Protein Structure and Function
Our lab studies how proteins function as molecular machines. We are interested in learning how proteins recognize each other, how they recognize specific ligands, and how they can act to carry out specific biological roles. Our main research tool is x-ray crystallography, but we use a variety of biophysical and biochemical techniques to investigate these questions.
Two main areas of investigation are how the BTB domain from PLZF acts as a transcriptional repressor, and how the lactose permease functions as a molecular pump to drive the transport of polar solutes across cell membranes.
Structure and function of the BTB domain: The BTB domain (also known as the POZ domain) is a common protein-protein interaction motif that is found in eukaryotes ranging from yeast to man. The human genome alone codes for over 60 distinct BTB/POZ proteins and many of these are implicated in development and/or in cancer. In man, the domain is most often found at the N-terminus of zinc finger transcription factors, although it can also be found in other proteins, notably in a class of actin binding proteins.
Examples of BTB/zinc finger proteins include the Promyelocytic Leukemia Zinc Finger protein (PLZF), which is implicated in Acute Promyelocytic Leukemia (APL), and BCL-6, whose expression is deregulated in nearly all Diffuse Large Cell Lymphomas (DLCL). In these oncoproteins, the BTB domain acts as an autonomous transcriptional repression module, and interacts directly with components of the histone deacetylase complex. Thus, the BTB domain is strongly implicated in the regulation of gene expression through chromatin remodelling effects.
Our goal is to understand the structural basis for the biological activities of the domain. Using the PLZF BTB domain as a prototype, we have used x-ray crystallography to solve the first three-dimensional structure of a BTB domain.
The structure reveals a domain-swapped dimer with an unusual protein fold. A groove is formed at the dimer interface, which we propose as a binding site for components of the histone deacetylase complex, including the nuclear co-receptors N-CoR, SMRT and mSin3A. We are currently pursuing functional questions that have arisen as a result of our structural model.
Structure of membrane proteins: Membrane proteins are critical components of all biological membranes, and can function as enzymes, receptors, channels and pumps. Even though membrane proteins represent 20-25% of most genomes, we know the three-dimensional structure of only very few of these. Given that over half of all human therapeutics target membrane proteins, it is clear that new methodologies are needed to accelerate discoveries in this field.
In particular, there are no structures known for the class of secondary transporter proteins that use coupled chemical gradients to drive the active transport of ligands across biological membranes. The single most limiting barrier to advancement is the production of crystals suitable for analysis by x-ray diffraction. We are addressing this problem by developing novel methodologies for the handling and crystallization of membrane proteins.
Using the lactose permease as a model system, we are pursuing protein-engineering strategies to increase the crystallization potential of this secondary transporter.
Fusion proteins are being produced as 'carriers' for crystallization, and these designed proteins have a higher potential for forming crystals than the native protein because of their radically different surface properties. In addition, the carrier fusion proteins maintain the normal activity of the native transport proteins, so that the structural machinery responsible for transport remains intact.
We are also developing unusual new detergents that we have designed specifically for the crystallization of membrane proteins.