PDBsum entry 1nqf

Transport protein

PDB id: 1nqf

Contents

Protein chain

544 a.a. *

Ligands

C8E ×6

Metals

_MG

Waters ×69

* Residue conservation analysis

PDB id:

1nqf

Name:

Transport protein

Title:

Outer membrane cobalamin transporter (btub) from e. Coli, methionine substiution construct for se-met sad phasing

Structure:

Vitamin b12 receptor. Chain: a. Synonym: outer membrane cobalamin transporter btub. Engineered: yes. Mutation: yes

Source:

Escherichia coli. Organism_taxid: 562. Gene: btub. Expressed in: escherichia coli. Expression_system_taxid: 562.

Resolution:

2.70Å R-factor: 0.246 R-free: 0.288

Authors:

D.P.Chimento,A.K.Mohanty,R.J.Kadner,M.C.Wiener

Key ref:

D.P.Chimento et al. (2003). Substrate-induced transmembrane signaling in the cobalamin transporter BtuB. Nat Struct Biol, 10, 394-401. PubMed id: 12652322 DOI: 10.1038/nsb914

Date:

21-Jan-03 Release date: 29-Apr-03

Protein chain

P06129  (BTUB_ECOLI) -  Vitamin B12 transporter BtuB from Escherichia coli (strain K12)

Seq: 614 a.a.

Struc: 544 a.a.*

* PDB and UniProt seqs differ at 6 residue positions (black crosses)

DOI no: 10.1038/nsb914

Nat Struct Biol 10:394-401 (2003)

PubMed id: 12652322

Substrate-induced transmembrane signaling in the cobalamin transporter BtuB.

D.P.Chimento, A.K.Mohanty, R.J.Kadner, M.C.Wiener.

ABSTRACT

The outer membranes of Gram-negative bacteria possess transport proteins essential for uptake of scarce nutrients. In TonB-dependent transporters, a conserved sequence of seven residues, the Ton box, faces the periplasm and interacts with the inner membrane TonB protein to energize an active transport cycle. A critical mechanistic step is the structural change in the Ton box of the transporter upon substrate binding; this essential transmembrane signaling event increases the affinity of the transporter for TonB and enables active transport to proceed. We have solved crystal structures of BtuB, the outer membrane cobalamin transporter from Escherichia coli, in the absence and presence of cyanocobalamin (vitamin B(12)). In these structures, the Ton box is ordered and undergoes a conformational change in the presence of bound substrate. Calcium has been implicated as a necessary factor for the high-affinity binding (K(d) approximately 0.3 nM) of cyanocobalamin to BtuB. We observe two bound calcium ions that order three extracellular loops of BtuB, thus providing a direct (and unusual) structural role for calcium.

Selected figure(s)

Figure 2.
Figure 2. Crystallographic structures of BtuB. a, apo BtuB; b, Ca^2+ -BtuB; and c, Ca^2+ -B[12] -BtuB. -barrel domains are shown in green; hatch domains, in purple. Bound calcium ions are shown in yellow in the Ca^2+ -BtuB and Ca^2+ -B[12] -BtuB structures. The bound cyanocobalamin (vitamin B[12]) substrate is shown in space-filling representation in the Ca^2+ -B[12] -BtuB structure. The left column depicts the structures with extracellular loops pointing upwards and periplasmic turns downwards; the -strands of the barrel domain span the outer membrane. The right column presents views, normal to the surface of the outer membrane, looking down into the extracellular side of the structures. The hatch domains extend from residues 6 -132, with the Ton box located at 6 -12. The hatch domain is formed around a core of four -strands. A short linker (133 -136) connects the hatch to the 22-stranded -barrel domain (137 -594). Extracellular loops that are disordered in the apo BtuB structure become partially ordered in the Ca^2+ -BtuB structure and fully ordered in the Ca^2+ -B[12] -BtuB structure. This ordering occurs in the vicinity of the bound calcium ions. Waters, detergent molecules and other (weakly) bound ions are not shown (for clarity).

Figure 5.
Figure 5. Calcium and cyanocobalamin binding in BtuB a, Stereo view of the 'aspartate cage' binding the two calcium ions (yellow) in the Ca^2+ -BtuB and Ca^2+ -B[12] -BtuB structures. b, Stereo view of cyanocobalamin bound to BtuB. The view is normal to the surface of the outer membrane, looking down into the extracellular side. Residues within 4.5 Å of the substrate are shown, with those of the hatch domain colored purple and those from the barrel colored green. The calcium ions are included to indicate the orientation of the molecule (which is the same as that of Fig. 1).

The above figures are reprinted by permission from Macmillan Publishers Ltd: Nat Struct Biol (2003, 10, 394-401) copyright 2003.

Figures were selected by an automated process.