Polymersomes with asymmetric membranes and self-assembled superstructures using pentablock quintopolymers resolved by electron tomography.

Polystyrene-block-poly(1,4-isoprene)-block-poly(dimethyl siloxane)-block-poly(tert-butyl methacrylate)-block-poly(2-vinyl pyridine), PS-b-PI-b-PDMS-b-PtBMA-b-P2VP, self-assembles in acetone into polymersomes with asymmetric (directional) PI-b-PDMS membranes. The polymersomes, in turn, self-assemble into superstructures. Analogically to supravesicular structures at a smaller length scale, we refer to them as suprapolymersome structures. Electron tomograms are shown to be invaluable in the structural assessment of such complex self-assemblies.

Effects of PEG on detergent micelles: implications for the crystallization of integral membrane proteins.

The solubilization of integral membrane proteins with detergents produces protein-detergent complexes (PDCs). Interactions between the detergent moieties of PDCs contribute significantly to their behavior. The effects of the precipitating agent polyethylene glycol (PEG) upon these detergent-detergent interactions have been examined, focusing on the detergent system used to crystallize the bacterial outer membrane protein OmpF porin. Static and dynamic light scattering were used to assess the effects of temperature and concentration upon the hydrodynamic size distribution and the aggregation state of detergent micelles and a phase diagram for micellar solutions was mapped. Estimates of the second osmotic virial coefficient obtained from static light-scattering measurements on micelles were shown to accurately reflect the thermodynamic quality of the solvent. Solvent quality decreases as the consolute boundary is approached, suggesting micelle-micelle attractive forces help to organize PDCs into crystalline aggregates near the cloud point. An apparent increase in micelle mass is observed as the solution approaches the cloud point. These results raise the possibility that the detergent-mediated aggregation of PDCs and/or slight changes in micelle geometry may prove to be important in the nucleation of membrane protein crystals.

The NMR structure of human beta-defensin-2 reveals a novel alpha-helical segment.

Human beta-defensin-2 (HBD-2) is a member of the defensin family of antimicrobial peptides. HBD-2 was first isolated from inflamed skin where it is posited to participate in the killing of invasive bacteria and in the recruitment of cells of the adaptive immune response. Static light scattering and two-dimensional proton nuclear magnetic resonance spectroscopy have been used to assess the physical state and structure of HBD-2 in solution. At concentrations of < or = 2.4 mM, HBD-2 is monomeric. The structure is amphiphilic with a nonuniform surface distribution of positive charge and contains several key structural elements, including a triple-stranded, antiparallel beta-sheet with strands 2 and 3 in a beta-hairpin conformation. A beta-bulge in the second strand occurs at Gly28, a position conserved in the entire defensin family. In solution, HBD-2 exhibits an alpha-helical segment near the N-terminus that has not been previously ascribed to solution structures of alpha-defensins or to the beta-defensin BNBD-12. This novel structural element may be a factor contributing to the specific microbicidal or chemokine-like properties of HBD-2.