No abstract

The structural study of membrane proteins requires detergents that can effectively mimic lipid bilayers, and the choice of detergent is often a compromise between detergents that promote protein stability and detergents that form small micelles. We describe lipopeptide detergents (LPDs), a new class of amphiphile consisting of a peptide scaffold that supports two alkyl chains, one anchored to each end of an alpha-helix. The goal was to design a molecule that could self-assemble into a cylindrical micelle with a rigid outer hydrophilic shell surrounding an inner lipidic core. Consistent with this design, LPDs self-assemble into small micelles, can disperse phospholipid membranes, and are gentle, nondenaturing detergents that preserve the structure of the membrane proteins in solution for extended periods of time. The LPD design allows for a membrane-like packing of the alkyl chains in the core of the molecular assemblies, possibly explaining their superior properties relative to traditional detergents in stabilizing membrane protein structures.

Protein S is a calcium-binding protein comprising two Greek key b-barrel domains. We have used NMR and optical spectroscopies to show that, in the absence of calcium, the N-terminal domain of protein S forms two equilibrium folding intermediates that are in slow exchange. The intermediates arise from differential calcium-dependent folding of subdomains which are not contiguous along the polypeptide chain. The structures of these intermediates are incompatible with several previously proposed folding mechanisms for Greek key b-barrel domains. We propose a different mechanism that involves multiple nucleation sites for folding and sequential acquisition of native long-range interactions.