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PDBsum entry 4y2o
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Structural protein
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PDB id
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4y2o
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References listed in PDB file
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Key reference
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Title
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Off-Pathway assembly of fimbria subunits is prevented by chaperone cfaa of cfa/i fimbriae from enterotoxigenic e. Coli.
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Authors
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R.Bao,
Y.Liu,
S.J.Savarino,
D.Xia.
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Ref.
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Mol Microbiol, 2016,
102,
975-991.
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PubMed id
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Abstract
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The assembly of the class 5 colonization factor antigen I (CFA/I) fimbriae of
enterotoxigenic E. coli was proposed to proceed via the alternate
chaperone-usher pathway. Here, we show that in the absence of the chaperone
CfaA, CfaB, the major pilin subunit of CFA/I fimbriae, is able to spontaneously
refold and polymerize into cyclic trimers. CfaA kinetically traps CfaB to form a
metastable complex that can be stabilized by mutations. Crystal structure of the
stabilized complex reveals distinctive interactions provided by CfaA to trap
CfaB in an assembly competent state through donor-strand complementation (DSC)
and cleft-mediated anchorage. Mutagenesis indicated that DSC controls the
stability of the chaperone-subunit complex and the cleft-mediated anchorage of
the subunit C-terminus additionally assist in subunit refolding. Surprisingly,
over-stabilization of the chaperone-subunit complex led to delayed fimbria
assembly, whereas destabilizing the complex resulted in no fimbriation. Thus,
CfaA acts predominantly as a kinetic trap by stabilizing subunit to avoid its
off-pathway self-polymerization that results in energetically favorable trimers
and could serve as a driving force for CFA/I pilus assembly, representing an
energetic landscape unique to class 5 fimbria assembly.
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