Two modes of ligand binding in maltose-binding protein of Escherichia coli. Electron paramagnetic resonance study of ligand-induced global conformational changes by site-directed spin labeling.

Thorgeir E. Thorgeirsson,Yeon-Kyun Shin,Hiroshi Nikaido,Jun Liu,Jason A. Hall

doi:10.1074/jbc.272.28.17610

Abstract

Binding of ligands to the maltose-binding protein (MBP) of Escherichia coli often causes a global conformational change involving the closure of its two lobes. We have introduced a cysteine residue onto each of these lobes by site-directed mutagenesis and modified these residues with spin labels. Using EPR spectroscopy, we examined the changes, caused by the ligand binding, in distance between the two spin labels, hence between the two lobes. The binding of both maltose and maltotetraose induced a considerable closure of the N- and C-terminal lobes of MBP. Little closure occurred upon the binding of maltotetraitol or beta-cyclodextrin. Previous study by fluorescence and UV differential absorbance spectroscopy (Hall, J. A., Gehring, K., and Nikaido, H. (1997) J. Biol. Chem. 272, 17605-17609) showed that maltose and a large portion of maltotetraose bound to MBP via one mode (R mode or "end-on" mode), which is physiologically active and leads to the subsequent transport of the ligands across the cytoplasmic membrane. In contrast, maltotetraitol and beta-cyclodextrin bound to MBP via a different mode (B mode or "middle" mode), which is physiologically inactive. The present work suggests that the B mode is nonproductive because ligands binding in this manner prevent the closure of the two domains of MBP, and, as a result, the resulting ligand-MBP complex is incapable of interacting properly with the inner membrane-associated transporter complex.

Highlights

Binding of ligands to the maltose-binding protein (MBP) of Escherichia coli often causes a global conformational change involving the closure of its two lobes
Maltose bound exclusively via one mode that involved a red shift of the intrinsic fluorescence emission spectrum of MBP as well as other characteristic changes in UV absorption spectrum (R mode, for red shift)
B-cyclodextrin as well as reduced or oxidized maltodextrins bound to MBP exclusively through another mode, which caused a blue shift of the fluorescence emission spectrum and other signature alterations in UV absorption spectrum (B mode, for blue shift)

Summary

Introduction

Binding of ligands to the maltose-binding protein (MBP) of Escherichia coli often causes a global conformational change involving the closure of its two lobes. Using EPR spectroscopy, we examined the changes, caused by the ligand binding, in distance between the two spin labels, between the two lobes. The binding of both maltose and maltotetraose induced a considerable closure of the Nand C-terminal lobes of MBP. 272, 17605–17609) showed that maltose and a large portion of maltotetraose bound to MBP via one mode (R mode or “end-on” mode), which is physiologically active and leads to the subsequent transport of the ligands across the cytoplasmic membrane. B-cyclodextrin as well as reduced or oxidized maltodextrins bound to MBP exclusively through another mode, which caused a blue shift of the fluorescence emission spectrum and other signature alterations in UV absorption spectrum (B mode, for blue shift). We have argued [1] that the R and B modes most likely correspond to the “end-on” and “middle” modes of binding, previously defined on the basis of NMR chemical shift of the 3H atom on the anomeric carbon of the ligand molecules [3]

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Conclusion