Capsular polysaccharide-mediated protein loading onto extracellular membrane vesicles of a fish intestinal bacterium, Shewanella vesiculosa HM13

Bacterial extracellular membrane vesicles (EMVs) play various physiologically important roles mediated by cargo proteins. However, our understanding of the molecular mechanism underlying cargo loading onto EMVs is limited. In this study, we analyzed the mechanism of cargo protein loading onto EMVs from a fish intestinal Gram-negative bacterium, Shewanella vesiculosa HM13. This strain secretes EMVs carrying a major cargo protein, P49. Near the P49 gene, we found genes having homology to genes involved in protein secretion and surface polysaccharide-chain synthesis. Among them, the deletion of genes encoding homologs of a flippase involved in bacterial extracellular polysaccharide synthesis (HM3343), phosphoethanolamine transferase (HM3344), and glycerophosphodiester phosphodiesterase (HM3345) resulted in the loss of capsular polysaccharide (CPS) of EMVs. We conducted an in vitro P49 loading assay onto P49-free EMVs to examine whether P49 was loaded onto the EMVs via its interaction with the CPS of the EMVs. We found that purified P49 was loaded onto EMVs harboring CPS in vitro, whereas it was not loaded onto EMVs from the mutants lacking CPS production due to the loss of HM3343, HM3344, and HM3345. Transmission electron microscopy of EMVs loaded with P49 in vitro and in vivo showed spherical nanoparticles around the EMVs, whereas such particles were not observed for EMVs without loaded P49, implying that P49 constitutes those particles on the surface of EMVs. These results indicate that P49 is loaded onto EMVs via its interaction with the CPS of EMVs. IMPORTANCEElucidating the mechanisms of cargo loading onto bacterial extracellular membrane vesicles (EMVs) is important to understand their biogenesis and to develop their applications. Here, we show that the major cargo protein of EMVs from a fish intestinal Gram-negative bacterium, Shewanella vesiculosa HM13, is loaded onto EMVs through its interaction with capsular polysaccharide (CPS) of EMVs. Genes involved in CPS synthesis were also identified. To our knowledge, there have been no reports describing the cargo protein-loading mechanism in which CPS serves as the protein-binding scaffold for EMVs. Thus, this study represents a new mode of protein loading onto EMVs. The results deepen our understanding of cargo loading onto EMVs and would contribute to development of their applications.