Functional Identification of Conserved Residues Involved in Lactobacillus rhamnosus GG Sortase Specificity and Pilus Biogenesis [Microbiology]

April 21st, 2014 by Douillard, F. P., Rasinkangas, P., von Ossowski, I., Reunanen, J., Palva, A., de Vos, W. M.

In Gram-positive bacteria, sortase-dependent pili mediate the adhesion of bacteria to host epithelial cells and play a pivotal role in pathogenicity, host signaling and biofilm formation. Lactobacillus rhamnosus strain GG, a well-known probiotic bacterium, also displays on its cell surface mucus-binding pilus structures, along with other LPXTG surface proteins, which are processed by sortases upon specific recognition of a highly conserved LPXTG motif. Bioinformatic analysis of all predicted LPXTG proteins encoded by the L. rhamnosus GG genome revealed a remarkable conservation of glycine residues juxtaposed to the canonical LPXTG motif. Here, we investigated and defined the role of this so-called triple glycine (TG) motif in determining sortase specificity during the pilus assembly and anchoring. Mutagenesis of the TG motif resulted in lack or alteration of the L. rhamnosus GG pilus structures, indicating that the TG motif is critical in pilus assembly and that they govern the pilin-specific and housekeeping sortase specificity. This allowed us to propose a regulatory model of the L. rhamnosus GG pilus biogenesis. Remarkably, the TG motif was identified in multiple pilin gene clusters of other Gram-positive bacteria, suggesting that similar signaling mechanisms occur in other, mainly pathogenic, species.
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