Research
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Research in our laboratory is focused primarily on three areas. Our most longstanding project concerns the roles for microtubules (MTs) and MT motor proteins in membrane trafficking. In addition, we are studying how actin filament organization is regulated by a protein known as IQGAP1, and how hyperphosphorylated forms of the MT-associated protein, tau, self assemble into paired helical filaments in Alzheimer's disease. To address these topics, we rely on the tools of biochemistry, cell and molecular biology, and biophysics, with the ultimate aim being to understand in vivo functions at the level of individual proteins.
Much
of our work on membrane trafficking has concentrated on the
MT motor protein, kinesin. Over the course of several years,
we demonstrated that kinesin is a MT stimulated ATPase which
moves organelles along MTs, and is a motor for anterograde
fast axonal transport in neurons, and for movement of
membranes from the Golgi apparatus to the endoplasmic
reticulum (ER). Now we are concentrating on how organelle
motility along MTs is regulated. We have obtained evidence
that kinesin-mediated membrane motility away from the Golgi
is regulated by multiple GTPases, including Cdc42 and Rac1.
We are now attempting to determine how those two proteins,
and whatever additional GTPases may prove to be relevant,
control membrane movement along MTs.
In a related project, we are
using live cell fluorescence microscopy to study the dynamic behavior
of membrane proteins fused to the naturally fluorescent protein, GFP.
Our efforts in this area are focused on proteins that normally reside
in specialized plasma membrane domains that are known as caveolae,
and represent sites at which many signalling moclecules are
concentrated. Our work on membrane trafficking has profited from
collaborations with several laboratories, including those of Scott
Brady and Dick Anderson (University of Texas Southwestern Medical
Center), Nobutaka Hirokawa (Univesity of Tokyo), Nancy Ratner
(University of Cincinnati) and Jennifer Lippincott-Schwartz
(NIH).
IQGAP1
is a protein which was shown by us to bind directly to actin
filaments, and by other labs to bind directly to calmodulin,
and to activated (GTP-bound) forms of Rac1 and Cdc42.
Because activated Rac1 and Cdc42 induce cortical actin
filaments to rearrange dramatically, we suspect that IQGAP1
serves as a direct molecular link between these GTPases and
cortical actin. We have purified native IQGAP1, and
demonstrated that it is a dimeric protein that cross-links
actin filaments into bundles and gels. We are now studying
how Cdc42, Rac1, calmodulin, GTP and calcium act
coordinately to regulate the actin filament binding and
cross linking activities of IQGAP1.
Our
work on tau has concentrated primarily on its interactions
with protein phosphatase 2A (PP2A). We have obtained
evidence that PP2A is a major tau phosphatase, and have
mapped the binding sites on PP2A for tau and microtubules,
and on tau and microtubules for PP2A. Our ultimate goal is
to understand the role of tau phosphorylation in the
assembly of paired helical filamnets in vivo. Labs
that have collaborated with us to study tau have included
those of Estelle Sontag and Marc Mumby (University of Texas
Southwestern Medical Center), Gloria Lee (University of
Iowa), Jeff Kuret (Ohio State University) and Roland Brandt
(University of Heidelberg).
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