Lipoproteins represent the biggest part of the PUL-encoded complexes and they are surface-exposed. This abundance of exposed lipoproteins is one of the main characteristics of Bacteroidetes. In Proteobacteria, the vast majority of outer membrane lipoproteins is inserted in the inner leaflet of the outer membrane and thus faces the periplasm.
Starting from the hypothesis that in Bacteroidetes surface exposed lipoproteins are endowed with a specific signal, we focused at identifying and characterizing this putative signal. In order to see if a specific amino acid motif would be responsible for the targeting of lipoproteins to the bacterial surface, we performed an in silico analysis on the sequences of the surface exposed lipoproteins of C. canimorsus. This led to the discovery of a consensus motif (QKDDE), located at the N-terminus of these proteins. In order to see whether this motif would represent an export signal, we grafted it to sialidase (SiaC), an OM-anchored periplasmic lipoprotein, and monitored its localization by immunofluorescence (IF) using both flow cytometry and microscopy. We observed that modified SiaC was exposed to the surface while normal SiaC was not. This showed that the consensus motif is indeed the lipoprotein export signal (LES) of C. canimorsus. We then analysed the motif in detail and found that the minimal LES consists of C-X-K-(D/E)2-X with the constraint that the overall charge must be negative (Fig.1). We also performed a similar in silico analysis in two other species of the phylum Bacteroidetes, namely B.acteroides fragilis and Flavobacterium johnsoniae, and identified the SDDDD and SDDFE consensus motifs respectively. Both motifs were grafted to SiaC and found to be sufficient to drive the protein to the surface. Taken together, these results show that the LES identified are quite conserved and confirm the existence of a shared novel pathway for lipoprotein export in this phylum of Gram-negative bacteria .Our current work focuses on unravelling this new pathway…
Publications:
- Lauber, F, Cornelis, G & Renzi, F# 2016, Identification of a New Lipoprotein Export Signal in Gram-Negative Bacteria mBio. DOI: 10.1128/mBio.01232-16 # corresponding author
