Leucine/isoleucine/valine-binding protein contracts upon binding of ligand

Small-angle x-ray scattering and computer modeling have been used to study the effects of ligand binding to the leucine/isoleucine/valine-binding protein, an initial component of the high-affinity active transport system for branched-chain aliphatic amino acids in Escherichia coli. Measurements were made with no ligand present and with either L-leucine or L-valine present. Upon binding of either leucine or valine, there is a decrease in the radius of gyration, from 23.2 +/- 0.2 to 22.2 +/- 0.2 A, and in the maximum particle dimension, from 82 +/- 3 to 73 +/- 3 A. The x-ray structure of the unbound form has been determined and gives a radius of gyration and a maximum dimension consistent with the values found for the solution structure in this study (Sack, J. S., Saper, M. A., and Quiocho, F. A. (1989) J. Mol. Biol. 206, 171-191). The reduction in the radius of gyration and maximum dimension upon ligand binding can be accounted for by a substrate-induced cleft closure in a combined "hinge-twist" motion. Modeling of the substrate-bound state was done by comparison of this protein with another periplasmic binding protein (L-arabinose-binding protein), which possesses a similar two-lobe structure and for which the x-ray structure is known in its ligand-bound form.