Robert Liddington's Research Focus
Dr. Liddington’s major research areas fall into two distinct but overlapping areas. The first is the structural basis of host-pathogen interactions. His work on the anthrax toxin components and their complexes has provided many important insights into the spatial and temporal regulation of toxin translocation into the host cell, and provided atomic resolution information that is being used by many groups for drug design. He is now working closely with Drs. Hanein and Volkmann to define the structures of the large translocation complexes using a combination of high resolution cryo-EM and fitting of crystal structures into the lower resolution EM images to create atomic models that can be explored by mutagenesis and complementary biophysical approaches.
Continuing on the biodefense theme, his group works closely with the groups of Drs. Strongin, Pellecchia, Salvesen and Smith, on structure-function correlates of virulence factors and essential genes from bacteria and viruses, including the processing proteases of the flaviviruses (e.g., West Nile and Dengue) and variola virus, the causative agent of smallpox. Building on this infrastructure, he is turning his attention to emerging diseases. In a collaboration with Dr. Wayne Marasco at the Dana-Farber Cancer Institute, he has characterized therapeutic neutralizing antibodies (nAbs) to treat potential pandemic viruses. Work on a SARS-nAb complex has been published, and his most recent work focuses on treatments for pandemic and seasonal influenza viruses.
A second area of research concerns the family of cell adhesion proteins called the integrins. The integrins are a family of plasma membrane proteins that mediate cell adhesion and migration, and thus play vital roles in the homing of leukocytes to sites of inflammation, hemostasis (through blood-clotting), and cancer metastasis. The versatility of integrin is linked to their ability to transduce bi-directional signals between the cytoskeleton and the extracellular matrix or other cells, and to set in motion intracellular signaling pathways that “cross-talk” with pathways triggered by receptors for soluble ligand.
In 1995, we determined the crystal structure of the first integrin domain, the extracellular I domain from the leukocyte integrin αMβ2, and proposed a general model of integrin-matrix binding at a site we called the MIDAS (Metal Ion Dependent Adhesion Site). We determined the first crystal structure of an I domain in complex with a 3-dimensional fragment of matrix – a triple-helical fragment of collagen. This structure confirmed our model of ligand-induced conformational changes in the integrin head that are linked structurally and thermodynamically to the intracellular integrin α and β “tails."
In collaboration with Drs. Mark Ginsberg at UCSD and Ian Campbell at Oxford University, U.K., we defined, structurally and functionally, the key role of the cytoskeletal protein, talin, in integrin activation; and developed a general model of activation in which talin binds to the membrane proximal β-tail of integrins, and breaks the bonds between the α and α-tails.
Now that we (think we) understand integrin activation in atomic detail we have turned our attention upstream, to understand how talin is activated. Our recent studies with Drs. Hanein and Critchley point to the existence of a cytoplasmic pool of globular auto-inhibited talin molecules, and Dr. Ginsberg has recently identified some of the players believed to activate talin by opening up its head-tail interaction so the constituent domains are able to bind other element of the cytoskeleton and associated signaling molecules with high affinity, and we are now trying to understand its structural basis.
Our model for talin is highly reminiscent of the model of “combinatorial activation” that we postulated for vinculin, another abundant cytoplasmic protein. Our most recent work aims to define the role of mechanical forces in the activation process, using the technique of Atomic Force Microscopy.
Robert Liddington's Research Report
Structural Basis of Integrin-Mediated Cell Adhesion
The integrins are a family of plasma membrane proteins that transduce bi-directional signals between the cytoskeleton and the extracellular matrix or other cells. My laboratory works principally though not exclusively on the structural basis of integrin-mediated signaling pathways.
In 1995, we determined the crystal structure of the first integrin domain, the extracellular I domain from the leukocyte integrin alpha Mbeta 2, and proposed a general model for metal ion-dependent ligand binding. We are now studying complexes between the extracellular domains of integrins and their ligands, and we have very recently determined the structure of the alpha 2 I domain in complex with a triple helical collagen fragment. This structure reveals ligand-induced conformational changes in the integrin domain that may represent the first step in the quaternary changes involved in signal transduction across the plasma membrane.
Quaternary changes in the integrin are thought to alter the interaction between its cytoplasmic tails, which in turn changes the affinity of the tails for cytoplasmic proteins involved in signaling or in assembly of the cytoskeleton. We are thus studying the structures and their complexes of proteins that bind integrin tails, including cytohesin-1 with integrin beta 2 tails, and beta 3-endonexin with beta 3 tails.
Structural targets within the cytoskeleton include talin and vinculin. We have recently determined the structure of a key domain from vinculin, a protein involved in the dynamic assembly of cell-matrix junctions termed "focal adhesions." The activation of vinculin is dependent on phosphorylation of inositol phospholipid in the plasma membrane that creates phosphatidylinositol 4,5 bis-phosphate (PIP-2). We have proposed a detailed model for PIP-2 induced activation of vinculin, and will test this hypothesis using NMR techniques in collaboration with Dr. Nuria Assa-Munt.
Finally, we plan to work in collaboration with Drs. Dorit Hanein and Niels Volkmann to fit our atomic models of the component proteins into lower resolution E.M. images of the actin cytoskeleton, thus building up an accurate model of this supramolecular complex.
Atomic space-filling model of the integrin alpha 2 I domain in complex with the collagen triple helix. In the upper panel, residues highlighted in red and pink are those that affect collagen binding when mutated, while those colored cyan do not (work in collaboration with T. Kamata and Y. Takada at The Scripps Research Institute). In the lower panel, residues in red are invariant among the collagen-binding integrin I domains (alpha 1, alpha 2 and alpha 10). Shown as transparent ribbon is an earlier model based on structural considerations alone.
Robert Liddington's Bio
Robert Liddington earned his Ph.D. degree from the University of York, United Kingdom, in 1986. He received postdoctoral training at Harvard University, prior to his appointment as Assistant Professor at Dana-Farber Cancer Institute and Harvard Medical School in 1990. In 1995, he was appointed Professor and Chair of Macromolecular Crystallography in the Department of Biochemistry at University of Leicester, U.K. Dr. Liddington was recruited to Sanford-Burnham Medical Research Institute July 1, 1999, as Co-Director of the Cell Adhesion–Extracellular Matrix Biology Program. In 2004 he was appointed Director of the Program on Infectious Diseases within the newly formed Infectious and Inflammatory Disease Center (IIDC), and in 2008 was appointed Center Director.