Journal
BIOCHEMISTRY
Volume 50, Issue 24, Pages 5465-5476Publisher
AMER CHEMICAL SOC
DOI: 10.1021/bi101869h
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Funding
- Lady Davis Foundation
- United States-Israel Binational Science Foundation
- University of Michigan Rackham Graduate School
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]
- Michigan Economic Development Corp.
- Michigan Technology Tri-Corridor [085P1000817]
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We report the 1.9 angstrom resolution crystal structure of enteropathogenic Escherichia colt GfcC, a periplasmic protein encoded by the gfc operon, which is essential for assembly of group 4 polysaccharide capsule (O-antigen capsule). Presumed gene orthologs of gfcC are present in capsule-encoding regions of at least 29 genera of Gram-negative bacteria. GfcC, a member of the DUF1017 family, is comprised of tandem beta-grasp (ubiquitin-like) domains (D2 and D3) and a carboxyl-terminal amphipathic helix, a domain arrangement reminiscent of that of Wza that forms an exit pore for group 1 capsule export. Unlike the membrane-spanning C-terminal helix from Wza, the GfcC C-terminal helix packs against D3. Previously unobserved in a beta-grasp domain structure is a 48-residue helical hairpin insert in D2 that binds to D3, constraining its position and sequestering the carboxyl-terminal amphipathic helix. A centrally located and invariant Arg115 not only is essential for proper localization but also forms one of two mostly conserved pockets. Finally, we draw analogies between a GfcC protein fused to an outer membrane beta-barrel pore in some species and fusion proteins necessary for secreting biofilm-forming exopolysaccharides.
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