title: Crystal structure of AlgQ2, a macromolecule (alginate)-binding protein of Sphingomonas sp. A1, complexed with an alginate tetrasaccharide at 1.6-A resolution.
abstract: Sphingomonas sp. A1 possesses a high molecular weight (HMW) alginate uptake system composed of a novel pit formed on the cell surface and a pit-dependent ATP-binding cassette (ABC) transporter in the inner membrane. The transportation of HMW alginate from the pit to the ABC transporter is mediated by the periplasmic HMW alginate-binding proteins AlgQ1 and AlgQ2. We determined the crystal structure of AlgQ2 complexed with an alginate tetrasaccharide using an alginate-free (apo) form as a search model and refined it at 1.6-A resolution. One tetrasaccharide was found between the N and C-terminal domains, which are connected by three extended hinge loops. The tetrasaccharide complex took on a closed domain form, in contrast to the open domain form of the apo form. The tetrasaccharide was bound in the cleft between the domains through van der Waals interactions and the formation of hydrogen bonds. Among the four sugar residues, the nonreducing end residue was located at the bottom of the cleft and exhibited the largest number of interactions with the surrounding amino acid residues, suggesting that AlgQ2 mainly recognizes and binds to the nonreducing part of a HMW alginate and delivers the polymer to the ABC transporter through conformational changes (open and closed forms) of the two domains.
authors.affiliation: Department of Basic and Applied Molecular Biotechnology, Division of Food and Biological Science, Graduate School of Agriculture, Kyoto University, Uji, Kyoto 611-0011, Japan.
authors.forename: Yumiko
authors.initials: Y
authors.lastname: Mishima
authors.forename: Keiko
authors.initials: K
authors.lastname: Momma
authors.forename: Wataru
authors.initials: W
authors.lastname: Hashimoto
authors.forename: Bunzo
authors.initials: B
authors.lastname: Mikami
authors.forename: Kousaku
authors.initials: K
authors.lastname: Murata
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008958:Models, Molecular; D009844:Oligosaccharides / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation; D011494:Protein Kinases / Q000737:chemistry* / Q000378:metabolism; D012680:Sensitivity and Specificity; D020578:Sphingomonas; D015534:Trans-Activators
chemical_list: C079502:AlgQ protein, Pseudomonas aeruginosa; D001426:Bacterial Proteins; D002352:Carrier Proteins; D009844:Oligosaccharides; D015534:Trans-Activators; D011494:Protein Kinases