title: The effect of calcium on the conformation of cobalamin transporter BtuB.
abstract: BtuB is a beta-barrel membrane protein that facilitates transport of cobalamin (vitamin B12) from the extracellular medium across the outer membrane of Escherichia coli. It is thought that binding of B12 to BtuB alters the conformation of its periplasm-exposed N-terminal residues (the TonB box), which enables subsequent binding of a TonB protein and leads to eventual uptake of B12 into the cytoplasm. Structural studies determined the location of the B12 binding site at the top of the BtuB's beta-barrel, surrounded by extracellular loops. However, the structure of the loops was found to depend on the method used to obtain the protein crystals, which-among other factors-differed in calcium concentration. Experimentally, calcium concentration was found to modulate the binding of the B12 substrate to BtuB. In this study, we investigate the effect of calcium ions on the conformation of the extracellular loops of BtuB and their possible role in B12 binding. Using all-atom molecular dynamics, we simulate conformational fluctuations of several X-ray structures of BtuB in the presence and absence of calcium ions. These simulations demonstrate that calcium ions can stabilize the conformation of loops 3-4, 5-6, and 15-16, and thereby prevent occlusion of the binding site. Furthermore, binding of calcium ions to extracellular loops of BtuB was found to enhance correlated motions in the BtuB structure, which is expected to promote signal transduction. Finally, we characterize conformation dynamics of the TonB box in different X-ray structures and find an interesting correlation between the stability of the TonB box structure and calcium binding.
authors.affiliation: Department of Physics, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: Binquan
authors.initials: B
authors.lastname: Luan
authors.forename: Rogan
authors.initials: R
authors.lastname: Carr
authors.forename: Martin
authors.initials: M
authors.lastname: Caffrey
authors.forename: Aleksei
authors.initials: A
authors.lastname: Aksimentiev
mesh_terms: D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D008969:Molecular Sequence Data; D011485:Protein Binding; D011487:Protein Conformation; D014805:Vitamin B 12 / Q000378:metabolism*; D014803:Vitamin B Complex / Q000378:metabolism*
chemical_list: D001425:Bacterial Outer Membrane Proteins; C044592:BtuB protein, E coli; D029968:Escherichia coli Proteins; D008051:Lipid Bilayers; D026901:Membrane Transport Proteins; D014803:Vitamin B Complex; D014805:Vitamin B 12; D002118:Calcium