Self-assembled lipid and membrane protein polyhedral nanoparticles

Abstract

We demonstrate that membrane proteins and phospholipids canself-assemble into polyhedral arrangements suitable for structuralanalysis. Using the Escherichia coli mechanosensitive channel ofsmall conductance (MscS) as a model protein, we prepared membrane protein polyhedral nanoparticles (MPPNs) with uniform radiiof ~20 nm. Electron cryotomographic analysis established thatthese MPPNs contain 24 MscS heptamers related by octahedralsymmetry. Subsequent single-particle electron cryomicroscopy yieldeda reconstruction at ~1-nm resolution, revealing a conformationclosely resembling the nonconducting state. The generality of thisapproach has been addressed by the successful preparation of MPPNsfor two unrelated proteins, the mechanosensitive channel of largeconductance and the connexon Cx26, using a recently devised micro-fluidics-based free interface diffusion system. MPPNs provide notonly a starting point for the structural analysis of membrane proteins in a phospholipid environment, but their closed surfacesshould facilitate studies in the presence of physiological transmem-brane gradients, in addition to potential applications as drug delivery carriers or as templates for inorganic nanoparticle formation.