PDBsum entry 1af6

Membrane protein

PDB id: 1af6

Contents

Protein chains

421 a.a. *

Ligands

GLC-FRU ×3

Metals

_MG ×4

Waters ×394

* Residue conservation analysis

PDB id:

1af6

Name:

Membrane protein

Title:

Maltoporin sucrose complex

Structure:

Maltoporin. Chain: a, b, c. Synonym: lamb. Engineered: yes

Source:

Escherichia coli. Organism_taxid: 562. Cellular_location: outer membrane. Gene: lamb. Expressed in: escherichia coli. Expression_system_taxid: 562.

Biol. unit:

Homo-Trimer (from PDB file)

Resolution:

2.40Å R-factor: 0.195 R-free: 0.214

Authors:

R.Dutzler,T.Schirmer

Key ref:

Y.F.Wang et al. (1997). Channel specificity: structural basis for sugar discrimination and differential flux rates in maltoporin. J Mol Biol, 272, 56-63. PubMed id: 9299337 DOI: 10.1006/jmbi.1997.1224

Date:

21-Mar-97 Release date: 25-Mar-98

Protein chains

P02943  (LAMB_ECOLI) -  Maltoporin from Escherichia coli (strain K12)

Seq: 446 a.a.

Struc: 421 a.a.

DOI no: 10.1006/jmbi.1997.1224

J Mol Biol 272:56-63 (1997)

PubMed id: 9299337

Channel specificity: structural basis for sugar discrimination and differential flux rates in maltoporin.

Y.F.Wang, R.Dutzler, P.J.Rizkallah, J.P.Rosenbusch, T.Schirmer.

ABSTRACT

Maltoporin (LamB) facilitates the diffusion of maltodextrins across the outer membrane of E. coli. The structural basis for the specificity of the channel is investigated by X-ray structure analysis of maltoporin in complex with the disaccharides sucrose, trehalose, and melibiose. The sucrose complex, determined to 2.4 A resolution, shows that the glucosyl moiety is partly inserted into the channel constriction, while the bulky fructosyl residue appears to be hindered to enter the constriction, thus interfering with its further translocation. One of the glucosyl moieties of trehalose is found in a similar position as the glucosyl moiety of sucrose, whereas melibiose appears disordered when bound to maltoporin. A comparison with the previously reported maltoporin-maltose complex sheds light on the basis for sugar discrimination, and explains the different permeation rates observed for the saccharides.

Selected figure(s)

Figure 1.
Figure 1. The structure of sucrose (Glc-α(1 → 2)β-Fru) in complex with maltoporin, and comparison with the corresponding maltose complex. (a) Stereo view of the cyclic averaged electron density of sucrose with the final model superimposed. To avoid any model bias only the protein model was used for the calculation of the initial (2 F[obs]−F[calc]) map. The methyl-hydroxyl groups of the fructosyl residue are labeled. For steric reasons the two sugar rings are arranged approximately at right angles to each other, while in maltose (see (d)), the angle between the sugar residues is considerably smaller. (b) Depiction of sucrose in relation to the greasy slide and the channel constriction. The extracellular vestibule of the channel is on top, and the periplasmic outlet at the bottom of the Figure. The barrel axis of maltoporin is oriented vertically and tilted by about 30° towards the viewer. The sugar model, the greasy slide residues (Trp#74, donated from an adjacent subunit; Tyr41; Tyr6; Trp420; Trp358; Phe227) and Tyr118 (right) are shown as stick models with the cyclic averaged electron density superimposed. The (clipped) C^α-tracing is colored in green. The glucosyl moiety of sucrose is in van der Waals contact with the greasy slide, while the fructosyl residue is found above the channel constriction. (c) Stereographic representation of the complex viewed from the vestibule at the extracellular side onto the constriction site, and along the pore axis. Three of the four hydroxyl groups of the glucosyl moiety are H-bonded to residues of the ionic tracks, hydroxyl O2-H is facing the channel entrance. The O6-H hydroxyl group of the fructosyl residue is bonded to Asp121, a residue that is not part of the constriction zone. During translocation, steric interactions of the fructosyl moiety with Tyr118 and Arg109 appear to hinder the movement of the saccharide across the pore constriction. (d) Superposition of the sucrose-maltoporin complex (atom colors) with the two maltose molecules (steel-blue) as observed in complex with maltoporin [Dutzler et al 1996]. The protein structures of both complexes are virtually identical. The view is the same as in (b). The individual glucosyl moieties of the maltose molecules are bound to subsites S1 to S4 (labeled in green). The glucosyl of sucrose adopts a position intermediate between S2 and S3.

Figure 2.
Figure 2. Trehalose (Glc-α(1 → 1)α-Glc) binding to maltoporin and a comparison of its interactions with those observed for sucrose. (a) Stereo view of the trehalose complex, with the sugar moiety colored in light blue, while the residues of the greasy slide and Tyr118 (right) are shown in atom colors. The extracellular side of the barrel is at the top, the periplasmic side at the bottom of the panel, with the channel axis in the plane of the paper. The averaged F[obs]−F[calc] vector difference map (see Materials and Methods) of the maltoporin-trehalose complex is contoured at +4 σ and −4 σ (green and red contours, respectively). Flat continuous density is found for one glucosyl moiety. Most of the remaining positive density peaks correspond to water molecules (red spheres) as determined in the maltoporin-sucrose complex. The water structure has not been used for the structure determination of the trehalose complex. The (clipped) C^α-trace of a maltoporin monomer is shown in yellow, and loop L#2, originating from the adjacent subunit and contributing Trp#74, in blue. (b) Superposition of the binding of trehalose (atom colors) and sucrose (green) to maltoporin, as obtained by structural alignment of the respective C^α-traces. The leading glucose moiety in trehalose (bottom) and the glucosyl moiety of sucrose exhibit essentially identical interactions with the protein. No density has been found for the other glucosyl residue of trehalose (a) which has been modeled based on stereochemical restraints.

The above figures are reprinted by permission from Elsevier: J Mol Biol (1997, 272, 56-63) copyright 1997.