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Title
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Crystal structure of Vigna radiata cytokinin-specific binding protein in complex with zeatin.
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Authors
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O.Pasternak,
G.D.Bujacz,
Y.Fujimoto,
Y.Hashimoto,
F.Jelen,
J.Otlewski,
M.M.Sikorski,
M.Jaskolski.
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Ref.
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Plant Cell, 2006,
18,
2622-2634.
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PubMed id
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Abstract
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The cytosolic fraction of Vigna radiata contains a 17-kD protein that binds
plant hormones from the cytokinin group, such as zeatin. Using recombinant
protein and isothermal titration calorimetry as well as fluorescence
measurements coupled with ligand displacement, we have reexamined the K(d)
values and show them to range from approximately 10(-6) M (for 4PU30) to 10(-4)
M (for zeatin) for 1:1 stoichiometry complexes. In addition, we have
crystallized this cytokinin-specific binding protein (Vr CSBP) in complex with
zeatin and refined the structure to 1.2 A resolution. Structurally, Vr CSBP is
similar to plant pathogenesis-related class 10 (PR-10) proteins, despite low
sequence identity (<20%). This unusual fold conservation reinforces the
notion that classic PR-10 proteins have evolved to bind small-molecule ligands.
The fold consists of an antiparallel beta-sheet wrapped around a C-terminal
alpha-helix, with two short alpha-helices closing a cavity formed within the
protein core. In each of the four independent CSBP molecules, there is a zeatin
ligand located deep in the cavity with conserved conformation and protein-ligand
interactions. In three cases, an additional zeatin molecule is found in variable
orientation but with excellent definition in electron density, which plugs the
entrance to the binding pocket, sealing the inner molecule from contact with
bulk solvent.
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