A novel effector secretion mechanism based on proton‐motive force‐dependent type III secretion apparatus rotation

Abstract

The type III secretion apparatus (T3SA) participates in the secretion of bacterial proteins called effectors, although the detailed mechanism of effector secretion remains unclear. T3SA and flagellum were shown to branch from a common ancestor and also show structural similarity. In addition, both T3SA-dependent effector secretion and flagellar rotation were reported to require proton-motive force (PMF) for activity. From these reports, we hypothesized that T3SA, like the flagellum, would rotate via PMF and that this rotation is responsible for effector secretion. To observe T3SA rotation, we constructed a novel observation system by modifying the tip of T3SA on bacterial cell membranes with an observation probe, which allowed documentation of T3SA rotation for the first time. T3SA rotation was stopped by the addition of a protonophore that decreases PMF. Moreover, increased viscosity of the observation medium inhibited both rotation of T3SA associated with beads and effector secretion. These results suggested that effector secretion would follow the PMF-dependent rotation of T3SA and could be inhibited by preventing T3SA rotation. Moreover, the motion-track analysis of bead rotation suggested that the T3SA needle might be flexible. Consequently, we propose a "rotational secretion model" as a novel effector secretion mechanism of T3SA.

Type III secretion systems: the bacterial flagellum and the injectisome

“…However, they are visible in spirochaetes with periplasmic filaments [48], which rotate at high load. No equivalent proteins have been detected in injectisomes, and although there is a report of rotation of the injectisome needle tip [49], this observation has so far not been repeated or functionally linked to any T3SS component.…”

The Bacterial Type III Secretion System as a Target for Developing New Antibiotics

“…These include polymers, proteins and polypeptide mimics. Ohgita et al (89) reported that the proton-motive force dependent rotation of the Pseudomonas T3SS was inhibited by the addition of the viscous polymer polyethylene glycol (PEG) 8000 and this was hypothesized to occur by the resistance of physical rotation due to solution viscosity. This hypothesis was further supported by a follow up study showing that other viscous polymers, such as alginate and mucin, inhibited T3SS rotation while low viscosity polymers such as PEG200 do not (90).…”

Building a secreting nanomachine: a structural overview of the T3SS

“…Intriguingly, especially in light of the ATPase complex structures, a recent study has suggested a proton-motive force dependent rotation of the NF-T3SS needle filament, thereby drawing further parallels to the flagellum [61 •• ] and the latest work proposing a general mechanism for flagellar export [62 •• ] may therefore also provide insight into the NF-T3SS. Future research concerning the molecular mechanisms of assembly and secretion by these nanomachines will undoubtedly provide further insight into the extent of the diversification of T3SSs.…”