PMID: 22993096 , Related PDB id: 3VOR

Authors:

Fukakusa S, Kawahara K, Nakamura S, Iwashita T, Baba S, Nishimura M, Kobayashi Y, Honda T, Iida T, Taniguchi T, Ohkubo T

Title:

Structure of the CFA/III major pilin subunit CofA from human enterotoxigenic Escherichia coli determined at 0.90 A resolution by sulfur-SAD phasing.

Journal:

Acta Crystallogr D Biol Crystallogr. 2012 Oct;68(Pt 10):1418-29. Epub 2012 Sep, 13.

Abstract:

CofA, a major pilin subunit of colonization factor antigen III (CFA/III), forms pili that mediate small-intestinal colonization by enterotoxigenic Escherichia coli (ETEC). In this study, the crystal structure of an N-terminally truncated version of CofA was determined by single-wavelength anomalous diffraction (SAD) phasing using five sulfurs in the protein. Given the counterbalance between anomalous signal strength and the undesired X-ray absorption of the solvent, diffraction data were collected at 1.5 A resolution using synchrotron radiation. These data were sufficient to elucidate the sulfur substructure at 1.38 A resolution. The low solvent content (29%) of the crystal necessitated that density modification be performed with an additional 0.9 A resolution data set to reduce the phase error caused by the small sulfur anomalous signal. The CofA structure showed the alphabeta-fold typical of type IVb pilins and showed high structural homology to that of TcpA for toxin-coregulated pili of Vibrio cholerae, including spatial distribution of key residues critical for pilin self-assembly. A pilus-filament model of CofA was built by computational docking and molecular-dynamics simulation using the previously reported filament model of TcpA as a structural template. This model revealed that the CofA filament surface was highly negatively charged and that a 23-residue-long loop between the alpha1 and alpha2 helices filled the gap between the pilin subunits. These characteristics could provide a unique binding epitope for the CFA/III pili of ETEC compared with other type IVb pili.