George S. Bloom, PhD
Regulation of cell adhesion, migration and cancer cell growth and metastasis by modulators of Rho family proteins
Cancer-related research in our laboratory is focused primarily on two areas: roles for microtubules (MTs) and MT motor proteins in membrane trafficking, and the IQGAP protein family. 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 IQGAP1 is directly aimed at the small G protein, Cdc42, consitutively active mutant forms of which are known to be oncogenic. The pathway by which dominant active Cdc42 causes cellular transformation remains to be elucidated, but could well involve its interactions with IQGAP1.
Much of our current work on membrane trafficking makes use of 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 where many signal transduction molecules become concentrated in response to appropriate extracellular stimulation. Included among such molecules are several, such as MAP kinase, Ras, and Src, that are directly involved in the control of cell proliferation. We recently found that both microtubules and actin filaments are required for proper trafficking of caveolar membrane proteins, and are beginning to explore how caveolar membranes interact with other components of the endomembrane system.
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 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. As mentioned above, we are also interested in learning whether IQGAP1 lies within the transforming pathway of constitutively active Cdc42.