A novel calcium binding site in the galactose-binding protein of bacterial transport and chemotaxis

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Abstract
The refined 1.9-Å resolution structure of the periplasmic D-galac-tose-binding protein (GBP) reveals a calcium ion surrounded by seven ligands, all protein oxygen atoms. A nine-residue loop (amino-acid positions 134–142), which is preceded by a /β-turn and followed by a β-strand, provides five ligands from every second residue. The last two ligands are supplied by the carboxylate group of Glu 205. The entire GBP Ca2+-binding site adopts a conformation very similar to the site in the 'helix-loop-helix' or 'EF-hand' unit commonly found in intracellular calcium-binding proteins1, but without the two helices. Structural analyses have also uncovered the sugar-binding site some 30 Å from the calcium and a site for interacting with the membrane-bound trg chemotactic signal transducer ∼45 Å from the calcium. Our results show that a common tight calcium binding site of ancient origin can be tethered to different secondary structures. They also provide the first demonstration of a metal-binding site in a protein which is involved in bacterial active transport and chemotaxis.

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