The surfaces of many Gram-negative bacteria are decorated with soluble
proteins anchored to the outer membrane via an acylated N-terminus; these
proteins are referred to as surface
lipoproteins or SLPs. In Neisseria meningitidis, SLPs such as
transferrin-binding protein B (TbpB) and
factor-H binding protein (fHbp) are essential for host colonization and
infection because of their essential roles in
iron acquisition and immune evasion, respectively. Recently, we identified a family of outer
membrane proteins called Slam (Surface
lipoprotein assembly modulator) that are essential for surface display of neisserial SLPs. In the present study, we performed a bioinformatics analysis to identify 832 Slam related sequences in 638 Gram-negative bacterial species. The list included several known human pathogens, many of which were not previously reported to possess SLPs. Hypothesizing that genes encoding SLP substrates of Slams may be present in the same gene cluster as the Slam genes, we manually curated neighboring genes for 353 putative Slam homologs. From our analysis, we found that 185 (~52%) of the 353 putative Slam homologs are located adjacent to genes that encode a
protein with an N-terminal lipobox motif. This list included genes encoding previously reported SLPs in Haemophilus influenzae and Moraxella catarrhalis, for which we were able to show that the neighboring Slams are necessary and sufficient to display these
lipoproteins on the surface of Escherichia coli. To further verify the authenticity of the list of predicted SLPs, we tested the surface display of one such Slam-adjacent
protein from Pasteurella multocida, a zoonotic pathogen. A robust Slam-dependent display of the P. multocida
protein was observed in the E. coli translocation assay indicating that the
protein is a Slam-dependent SLP. Based on multiple sequence alignments and domain annotations, we found that an eight-stranded beta-barrel domain is common to all the predicted Slam-dependent SLPs. These findings suggest that SLPs with a TbpB-like fold are found widely in Proteobacteria where they exist with their interaction partner Slam. In the future, SLPs found in pathogenic bacteria can be investigated for their role in virulence and may also serve as candidates for
vaccine development.