Protein selection and export via outer membrane vesicles

Abstract

Outer membrane vesicles (OMVs) are constitutively produced by all Gram-negative bacteria. OMVs form when buds from the outer membrane (OM) of cells encapsulate periplasmic material and pinch off from the OM to form spheroid particles approximately 10 to 300nm in diameter. OMVs accomplish a diversity of functional roles yet the OMV's utility is ultimately determined by its unique composition. Inclusion into OMVs may impart a variety of benefits to the protein cargo, including: protection from proteolytic degradation, enhancement of long-distance delivery, specificity in host-cell targeting, modulation of the immune response, coordinated secretion with other bacterial effectors, and/or exposure to a unique function-promoting environment. Many enriched OMV-associated components are virulence factors, aiding in host cell destruction, immune system evasion, host cell invasion, or antibiotic resistance. Although the mechanistic details of how proteins become enriched as OMV cargo remain elusive, recent data on OM biogenesis and relationships between LPS structure and OMV-cargo inclusion rates shed light on potential models for OM organization and consequent OMV budding. In this review, mechanisms based on pre-existing OM microdomains are proposed to explain how cargo may experience differing levels of enrichment in OMVs and degrees of association with OMVs during extracellular export. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Keywords: Bacterial envelope; Cargo selection; Gram-negative bacteria; Outer membrane vesicle; Secretion.

Fig. 1

Models of envelope organization prior to outer membrane vesicle (OMV) budding that could lead to cargo enrichment and exclusion. A. Outer membrane proteins (OMPs) and lipopolysaccharide (LPS) are likely found in pre-existing microdomains in the outer membrane, resulting directly from the membrane biogenesis processes. The content clustered in one area may be inherently more prone to budding. This organization can lead to the enrichment of particular OMPs into OMVs. Subsequent partitioning of envelope components can lead to enrichment of other envelope factors (B–F). Envelope features for parts B–F are as indicated in part A: outer membrane (OM); periplasm (PP); inner membrane (IM); and individual envelope components coded by shape (indicated in the side legends) and color (OMV excluded cargo: red symbols; enriched cargo: green symbols; neither enriched/excluded cargo: blue symbols). B. The localization of periplasmic proteins (PP proteins) to the external face of the inner membrane (IM) may lead to exclusion from OMVs while localization to the OM may enhance incorporation into OMVs. C. Recruitment factors, potentially OM lipoproteins (OMLPs) or OMPs (green symbols), may bind to or interact with OMV cargo to enhance its localization to future budding sites. D. Exclusion factors, such as OMPs or OMLPs excluded from OMVs (red symbols) may interact with or bind to PP proteins in order to hinder their incorporation into OMVs. E. Protein secretion apparatuses may exist near potential sites of budding. If their secretion substrate binds an OM component or OM-associated component, then these secreted proteins may cluster on the external surface. These areas of high local protein concentration could cause crowding and thereby provide the energy for establishment of positive curvature. F. A combination of all of the previously-described mechanisms most likely is at work to promote budding and OMV formation. In each of the models, lipoprotein (LPP) – peptidoglycan (PG) crosslinks are notably absent from potential OMV bud sites, either by an exclusion mechanism or by conformational changes to the LPP.