Crystal structures of the bacterial solute receptor AcbH displaying an exclusive substrate preference for beta-D-galactopyranose

Item Type:	Article
Title:	Crystal structures of the bacterial solute receptor AcbH displaying an exclusive substrate preference for beta-D-galactopyranose
Creators Name:	Licht, A. and Bulut, H. and Scheffel, F.M. and Daumke, O. and Wehmeier, U.F. and Saenger, W. and Schneider, E. and Vahedi-Faridi, A.
Abstract:	Solute receptors or binding proteins are indispensable components of canonical ATP-binding cassette importers in prokaryotes. Here, we report on the characterization and crystal structures in the closed and open conformations of AcbH, the solute receptor of the putative carbohydrate transporter AcbFG which is encoded in the acarbose (acarviosyl-1,4-maltose) biosynthetic gene cluster from Actinoplanes sp. SE50/110. Binding assays identified AcbH as a high affinity monosaccharide-binding protein with a dissociation constant (K(d)) for beta-D-galactopyranose of 9.8 ± 1.0 nM. Neither galactose-containing di- and trisaccharides, such as lactose and raffinose, nor monosaccharides including D-galacturonic acid, L-arabinose, D-xylose and L-rhamnose competed with (14)C-galactose for binding to AcbH. Moreover, AcbH does not bind D-glucose which is a common property of all but one D-galactose-binding proteins characterized to date. Strikingly, determination of the X-ray structure revealed that AcbH is structurally homologous to maltose binding proteins rather than to glucose binding proteins. In the substrate binding pocket, two helices are inserted which reduce the cavity size and allow the exclusive binding of monosaccharides, specifically beta-D-galactopyranose, in the (4)C(1) conformation. Site directed mutagenesis of three residues from the binding pocket (Arg82, Asp361, Arg362) which interact with the axially oriented O4-H hydroxyl of the bound galactopyranose and subsequent functional analyses indicate that these residues are crucial for galactose binding. To our knowledge, this is the first report on the tertiary structure of a solute receptor with exclusive affinity for beta-D-galactopyranose. The putative role of a galactose import system in the context of acarbose metabolism in Actinoplanes sp. is discussed.
Keywords:	ABC Transporter, D-Galactose-Binding Protein, AcbH, Protein Crystallography, Actinoplanes sp. SE50/110
Source:	Journal of Molecular Biology
ISSN:	0022-2836
Publisher:	Elsevier
Volume:	406
Number:	1
Page Range:	92-105
Date:	11 February 2011
Official Publication:	https://doi.org/10.1016/j.jmb.2010.11.048
PubMed:	View item in PubMed

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