title: The Corynebacterium diphtheriae shaft pilin SpaA is built of tandem Ig-like modules with stabilizing isopeptide and disulfide bonds.
abstract: Cell-surface pili are important virulence factors that enable bacterial pathogens to adhere to specific host tissues and modulate host immune response. Relatively little is known about the structure of Gram-positive bacterial pili, which are built by the sortase-catalyzed covalent crosslinking of individual pilin proteins. Here we report the 1.6-A resolution crystal structure of the shaft pilin component SpaA from Corynebacterium diphtheriae, revealing both common and unique features. The SpaA pilin comprises 3 tandem Ig-like domains, with characteristic folds related to those typically found in non-pilus adhesins. Whereas both the middle and the C-terminal domains contain an intramolecular Lys-Asn isopeptide bond, previously detected in the shaft pilins of Streptococcus pyogenes and Bacillus cereus, the middle Ig-like domain also harbors a calcium ion, and the C-terminal domain contains a disulfide bond. By mass spectrometry, we show that the SpaA monomers are cross-linked in the assembled pili by a Lys-Thr isopeptide bond, as predicted by previous genetic studies. Together, our results reveal that despite profound dissimilarities in primary sequences, the shaft pilins of Gram-positive pathogens have strikingly similar tertiary structures, suggesting a modular backbone construction, including stabilizing intermolecular and intramolecular isopeptide bonds.
authors.affiliation: Maurice Wilkins Centre for Molecular Biodiscovery and School of Biological Sciences, University of Auckland, Auckland 1020, New Zealand.
authors.forename: Hae Joo
authors.initials: HJ
authors.lastname: Kang
authors.forename: Neil G
authors.initials: NG
authors.lastname: Paterson
authors.forename: Andrew H
authors.initials: AH
authors.lastname: Gaspar
authors.forename: Hung
authors.initials: H
authors.lastname: Ton-That
authors.forename: Edward N
authors.initials: EN
authors.lastname: Baker
mesh_terms: D000595:Amino Acid Sequence; D001216:Asparagine / Q000737:chemistry / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003353:Corynebacterium diphtheriae / Q000235:genetics / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004220:Disulfides / Q000737:chemistry / Q000378:metabolism; D036781:Fimbriae Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007136:Immunoglobulins / Q000737:chemistry / Q000378:metabolism; D008239:Lysine / Q000737:chemistry / Q000378:metabolism; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D013912:Threonine / Q000737:chemistry / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004220:Disulfides; D007136:Immunoglobulins; D036781:Fimbriae Proteins; D013912:Threonine; D001216:Asparagine; D008239:Lysine