Molecular identification and characterization of an alginate-binding protein on the cell surface of Sphingomonas sp. A1.

Cells of Sphingomonas sp. A1 (strain A1) directly incorporate a macromolecule, alginate, into cytoplasm through a biosystem, or "super-channel," consisting of a pit on the cell surface, alginate-binding proteins in periplasm, and an ABC transporter in the inner membrane. The pit functions as a concentrator for extracellular alginate. Through differential display analysis, a protein (p8) with a molecular mass of 20kDa and a pI of 7.4 was found to be inducibly expressed in the outer membrane of alginate-grown cells. The gene coding for p8 was identified in the genome of strain A1 and shown to be similar to that for the polyhydroxyalkanoate granule-associated protein of Ralstonia eutropha. The disruptant of p8 gene showed significant growth retardation in the alginate medium. An overexpression system for p8 was constructed in Escherichia coli, and the protein was purified and characterized. Surface plasmon resonance biosensor analysis indicated that p8 is able to bind alginate most efficiently at pH 4.0. The above results indicate that p8 is a cell surface protein able to bind alginate and facilitates the concentration of alginate in the pit on the cell surface of strain A1.