engleski

F-19 NMR of the E. coli periplasmic binding proteins

Leucine-isoleucine-valine binding protein (LIV) and Leucine-specific binding protein (LS) are soluble, hydrophobic amino-acid binding receptors in the osmotic shock-sensitive transport system of Escherichia coli. Despite their high similarity in amino-acid sequence (~80%) and almost identical tertiary structure, the two periplasmic receptors have differing specificities for ligands: LIV binds leucine, isoleucine, valine, and to a lesser extent, threonine, serine, and alanine, while LS shows specificity for leucine and phenylalanine as well as the fluorinated analogs of both of these amino acids. Ligand binding induces a conformational change, which enables the membrane proteins to recognize the 'charged' receptor. In order to examine conformational changes and protein dynamics upon ligand binding, single Trp to Phe mutants of both proteins were generated. Fluorinated tryptophan (5F-Trp) was biosynthetically incorporated into LIV and LS proteins and F-19 NMR spectra were obtained at 470 MHz on a Varian 500 spectrometer. Incorporation of 5F-Trp was between 80 and 100% in both examined proteins. Single Trp to Phe mutants did not cause perturbation in protein structure. Comparing spectra of single mutants to spectra of wild-type proteins, three Trp residues were assigned in LIV protein at the positions 278, 318 and 334, and four Trp residues were assigned in LS protein at the positions 18, 278, 320 and 336. In both proteins the spectra of the open form showed broadened resonances, which sharpened and shifted upon ligand binding. The main shift was observed in the binding pocket (Trp 18 in LS protein) and in the hinge region (Trp 278 in both proteins).

Leucine-isoleucine-valine binding protein; leucine-specific binding protein; Escherichia coli; 5-fluorotryptophan labeling; fluorine (F-19) NMR; ligand-induced conformational change

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