Off-pathway assembly of fimbria subunits is prevented by chaperone CfaA of CFA/I fimbriae from enterotoxigenic E. coli.
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.
Division of Infectious Diseases, National Key Laboratory of Biotherapy/Collaborative Innovation Center of Biotherapy, West China Hospitals, Sichuan University, Chengdu, 610041, China.|Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD, 20892, USA.
Rui
R
Bao
Enteric Diseases Department, Infectious Diseases Directorate, Naval Medical Research Center, Silver Spring, MD, 20910-7500, USA.
Yang
Y
Liu
Enteric Diseases Department, Infectious Diseases Directorate, Naval Medical Research Center, Silver Spring, MD, 20910-7500, USA.|Department of Pediatrics, Uniformed Services University of the Health Sciences, Bethesda, MD, 20814-4799, USA.
Stephen J
SJ
Savarino
Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD, 20892, USA.
Di
D
Xia
D020816:Amino Acid Motifs; D054307:Enterotoxigenic Escherichia coli / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D004926:Escherichia coli / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000378:metabolism; D036781:Fimbriae Proteins / Q000235:genetics / Q000378:metabolism*; D010861:Fimbriae, Bacterial / Q000378:metabolism; D018832:Molecular Chaperones / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000378:metabolism; D013329:Structure-Activity Relationship
D029968:Escherichia coli Proteins; D018832:Molecular Chaperones; D021122:Protein Subunits; C022924:colonization factor antigens; D036781:Fimbriae Proteins