Abstract
The Gram-negative bacterium Porphyromonas gingivalis is a secondary colonizer of the oral biofilm and is involved in the onset and progression of periodontitis. Its fimbriae, of type-V, are important for attachment to other microorganisms in the biofilm and for adhesion to host cells. The fimbriae are assembled from five proteins encoded by the mfa1 operon, of which Mfa5 is one of the ancillary tip proteins. Here we report the X-ray structure of the N-terminal half of Mfa5, which reveals a von Willebrand factor domain and two IgG-like domains. One of the IgG-like domains is stabilized by an intramolecular isopeptide bond, which is the first such bond observed in a Gram-negative bacterium. These features make Mfa5 structurally more related to streptococcal adhesins than to the other P. gingivalis Mfa proteins. The structure reported here indicates that horizontal gene transfer has occurred among the bacteria within the oral biofilm.
Highlights
The Gram-negative bacterium Porphyromonas gingivalis is a secondary colonizer of the oral biofilm and is involved in the onset and progression of periodontitis
Fimbriae are protein polymers projecting from the bacterial surface, and these make the first contact with the targeted host
It is of great interest to understand the structure, assembly mechanism, and ligand specificity of fimbriae because their adherence mechanisms and biogenesis are potential targets for the development of novel targeted antibacterials[43]
Summary
The Gram-negative bacterium Porphyromonas gingivalis is a secondary colonizer of the oral biofilm and is involved in the onset and progression of periodontitis. Fimbriae ( called pili) are long filamentous protein polymers that project from the bacterial surface and are crucial for attachment to other microorganisms, host cells, and surfaces[1] They usually contain several protein subunits encoded by the same gene cluster, resulting in the assembly of a long shaft of repetitive proteins decorated by ancillary tip proteins[2,3,4]. Gram-positive fimbrial proteins are covalently linked to each other by intermolecular isopeptide bonds, which are amide bonds between a lysine side chain of one subunit and the carboxyl group of a C-terminal threonine of the subunit The formation of these bonds is mediated by specific sortases that are encoded by the same gene cluster[6,7]. The proteins are acylated, processed by signal peptidase II, and transported to the outer membrane by the localization of lipoprotein export pathway[31]
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