NMR STRUCTURES OF PLATELET FACTOR 4 & B-THROMBOGLOBULIN

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Platelet factor (PF4) and B-thromboglobulin-related proteins (BTG) are low- molecular weight heparin binding proteins generally considered to be platelet specific and are involved in blood clotting, wound healing, tissue repair and cell proliferation. PF4 is known to strongly bind heparin thereby inhibiting the formation of thrombin-antithrombin III complexes at vascular sites. Other physiological activities of PF4 include stimulation of fibroblast attachment to the substrate, stimulation of histamine release from human basophils, chemotactic activity, and potentiation of platelet aggregation. While BTG is about 50% sequentially homologous to PF4, its anti-heparin activity is less, and it possesses significant mitogenic activity in certain cell types and inhibits the catalytic effect heparin on Factor Xa neutralization by anti-thrombin IIIa more than does PF4. Although the structures of PF4 and BTG are considered critical to biological function, little is known about their structure-function, relationships, about how their conformations are related, or about any specific structural domain(s) essential for interaction with heparin or glycosaminoglycans in general. The immediate goal of this project is to elucidate the solution structures of PF4 and low affinity-Pf4 (LA-PF4, the parent protein of the more commonly known BTG), and the long range goal is to identity the protein- heparin binding domain(s). Comparison of PF4 and BTG structures will give initial insight into this latter point. The primary technique to be used towards these goals is high-resolution, two-dimensional proton NMR spectroscopy in combination with computer modeling studies. By using COSY- like and NOESY NMR experiments, sequence-specific proton resonance assignments will be made for PF4 and BTG at low ph under conditions where these proteins exist as monomers (7,800 daltons). NOESY experiments will then be used to establish distance tetrameric structures at low ph than at more physiological conditions will be investigated by NMR methods just described. The tight association of four, tetrahedral-related, identical monomers makes structural elucidation of the 32 kD tetramer feasible by NMR methods.