Abstract
ABSTRACTBacteria of the phylum Bacteroidetes, including commensal organisms and opportunistic pathogens, harbor abundant surface-exposed multiprotein membrane complexes (Sus-like systems) involved in carbohydrate acquisition. These complexes have been mostly linked to commensalism, and in some instances, they have also been shown to play a role in pathogenesis. Sus-like systems are mainly composed of lipoproteins anchored to the outer membrane and facing the external milieu. This lipoprotein localization is uncommon in most studied Gram-negative bacteria, while it is widespread in Bacteroidetes. Little is known about how these complexes assemble and particularly about how lipoproteins reach the bacterial surface. Here, by bioinformatic analyses, we identify a lipoprotein export signal (LES) at the N termini of surface-exposed lipoproteins of the human pathogen Capnocytophaga canimorsus corresponding to K-(D/E)2 or Q-A-(D/E)2. We show that, when introduced in sialidase SiaC, an intracellular lipoprotein, this signal is sufficient to target the protein to the cell surface. Mutational analysis of the LES in this reporter system showed that the amino acid composition, position of the signal sequence, and global charge are critical for lipoprotein surface transport. These findings were further confirmed by the analysis of the LES of mucinase MucG, a naturally surface-exposed C. canimorsus lipoprotein. Furthermore, we identify a LES in Bacteroides fragilis and Flavobacterium johnsoniae surface lipoproteins that allow C. canimorsus surface protein exposure, thus suggesting that Bacteroidetes share a new bacterial lipoprotein export pathway that flips lipoproteins across the outer membrane.
Highlights
Bacteria of the phylum Bacteroidetes, including commensal organisms and opportunistic pathogens, harbor abundant surface-exposed multiprotein membrane complexes (Sus-like systems) involved in carbohydrate acquisition
We extend our findings to other Bacteroidetes species, namely, Flavobacterium johnsoniae and Bacteroides fragilis, identifying their specific lipoprotein export signal (LES), showing that they share a new bacterial lipoprotein export pathway that flips lipoproteins across the outer membrane
In order to see whether a specific amino acid motif would be responsible for the targeting of lipoproteins to the bacterial surface, we examined in detail the sequences of the lipoproteins detected at the surface of Capnocytophaga canimorsus strain 5 [17]
Summary
Bacteria of the phylum Bacteroidetes, including commensal organisms and opportunistic pathogens, harbor abundant surface-exposed multiprotein membrane complexes (Sus-like systems) involved in carbohydrate acquisition. These complexes have been mostly linked to commensalism, and in some instances, they have been shown to play a role in pathogenesis. The wide distribution of these organisms reflects their high adaptability, partially due to their vast array of glycosylhydrolases allowing them to degrade most types of carbohydrates they can encounter [7, 16,17,18,19] These enzymes are often surface-exposed lipoproteins and are part of multiprotein outer membrane (OM) complexes devoted to nutrient acquisition. The surface localization of OM lipoproteins in Bacteroidetes implies the existence of a yet unknown dedicated recognition and transport mechanism
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