| Abstract
| - Proteins imbedded in solid-supported lipid bilayers can serve as model systems for investigations of cellular membranesand protein behavior on surfaces. We have investigated the self-assembly of streptavidin on mica-supported bilayermembranes. Using fluorescence microscopy and atomic force microscopy, our studies reveal that the concentrationof surface ligand influences the molecular packing of the resulting protein arrays, which in turn affects overall crystalmorphology. Two-dimensional streptavidin crystals are obtained when the biotinylated lipid density on the substratereaches 1.5% mole fraction, yielding high-aspect morphologies that comprise primarily of crystals with P1 symmetry.At 3% and above, crystals with C222 symmetry are formed and result in H-shaped and confluent structures. Inintermediate densities between 2 and 3%, a coexistence of P1 and C222 crystal forms is observed. The relationshipbetween macroscopic morphology and molecular configuration is similar to previously reported data obtained at theair/water interface. This suggests that, under our experimental conditions, protein interactions with the supportingsubstrate are less significant for defining self-assembly behavior than interactions between protein molecules. Ligand-inhibition and fluorescence recovery after photobleaching were used to elucidate the concentration-dependent mechanismfor the divergent crystal forms. We have measured the diffusion coefficient of molecules in P1-forming conditionsto be approximately twice that of molecules in C222-forming concentrations, which is consistent with proteins boundto the surface through one and two ligands, respectively. The differential flexibility associated with the binding stateis therefore likely to alter the subtle protein interactions involved in crystallization.
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