Our lab studies the role of cytoskeletal proteins in cell adhesion and motility. In particular we’re interested in the assembly, structure, and function of cell:matrix adhesions, the protein complexes that form transmembrane connections to the actin cytoskeleton. Interactions between the proteins of these complexes transmit mechanical force across the membrane as well as biochemical signals that coordinate cell division, motility, cell adhesion, and differentiation. The roles of different components of matrix adhesions in these various processes are not well-appreciated. Our recent studies have focussed on vinculin and talin, two proteins found in the cytoplasmic, actin-associated plaque of cell:matrix adhesions. They are ligands for each other, are essential for mammalian development, and are involved in control of integrin-mediated cell adhesion and motility. In a bare bones model, the cell surface portion of transmembrane integrins bind to the extracellular matrix that holds cells of tissues together, while the intracellular portion of the integrin binds to the internal contractile machinery. We use a combination of biochemical, biophysical, and cell biological approaches to understand how these proteins are recruited to cell:matrix junctions, how they function to control adhesion and signaling, and how their interactions with each other and the actin cytoskeleton are regulated.

We’ve learned that an intramolecular interaction in cytoplasmic vinculin masks binding sites for talin, F-actin, acidic phospholipids and several other adhesion site proteins. The presence of these cryptic sites suggests that assembly of vinculin into a cell adhesion site requires "activation" of cytoplasmic vinculin to disrupt an intramolecular interaction that maintains vinculin in an adhesion-incompetent state. In collaboration with Robert Liddington's lab, we determined the atomic structure of vinculin which showed how vinculin is maintained in an inactive state and suggested a mechanism of vinculin activation. Our lab then found that activation of vinculin requires spatial and temporal coincidence of both talin and actin filaments at sites of cell:cell and cell matrix adhesion, because both proteins must bind to vinculin simultaneously in order to activate the scaffolding function of vinculin. It is the scaffolding function that could enable vinculin to build structures that relay force across cell membranes. Using optical probes we developed to report on activation of vinculin, we’ve begun to determine when and where vinculin is activated in living cells.

Many questions remain including :
What other ligand pairs can activate vinculin?
How does vinculin get recruited from cytoplasm to adhesion sites?
Does activation of vinculin result in increased transmembrane force at adhesion sites?
Does the vinculin:talin interaction promote or inhibit adhesion?
What are the signaling pathways that control vinculin activation in living cells?
What is the functional role of individual vinculin:ligand pairs?

Publications:

Chen, H., Choudhury, D., and S.W. Craig. 2006. Coincidence of talin and actin filaments is required to activate vinculin. J. Biol. Chem. 281:40389-398.
PubMed Reference

Cohen, D.M., Kutscher, B., Chen, H., Murphy, D.M., and S.W. Craig. 2006. A Conformational switch in vinculin drives formation and dynamics of a talin-vinculin complex at focal adhesions. J. Biol. Chem. 281:16006-16015.
PubMed Reference

Chen, H., D.M. Cohen, D.M. Choudhury, Kioka, and S.W. Craig. 2005. Spatial distribution and functional significance of activated vinculin in living cells. J. Cell Biol. 169:459-470.
PubMed Reference

Cohen, D.M., Chen, H., Johnson, R.P., Choudhury, B., and S.W. Craig. 2005. “ Two distinct head-tail Interfaces cooperate to suppress activation of vinculin by talin”. J Biol Chem. 280: 17109-17117.
PubMed Reference

Bakolitsa C, Cohen DM, Bankston LA, Bobkov AA, Cadwell GW, Jennings L, Critchley DR, Craig, SW and Liddington RC. 2004. Structural basis for vinculin activation at sites of cell adhesion. Nature. 430:583-6. Epub 2004 Jun 13.
PubMed Reference