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College Park, Maryland      June 6 - 10 , 2004

MP2: Studying Protein Dimerization on Membranes by Small Angle Neutron Scattering

K. Gupta (Dept. of Chemical and Biomolecular Eng.; Program in Computational Biology, Johns Hopkins Univeristy, Baltimore, MD), J. Pencer (Dept of Chemical and Biomolecualr Engg., Johns Hopkins University, Baltimore, MD.; NIST Center for Neutron Research), D. Bossev, S. Krueger (NIST Center for Neutron Research), M. Paulaitis (Dept. of Chemical and Biomolecular Engg.; Program in Computational Biology, Johns Hopkins Univeristy, Baltimore, MD)

Clustering/oligomerization of membrane bound receptors is found to be important in cellular responses. It has been suggested that oligomerization is induced both by lipid membrane environment and by ligand binding. Though many studies have been focused on receptor clustering, the structural features of these oligomers are still lacking. We have developed a model system to study the role of membrane environment and ligand binding on the structural organization of membrane anchored proteins using small angle neutron scattering (SANS). Our model system consists of a mouse IgG Fab fragment covalently attached to unilamellar DMPC lipid vesicles on which dimerization/oligomerization is induced by anti-Fab antibody. Contrast matching of isotopically substituted phospholipids eliminates the contribution of the bilayer to the observed scattering, resulting in a scattering profile dependent on the structural organization of protein complexes on the bilayer. By comparing the scattering profiles from protein complexes in solution and on the bilayer, we have found that the complexes on the membrane have a smaller physical size and less flexibility than in the solution. Our results also suggest that anchoring proteins on a single component bilayer system does not increase their dimerization/oligomerization potential in the presence of a ligand. One implication of these results is that lipid domains (rafts) or other cellular components may be required in facilitating the oligomerization of membrane bound receptors.

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