Periplasmic-binding component of alginate-specific ABC uptake system-like; contains the type 2 periplasmic binding fold.
This family represents the periplasmic-binding component of high molecular weight (HMW) alginate uptake system found in gram-negative soil bacteria and related proteins. The HMW alginate uptake system is composed of a novel pit formed on the cell surface and a pit-dependent ATP-binding cassette (ABC) transporter in the inner membrane. In Sphingomonas sp. A1, the transportation of HMW alginate from the pit to the ABC transporter is mediated by periplasmic HMW alginate-binding proteins AlgQ1 and AlgQ2. Alginate is an anionic polysaccharide that is made up of alpha-L-mannuronate and its 5'-epimer, alpha-L-guluronate. Alginate is present in the cell walls of brown seaweeds, where it forms a viscous gum by binding water. Alginate is also produced by two bacteria genera Pseudomonas and Azotobacter. AlgQ1 and AlgQ2 belong to the type 2 periplasmic-binding fold superfamily. PBP2 is comprised of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap. However, unlike other bacterial periplasmic-binding proteins that deliver small solutes to ABC transporters, AlgQ1/2 can bind a macromolecule and may have specificity for either sugar or a certain type of polysaccharide.
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