Solution structure and oligomeric state of the E. coliglycerol facilitator

Abstract

Protein dynamics at atomic resolution can provide deep insights into the biological activities of proteins and enzymes but they can also make structure and dynamics studies challenging. Despite their well-known biological and pharmaceutical importance, integral membrane protein structure and dynamics studies lag behind those of water-soluble proteins mainly owing to solubility problems that result upon their removal from the membrane. Escherichia coli glycerol facilitator (GF) is a member of the aquaglyceroporin family that allows for the highly selective passive diffusion of its substrate glycerol across the inner membrane of the bacterium. Previous molecular dynamics simulations and hydrogen-deuterium exchange studies suggested that protein dynamics play an important role in the passage of glycerol through the protein pore. With the aim of studying GF dynamics by solution and solid-state nuclear magnetic resonance (NMR) spectroscopy we optimized the expression of isotope-labelled GF and explored various solubilizing agents including detergents, osmolytes, amphipols, random heteropolymers, lipid nanodiscs, bicelles and other buffer additives to optimize the solubility and polydispersity of the protein. The GF protein is most stable and soluble in lauryl maltose neopentyl glycol (LMNG), where it exists in a tetramer-octamer equilibrium. The solution structures of the GF tetramer and octamer were determined by negative-stain transmission electron microscopy (TEM), size-exclusion chromatography small-angle X-ray scattering (SEC-SAXS) and solid-state magic-angle spinning NMR spectroscopy. Although NMR sample preparation still needs optimization for full structure and dynamics studies, negative stain TEM and SEC-SAXS revealed low-resolution structures of the detergent-solubilized tetramer and octamer particles. The non-native octamer appears to form from the association of the cytoplasmic faces of two tetramers, the interaction apparently mediated by their disordered N- and C-termini. This information may be useful in future studies directed at reducing the heterogeneity and self-association of the protein.

Keywords: Glycerol facilitator; Membrane protein; Negative stain transmission electron microscopy; Size-exclusion chromatography multi-angle light scattering; Small angle X-ray scattering; Solid-state NMR spectroscopy.