This proposal, entitled "Structural studies of leukocyte integrin interactions", aims to understand, at atomic resolution, the mechanism by which leukocyte Integrins, Mac-1 and aXB2, interact with diverse ligands via the a I domains. The major focus will be how Mac-1 binds ligands of very different categories to perform its crucial biological functions. Supported by NIH funds, in a continuous collaboration with Dr. Springer, we have successfully defined the structural basis of leukocyte integrin LFA-1 interaction with the ICAM family. We now will turn our efforts to Mac-1. Despite belonging to the same leukocyte integrin family, structurally and functionally Mac-1 is distinct from LFA-1. Expressed on neutrophils and other myeloid cells, Mac-1 binding to very diverse ligands not only mediates the leukocytes recruitment in a differential fashion from LFA-1, but also facilitates phagocytosis during inflammation and the clearance of apoptotic cells. We will study Mac-1's interaction with ICAM-1, and compare it with LFA-1/ICAM interactions to investigate their sequential engagement of ICAM-1 for leukocyte recruitment. We will study Mac-1 (and also aX(32) interactions with complement component, iC3b, the major opsonin in facilitating pathogen phagocytosis by neutrophils. We will study Mac-1 interactions with the endothelium-secreted extracellular matrix ligand, Del-1, the suggested bridge between apoptotic cells and phagocytes. We will employ several affinity-enhanced Mac-1 I domain mutants for the structural studies with these ligands'Mac-1-binding domains. We will test the hypothesis that compared to relatively flat binding interface LFA-1 associates, Mac-1's ligands may have their protruding loops to bind "bumpier" Mac-1 I domain. We will perform comparison of RGD-containing ligands bound to integrins with and without I domain. The investigation on the seemingly promiscuous binding ability of Mac-1 will lay a good foundation for therapeutic exploration to treat many relevant diseases.
This proposal aims to carry out structural investigation of the leukocyte integrin Mac-1's interactions with three distinct ligands. These interactions mediate Mac-1's important functions in phagocytosis and neutrophils apoptosis, as well as in leukocyte recruitment. It is extremely important for vascular biology and immunology, and may lead to therapeutic intervention of many diseases.
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