9728122 Hantgan The objective of this research is to answer a central question in cell biology: What structural changes convert "inert" integrins into "activated" high affinity receptors? Integrins are a superfamily of membrane-bound proteins that, when activated, regulate many essential cellular processes. Despite major advances in molecular biology, the structural bases for integrin activation are not well understood. This work will test the new hypothesis that activation is a dynamic multistep process, leading to integrin clusters that form tight multivalent links with adhesive proteins. Central to this research is a novel positive feedback mechanism in which receptor occupancy promotes self-association and self-association promotes high-affinity receptor ligand:interactions. The hypotheses that follow will be tested by biophysical studies of the prototypical integrin, the human platelet IIb-beta-3 complex. Hypothesis 1: The IIb-beta-3 integrin is an inherently "sticky" protein; i.e., these receptors self-associate to form higher order (IIb-beta-3)n oligomers. Hypothesis 2: "Activated" IIb-beta-3 complexes exhibit an even greater tendency to oligomerize. A corollary of this hypothesis is that receptor occupancy induces a conformational change in IIb-beta-3, one which enhances self-association. Hypothesis 3: High molecular weight adhesive proteins, such as the dimeric macromolecule fibrinogen, bind tightly to (IIb-beta-3)n oligomers, by forming multiple ligand:receptor contacts. This research focuses on the question: How do living cells recognize each other? Many mammalian cells use large protein molecules, termed integrins, that protrude from their surface as "molecular velcro". Smaller molecules, termed ligands, then adhere to the integrins and cause cells to clump. This investigation's goal is to understand the steps that control cell clumping at the molecular level, by exploring how integrin molecules become "sticky". Optical measurements will determine what events cau se to ligands to stick to integrins and slow down their movement. This data will provide new insights into how integrins work as "molecular glue".
