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PDBsum entry 3vdz
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Metal binding protein, de novo design
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PDB id
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3vdz
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References listed in PDB file
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Key reference
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Title
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Tailoring encodable lanthanide-Binding tags as mri contrast agents.
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Authors
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K.D.Daughtry,
L.J.Martin,
A.Sarraju,
B.Imperiali,
K.N.Allen.
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Ref.
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Chembiochem, 2012,
13,
2567-2574.
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PubMed id
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Abstract
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Lanthanide-binding tags (LBTs), peptide-based coexpression tags with high
affinity for lanthanide ions, have previously been applied as luminescent probes
to provide phasing for structure determination in X-ray crystallography and to
provide restraints for structural refinement and distance information in NMR.
The native affinity of LBTs for Gd(3+) indicates their potential as the basis
for engineering of peptide-based MRI agents. However, the lanthanide
coordination state that enhances luminescence and affords tightest binding would
not be ideal for applications of LBTs as contrast agents, due to the exclusion
of water from the inner coordination sphere. Herein, we use structurally defined
LBTs as the starting point for re-engineering the first coordination shell of
the lanthanide ion to provide for high contrast through direct coordination of
water to Gd(3+) (resulting in the single LBT peptide, m-sLBT). The effectiveness
of LBTs as MRI contrast agents was examined in vitro through measurement of
binding affinity and proton relaxivity. For imaging applications that require
targeted observation, fusion to specific protein partners is desirable. However,
a fusion protein comprising a concatenated double LBT (dLBT) as an N-terminal
tag for the model protein ubiquitin had reduced relaxivity compared with the
free dLBT peptide. This limitation was overcome by the use of a construct based
on the m-sLBT sequence (q-dLBT-ubiquitin). The structural basis for the enhanced
contrast was examined by comparison of the X-ray crystal structure of
xq-dLBT-ubiquitin (wherein two tryptophan residues are replaced with serine), to
that of dLBT-ubiquitin. The structure shows that the backbone conformational
dynamics of the MRI variant may allow enhanced water exchange. This engineered
LBT represents a first step in expanding the current base of
specificity-targeted agents available.
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