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The 3 angstrom Resolution Structure of a D-galactose-Binding Protein for Transport and Chemotaxis in Escherichia coli
N. K. Vyas, M. N. Vyas and F. A. Quiocho
Proceedings of the National Academy of Sciences of the United States of America
Vol. 80, No. 7, [Part 1: Biological Sciences] (Apr. 1, 1983), pp. 1792-1796
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/13378
Page Count: 5
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X-ray diffraction studies of a D-galactose-binding protein essential for transport and chemotaxis in Escherichia coli have yielded a model of the polypeptide chain backbone. An initial polyalanine backbone trace was obtained at 3.2 angstrom resolution by the molecular replacement technique, using a polyalanine search model derived from the refined structure of the L-arabinose-binding protein. Concurrently, a 3 angstrom resolution electron-density map of the D-galactose receptor was determined from multiple isomorphous replacement (MIR) phases. The properly transformed initial polyalanine model superimposed on the MIR electron-density map proved to be an excellent guide in obtaining a final trace. The few changes made in the polyalanine model to improve the fit to the density were confined primarily to the COOH-terminal peptide and some loops connecting the elements of the secondary structure. Despite the lack of significant sequence homology, the overall course of the polypeptide backbone of the D-galactose-binding protein is remarkably similar to that of the L-arabinose-binding protein, the first structure in a series to be solved from this family of binding proteins. Both structures are elongated (axial ratios of 2:1) and composed of two globular domains. For both proteins, the arrangements of the elements of the secondary structure in both domains are identical; both lobes contain a core of β -pleated sheet with a pair of helices on either side of the plane of the sheet. The four major hydrophobic clusters that stabilize the structure of the L-arabinose-binding protein are also present in the D-galactose-binding protein.
Proceedings of the National Academy of Sciences of the United States of America © 1983 National Academy of Sciences