Vibrio effector protein VopQ inhibits fusion of V-ATPase–containing membranes

Abstract

Vesicle fusion governs many important biological processes, and imbalances in the regulation of membrane fusion can lead to a variety of diseases such as diabetes and neurological disorders. Here we show that the Vibrio parahaemolyticus effector protein VopQ is a potent inhibitor of membrane fusion based on an in vitro yeast vacuole fusion model. Previously, we demonstrated that VopQ binds to the V o domain of the conserved V-type H + -ATPase (V-ATPase) found on acidic compartments such as the yeast vacuole. VopQ forms a nonspecific, voltage-gated membrane channel of 18 Å resulting in neutralization of these compartments. We now present data showing that VopQ inhibits yeast vacuole fusion. Furthermore, we identified a unique mutation in VopQ that delineates its two functions, deacidification and inhibition of membrane fusion. The use of VopQ as a membrane fusion inhibitor in this manner now provides convincing evidence that vacuole fusion occurs independently of luminal acidification in vitro.V esicle fusion governs many important physiological processes including neurotransmitter release and exocytosis. As such, many studies have focused on understanding this process and the proteins involved in fusion using various models such as yeast vacuoles and Drosophila synaptic vesicles (1, 2). Yeast vacuoles are an established and elegant model to study eukaryotic membrane fusion because of the ease of their isolation and the conserved nature of the fusion machinery required for their homotypic fusion (3). Although the core SNARE and Rab GTPase fusion machinery alone can drive the physiologically relevant fusion of liposomes in vitro (2), genetic and biochemical experiments have identified a number of additional regulators of vacuole fusion, including the membrane sector of the highly conserved V-type H + -ATPase (V-ATPase) (4, 5). The eukaryotic V-ATPase is the main electrogenic proton pump involved in the acidification of many intracellular organelles such as endosomes, lysosomes, and the yeast vacuole (6). The V-ATPase consists of two conserved, multisubunit domains: the cytoplasmic V 1 domain and the membrane bound V o domain. The V 1 domain hydrolyzes ATP, providing the energy for proton translocation through the membrane-bound V o proteolipid proton channel, thus acidifying the lumen of the vesicle. The loss of V-ATPase subunits is lethal in higher eukaryotes, highlighting the importance of this vital protein complex for normal eukaryotic physiology. However, yeast that lack subunits of the V-ATPase exhibit conditional lethality that is rescued by growth on acidic media, thus providing a unique and powerful system for the study of V-ATPase functions in vivo. In addition to its acidification function, the V-ATPase has been implicated in a broad range of biological processes, including the proper trafficking of secreted and endocytosed cargos (7), viral fusion (8), exocytosis (1, 9, 10), and the SNARE-dependent membrane fusion of yeast vacuoles (4,5,11,12). Even though the role of V-ATPase in fusion has been demons...

Organelle size control – increasing vacuole content activates SNAREs to augment organelle volume through homotypic fusion

“…The fusion defect of vtcΔ vacuoles had initially been ascribed to a physical interaction of VTC with the vacuolar H + -ATPase (Müller et al, 2002(Müller et al, , 2003, which is required for vacuole fusion Peters et al, 2001;Sreelatha et al, 2014;Strasser et al, 2011). A function of VTC as the vacuolar polyP polymerase had been dismissed owing to the fact that the complex is entirely oriented towards the cytosol, whereas polyP is located inside the vacuoles.…”

Vibrio parahaemolyticus virulence determinants

“…Lysosomal deacidification caused by VopQ would impair the degradation of autophagic vesicles and therefore block their turnover. Recently, [88] demonstrated that VopQ also inhibits homotypic fusion of yeast vacuoles. Interestingly, the inhibitory activity was shown to be dependent only on VopQ binding to the V-ATPase but not on the effectors' pore forming activity, i.e., VopQ inhibits yeast vacuole fusion in an acidification-independent manner.…”

What are the roles of V-ATPases in membrane fusion?

“…This complexity has made it hard to resolve how V-ATPases might function in fusion. Now, Sreelatha et al (6) report studies of yeast lysosomal vacuoles and VopQ, a type III effector secreted by Vibrio parahaemolyticus. VopQ binds the V-ATPase, collapses ΔpH and ΔΨ, and inhibits fusion.…”

“…Second, the fusion phenotype of the Cmd1-3 mutant is contested, with a newer study, indicating no defect in fusion (21,24). Third, neither a ΔCa 2+ gradient nor Ca 2+ at the cytosolic face of the vacuole membrane is needed for fusion (6,24,25).…”

“…VopQ blocks fusion at an early stage, either before or during docking (6). If VopQ indeed inhibits fusion through its effects on V 0 , the results of Sreelatha et al (6) are difficult to reconcile with models where V 0 acts late (17). Screens in Drosophila revealed neurotransmission and trafficking defects when the V 0 subunit V100 was mutated (26).…”