Transcriptional repression complexes

Transcriptional regulation complexes are large, multicomponent  assemblies involving proteins with various enzymatic activities, adaptor  functions and DNA recognition modules. We are looking at interplay  between the components of these assemblies in order to understand the  structural “logic” of how these complexes carry out intricate biological  activities.
We are studying a family of BTB-zinc finger transcriptional regulators  that include proteins implicated in development and/or in cancer. In  these proteins, the BTB domain is a protein-protein interaction module  that recruits activator and/or corepressor complexes to promoter sites  recognized by the C-terminal zinc-finger regions. Our objective is to  understand and characterize the protein-protein interaction network of  these proteins.
For example, we have determined crystal structures of complexes between  the BTB domain of BCL6 and the minimal binding region of the SMRT, NCoR  and BCoR corepressors. BCL6 is a key oncoprotein in B-cell lymphoma and  exerts its biological effects through its interactions with  HDAC-associated corepressor complexes.  The structures reveal a peptide  binding groove on the side of the BCL6 BTB domain dimer that forms the  binding site for the corepressors. This detailed atomic view of the  interactions between the two proteins identifies a key pocket that can  be targeted with small molecule inhibitors. 


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Assembly and function of ubiquitin ligase E3 complexes
In addition to transcription, a key regulatory mechanism for the  control of cellular programs involves the targeted modification of  proteins with ubiquitin and ubiquitin-like proteins. One of the largest  families of proteins involved with this process are the  Cullin3-Ring-Ligases (CRL3s). This E3 ligase family uses  Cullin3/Rbx2/BTB adaptor protein complexes to bring a ubiquitin-charged
E2 protein together with a substrate protein destined for  ubiquitination. Driven by BTB domain self-association, the CRL3s  assemble with multiple copies of each of the protein components. This  produces complexes with multiple E2~ubiqutin binding sites and multiple  substrate binding sites, and we are studying how the multivalent  architecture of the CRL3s affect substrate ubiquitination.  
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Structural biology of proteins involved in sphingolipid metabolism
Another area of focus in the lab is the mechanism of sphingolipid  metabolism in the cell. Sphingolipids have roles in cell integrity and  cell-cell recognition but can also can act as signaling molecules in  cell growth and apoptosis. As a result, lipids such as ceramide and  sphingosine-1-phospate have been implicated in tumor promotion.
Sphingolipids are in constant flux in cells, and the enzymes that modify  and breakdown these lipids generally require a “sphingolipid activator  protein”, or saposin. These saposin proteins interact with both  membranes and proteins to assemble catalytically active complexes, and  we are interested in the mechanism of the saposin activation reaction.
The small saposin proteins can access a surprisingly wide range of  physical states with lipids, bilayers and proteins. Some saposins act as  “physiological  detergents” and can solubilize target sphingolipids,  while others assemble membrane-bound enzyme complexes at the bilayer  surface in a form of interfacial catalysis.