+TIPs can perform at least two functions: 1) they regulate
microtubule dynamics; and 2) they can couple movement of
other proteins to microtubule polymerization. Several
projects in our lab are designed to test the hypothesis
that +TIPs play an important function in signaling events
at the cell cortex and that microtubule plus ends serve as
motile “platforms” that selectively recruit signaling
molecules and cytoskeletal crosslinkers for transport to
the plasma membrane.
We
previously identified Drosophila RhoGEF2 as a +TIP protein
that associates with microtuble plus ends in an
EB1-dependent manner in S2 cells. RhoGEF2 is an activator
of the Rho1 small G protein and participates in epithelial
tissue remodeling throughout morphogenesis in flies.
Developmentally important events such as gastrulation and
imaginal disc folding are mediated by actin- and myosin
II-mediated contractility downstream of RhoGEF2 and Rho1.
We propose that RhoGEF2 exploits microtubule dynamics to
search the cortex for sites of upstream receptor activation
which, in epithelia, should occur in a polarized and
spatially restricted manner. This is an exciting hypothesis
because it represents a novel cellular function for
microtubule dynamics and implies that they could be an
essential, dynamic component to this signaling pathway.
Another
EB1-binding protein that we are studying is the giant
spectraplakin, Short stop (Shot). Shot is a multi-domain
protein that has an N-terminal actin-binding site and a
C-terminal microtubule binding domain and, thus, has the
capacity to act as a crosslinking factor for actin
filaments and microtubules. Mutant analysis in
Drosophila
has
shown that Shot is important at
multiple
stages of embryonic development and is involved in
cell-cell and cell-ECM adhesion, cellular movements, and
regulated protein translation. Loss-of-function studies in
flies and in cultured mouse cells directly implicated this
family of proteins in the regulation of actin and
microtubule dynamics, as well. We are studying the cell
biology of Shot in order to understand the significance of
its association with the microtubule plus end, to determine
the mechanisms by which it regulates cytoskeletal behavior,
and to integrate these insights into a larger model of
cytoskeletal function during cell motility.
