Our laboratory has been primarily engaged in structural studies on cell surface receptors in immune and neural systems using X-ray crystallography and some other biochemical and biophysical techniques.

1: Structural investigation of cell surface receptors and their biological significance.

a) The central issue of T cell immunity is that a T cell receptor recognizes a cognate antigenic peptide presented by a MHC molecule on antigen-presenting surface. The interaction is aided by co-receptors CD4 and CD8 in helper T cell (class II) and cytotoxic T cell (class I), respectively. We have been working out structures of T cell receptor (TCR) and its complex with antigenic peptides bound to MHC molecule in both class I and II systems. We have also been worked out the structures of co-receptors CD8 and CD4 in complex with classic or non-classic MHC molecules. We have provided the structural basis for cellular immune recognition. In the future, we will explore the developmental aspect of cellular immunity and mechanism of autoimmunity from structural point of view.
b) Another important research line has to do with the structural basis of cell adhesion. Cell adhesion is extremely important in immune surveillance and the formation of immunological synapse, the key local contacts between immune cells and antigen-presenting cells.  For example, we have been studying how ICAM family members interact with integrin LFA-1 at atomic resolution detail and CD2 interacts with CD58 from opposing cell surfaces.  Cell adhesion is also vital for tissue maintenance and nervous system development. We have determined structure of N-terminal two-domain of E-cadherin, the molecule that plays a key role for epithelial tissue formation. More recently, we have solved the structure of a headpiece of a fascinating receptor molecule Dscam from Drosophila. Dscam has unprecedented 38,000 isoforms generated by alternative splicing from a single gene. The molecule serves both for neuronal wiring process and immune response. We have identified two epitopes on the structure, which may explain how this molecule works in homophilic adhesion to determine the network wiring process of a neural system, and possible heterophilic interaction with pathogen.
From these works we have formulated some principal of protein-protein interactions from opposing cell surfaces. We have summarized the unique structural features of this interaction as opposed to protein-protein interaction in solution.
These works have been extensive collaborations with colleagues within Dana-Farber and Harvard Medical School.

2: Structural virology.

a) Structures of virus receptors.
We have already worked out four virus receptor structures, including CD4, the receptor for HIV, ICAM-1, the receptor for human rhinovirus, VCAM-1, the receptor for mouse encephalomyocarditis, and CEACAM1a, the mouse hepatitis virus receptor. We have discovered how these similar-looking proteins are subverted to receptors for different viruses. Our structural studies on these extremely important receptors have had a good impact on the medical research and pharmaceutical circle. We would like to go on this line, working on other virus receptors.
b) Structures of viral fusion proteins.
After binding to receptor, those enveloped viruses have a mechanism to fuse their membrane with cellular membrane, resulting in the entry of their genome into host cell. We have solved structures of a core part of fusion protein gp41 from HIV and SIV surface glycoprotein. We will be working on other viral fusion proteins.
c) More recently the structure of nucleoprotein (NP) of influenza virus H5N1 has been resolved. We will work with our collaborators to expand structural and biochemical studies into how NP interacts with other viral and host cell molecules. This should form a good basis for drug design to combat the threatening virus. This work is a collaboration with scientist at Chinese University of Hong Kong.

3: Structural neuroscience.

a) Netrin-1 has been known to be an important axon guidance cue that plays a key role in neuronal wiring. We have determined crystal structure of netrin-1 in complex with one of its receptor, DCC (Deleted in Colorectal Cancer). Netrin-1 is in fact a bi-functional ligand. When it binds to DCC, which is constitutively expressed on the surface of axonal growth cone, it will trigger axon attraction toward the source of netrin-1. When another receptor UNC5 co-exists on the surface of growth cone, netrin-1 will bring DCC and UNC5 together, resulting in axon repulsion. DCC receptor is composed of 4 Ig-like domains at N-terminal, followed by 6 fibronectin (FN) domains. Since netrin-1 is known to bind membrane-proximal FN domains, we used FN5-FN6 for co-crystallization. Our structure demonstrates that netrin-1 simultaneously binds two DCC. The binding site 1 is DCC-specific whereas the site 2 is more generic. We propose that the more generic binding site can be replaced by other receptor, like UNC5. Functional data confirm that UNC5 is indeed able to out-compete DCC at the site 2 for repulsion outcome.

b) We have determined structures of N-terminal 4 Ig-like domains of a cell adhesion molecule, Dscam from Drosophila. The four domains fold into a horseshoe configuration. We unveil that the homophilic dimerization of this horseshoe is physiological. Drosophila Dscam has astonishing 40 thousands isoforms! Each neuron only expresses about 25 unique isoforms. During neuronal wiring, a self-avoidance mechanism ensures that the extension from the same neuron will not connect, only extension from different neuron is possibly connected. We show structurally that this is mediated by homophilic dimerization of horseshoe unit between the same Dscam isoform.

c) We have determined structure of DCC N-terminal four Ig-like domains, which also fold into horseshoe configuration. We have compared 4 published structures of Ig superfamily members that have similar horseshoe unit, and identified structural elements that determine the horseshoe conformation. We further predict the existence of 23 horseshoe-like receptor structures in the whole human genome. These receptors overwhelmingly serve as neuroreceptors!


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