Subcellular localisation of lipoproteins of Vibrio vulnificus by the identification of outer membrane vesicles components

Vibrio vulnificus, a Gram-negative halophilic bacterium, is an opportunistic human pathogen that is responsible for the majority of seafood-associated deaths worldwide. Lipoproteins are important components of the bacterial cell envelope and have been shown to be involved in a wide variety of cellular processes. Little is known about the localisation or transport mechanism of lipoproteins in V. vulnificus. To assess the localisation of lipoproteins in V. vulnificus, we tested two established techniques for the rapid separation of membrane-associated proteins: detergent extraction with Sarkosyl and outer membrane vesicles (OMVs) preparation. The results showed that Sarkosyl extraction was not useful for the separation of lipoproteins from the different membranes of V. vulnificus. On the other hand, we confirmed that OMVs produced by V. vulnificus contained lipoproteins from the outer but not the inner membrane. Analysis of the OMVs components confirmed the localisation of several well-known lipoproteins to membranes that were different from expected, based on their predicted functions. Using this technique, we found that Asp at position +2 of mature lipoproteins can function as an inner membrane retention signal in V. vulnificus. Interestingly, the Escherichia coli +2 rule does not apply to the V. vulnificus lipoprotein IlpA (G2D) mutant, as a Ser to Asp mutation at position +2 of IlpA did not affect its outer membrane localisation. Furthermore, an IlpA tether-mRFP chimeric lipoprotein and its G2D mutant also behaved like IlpA. Together, these results suggest that the sorting rule of lipoproteins in V. vulnificus might be different from that in E. coli.