title: Role of the Native Outer-Membrane Environment on the Transporter BtuB.
abstract: BtuB is a TonB-dependent transporter that permits the high-affinity binding and transport of cobalamin (CBL), or vitamin B12, across the asymmetric outer membrane (OM) of Gram-negative bacteria. It has been shown that Ca2+binding is necessary for high-affinity binding of CBL to BtuB, and earlier simulations suggested that calcium ions serve to stabilize key substrate-binding extracellular loops. However, those simulations did not account for the lipopolysaccharides in the OM. To illuminate the roles of both Ca2+and lipopolysaccharides in protein functionality, we performed simulations of apo and Ca2+-loaded BtuB in symmetric and asymmetric bilayers. The simulations reveal that the oligosaccharides of LPS stabilize the extracellular loops to some degree, apparently obviating the need for Ca2+. However, it is shown that Ca2+ions stabilize a key substrate-binding loop to an even greater degree, as well as reposition specific CBL-binding residues, bringing them closer to the organization found in the CBL-bound structure. These results indicate the importance of including realistic membrane models when simulating outer-membrane proteins.
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, Georgia.
authors.forename: Curtis
authors.initials: C
authors.lastname: Balusek
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, Georgia. Electronic address: gumbart@physics.gatech.edu.
authors.forename: James C
authors.initials: JC
authors.lastname: Gumbart
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D002413:Cations, Divalent / Q000737:chemistry / Q000378:metabolism; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D004926:Escherichia coli; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008051:Lipid Bilayers; D008070:Lipopolysaccharides / Q000737:chemistry / Q000378:metabolism; D056004:Molecular Dynamics Simulation; D000072417:Protein Domains; D055550:Protein Stability; D017433:Protein Structure, Secondary; D014867:Water / Q000737:chemistry
chemical_list: D001425:Bacterial Outer Membrane Proteins; D002413:Cations, Divalent; D029968:Escherichia coli Proteins; D008051:Lipid Bilayers; D008070:Lipopolysaccharides; D014867:Water; D002118:Calcium