Current Research and Scholarly Interests
Several distinct intercellular junctions connect epithelial cells. Two of these, the adherens junction and the desmosome, contain cadherin cell adhesion molecules. The extracellular regions of these transmembrane proteins mediate intercellular binding, while their cytoplasmic domains are linked to the actin- (adherens junction) or intermediate filament- (desmosome) based cytoskeletons. In this way the cytoskeletons of cells comprising a tissue are linked, imparting particular morphologies and mechanical strength to the tissue. The dynamics of these complex assemblies underlie changes in cell and tissue architecture that occur during development and in many cancers. Our current work aims to understand how junctional assemblies respond to and are regulated by mechanical force. We are also studying how the multiprotein junctional assemblies have changed as part of the the evolution of more complex multicellular animals.
The Wnt signaling pathway controls cell fate determination during embryogenesis and in the normal renewal of tissues in the adult. beta-catenin is the central component of this pathway, where it serves as a transcriptional coactivator. In the absence of a secreted Wnt protein, non-junctional beta-catenin is bound in a multiprotein ôdestruction complexö. Formation of this complex promotes phosphorylation of beta-catenin, which targets it for degradation by the ubiquitin/proteosome pathway. Binding of a Wnt to cell surface receptors prevents phosphorylation of beta-catenin. The resulting stabilized beta-catenin enters the nucleus and activates transcription of Wnt target genes through its interactions with Tcf-family transcription factors, proteins that contain a beta-catenin-binding domain and a sequence-specific DNA-binding domain.
We are trying to understand the molecular mechanisms by which the destruction complex degrades beta-catenin, how binding of Wnts to their receptors turns off beta-catenin destruction, and how beta-catenin serves as a scaffold to link the sequence-specific Tcfs to components of the general transcription machinery. We are attempting to biochemically reconstitute these complexes for mechanistic and structural studies.
Intracellular vesicle trafficking and cell polarity
The directed movement of membranous vesicles is essential for maintaining the compartmentalized structure of the eukaryotic cell. The machinery responsible for this process is highly conserved amongst different intracellular trafficking pathways and amongst eukaryotes. An important example is the delivery of vesicles to particular regions of the plasma membrane, which is essential for maintaining the structure of polarized cells. We are studying proteins involved in the regulated movement, docking, and fusion of vesicles with their target membranes, and how this machinery interfaces with the cell adhesion machinery as part of establishing cell polarity.