Biological functions and biogenesis of secreted bacterial outer membrane vesicles

Abstract

Gram-negative bacteria produce outer membrane vesicles (OMVs) that contain biologically active proteins and perform diverse biological processes. Unlike other secretion mechanisms, OMVs enable bacteria to secrete insoluble molecules in addition to and in complex with soluble material. OMVs allow enzymes to reach distant targets in a concentrated, protected, and targeted form. OMVs also play roles in bacterial survival: Their production is a bacterial stress response and important for nutrient acquisition, biofilm development, and pathogenesis. Key characteristics of OMV biogenesis include outward bulging of areas lacking membrane-peptidoglycan bonds, the capacity to upregulate vesicle production without also losing outer membrane integrity, enrichment or exclusion of certain proteins and lipids, and membrane fission without direct energy from ATP/GTP hydrolysis. Comparisons of similar budding mechanisms from diverse biological domains have provided new insight into evaluating mechanisms for outer membrane vesiculation.

Figure 1

General schemes for purifying native (left) and nonnative (right) vesicles. Detailed explanations for each stage are provided along the left side of the figure. OMVs, outer membrane vesicles.

Figure 2

Comparison of soluble and outer membrane vesicle (OMV)-based secretion and delivery. Soluble secretion pathways (a) result in general diffusion of secreted enzymes. Soluble secretion is most effective over short distances, especially if a threshold concentration is required for activity. Also, if the secreted enzymes are sensitive to proteases and other degradative factors in the environment, fewer enzymes reach target cells. OMV-mediated secretion pathways (b) generate OMVs that contain sufficient concentrations of enzymes to affect the substrate. Further, other molecules on the surface of OMVs can bind to host receptors, thereby allowing the enzyme to be targeted to specific cells or distal sites. OMVs protect the associated enzymes from extracellular degradation, allowing the enzymes to have a greater effective range.

Figure 3

Events in outer membrane vesicle (OMV) biogenesis. Step 1: Unbudded gram-negative envelope. The overall homogenous distribution of envelope proteins, including outer membrane (OM)-peptidoglycan-linking proteins (purple ovals or orange hexagons), does not lead to significant areas of OM unlinked from peptidoglycan. Steps 2 and 3: Initial stages of vesiculation. In various areas, links between the OM and the peptidoglycan are lost, either by movement of the linking protein (purple ovals) or by breaking the connections directly (orange half hexagons). This could be sufficient for basal OMV production (column a). Gathering of periplasmic proteins ( yellow stars) (column b) and/or accumulation of curvature-inducing OM proteins (red wedges) (column c) could induce additional budding events. Step 4: Released OMVs. OMVs that form without an inducing force (column a) may not have particularly enriched cargo. If OMVs are formed because of localized pressure on the OM, then the proteins causing the pressure (yellow stars) would be enriched in the OMVs (column b). If curvature-inducing molecules (red wedges) are involved, then these molecules and molecules associated with them (green circles) would also be enriched in the OMVs (column c). In all cases, inactive or cleaved OM-peptidoglycan-bridging proteins may be present in the OMVs. These multiple budding mechanisms are not exclusive, so OMVs released in a culture could be a mixed population and would make cargo enrichment difficult to detect.