A Rab-centric perspective of bacterial pathogen-occupied vacuoles

Abstract

The ability to create and maintain a specialized organelle that supports bacterial replication is an important virulence property for many intracellular pathogens. Living in a membrane-bound vacuole presents inherent challenges, including the need to remodel a plasma membrane-derived organelle into a novel structure that will expand and provide essential nutrients to support replication, while also having the vacuole avoid membrane transport pathways that target bacteria for destruction in lysosomes. It is clear that pathogenic bacteria use different strategies to accomplish these tasks. The dynamics by which host Rab GTPases associate with pathogen-occupied vacuoles provide insight into the mechanisms used by different bacteria to manipulate host membrane transport. In this review we highlight some of the strategies bacteria use to maintain a pathogen-occupied vacuole by focusing on the Rab proteins involved in biogenesis and maintenance of these novel organelles.

Figure 1. Common Rab signatures displayed by cellular organelles

Rab proteins play key roles in regulating vesicle traffic along the endocytic and secretory pathways. Localization of a subset of these Rab GTPases (Grey hexagons) and transport routes (black arrows) are depicted. Early endosomal vesicles display a Rab5 signature and the association of other Rab proteins with this organelle promotes sorting of cargo to a recycling pathway or to late endosomes or to create other organelles such as melanosomes. Acidified late endosomal and lysosomal organelles display Rab7 and Rab 9 and the proteins LAMP1 and CD63. Autophagosomes that display LC3 and Rab24 fuse with lysosomes to create degradative autolysosomes. Rab1 and Rab2 are found on early secretory vesicles in transit between the ER and Golgi. Specific Rab signatures are also present on Golgi-derived secretory vesicles.

Figure 2. Rab signatures displayed by pathogen-occupied vacuoles

Rab signatures displayed by pathogen-occupied vacuoles are depicted. The ER-like vacuoles harboring Legionella (LCV) and Brucella (BCV) can be differentiated based on Rab1 and Rab2 localization. The inclusion containing Chlamydia (CI) displays Rab proteins found on early endocytic organelles and on secretory organelles, suggesting that this pathogen-occupied vacuole selectively interacts with multiple host compartments. Mycobacterium-containing vacuoles (MCV) have a Rab signature consistent with maturation being stalled at an early endosomal stage. Salmonella-containing vacuoles (SCV) display a Rab signature that indicates maturation being stalled at a late endosomal stage. The destruction of Rab29, Rab32, and Rab38 by the effector protein GtgE eliminates these proteins from the SCV formed by Typhimurium but these Rab proteins are displayed by the SCV formed by Typhi due to the absence of GtgE. The vacuole containing Coxiella displays a Rab signature suggestive of both autophagosomes and lysosomes interacting with this compartment.

Figure 3. A cascade of Legionella effectors controls Rab1 localization to the LCV

A model proposing a sequence of activities mediated by Legionella effectors that will control the cycling of Rab1 on the pathogen-occupied vacuole. Depicted are five different Legionella effector proteins that target Rab1 (DrrA, AnkX, SidD, Lem3, LepB), which are delivered into host cells by the Dot/Icm secretion system (T4SS). The effector protein DrrA is a GEF that recruits Rab1 to the LCV membrane by stimulating the exchange of GDP for GTP, which dissociates the chaperone protein RabGDI. Rab1-GTP is modified by the AMPylation domain of DrrA or the PCylation domain of AnkX. The active Rab1 protein in association with a tethering factor recruits ER-derived vesicles that fuse with the LCV. The de-modifying enzymes SidD and Lem3 accumulate on the LCV and remove AMP and PC from Rab1. The unmodified Rab1 protein is susceptible to the GAP activity of LepB, which stimulates GTP hydrolysis. RabGDI removes Rab1-GDP from the LCV membrane.