Regulation of the Yersinia type III secretion system: traffic control

Abstract

Yersinia species, as well as many other Gram-negative pathogens, use a type III secretion system (T3SS) to translocate effector proteins from the bacterial cytoplasm to the host cytosol. This T3SS resembles a molecular syringe, with a needle-like shaft connected to a basal body structure, which spans the inner and outer bacterial membranes. The basal body of the injectisome shares a high degree of homology with the bacterial flagellum. Extending from the T3SS basal body is the needle, which is a polymer of a single protein, YscF. The distal end of the needle serves as a platform for the assembly of a tip complex composed of LcrV. Though never directly observed, prevailing models assume that LcrV assists in the insertion of the pore-forming proteins YopB and YopD into the host cell membrane. This completes a bridge between the bacterium and host cell to provide a continuous channel through which effectors are delivered. Significant effort has gone into understanding how the T3SS is assembled, how its substrates are recognized and how substrate delivery is controlled. Arguably the latter topic is the least understood; however, recent advances have provided new insight, and therefore, this review will focus primarily on summarizing the current state of knowledge regarding the control of substrate delivery by the T3SS. Specifically, we will discuss the roles of YopK, as well as YopN and YopE, which have long been linked to regulation of translocation. We also propose models whereby the YopK regulator communicates with the basal body of the T3SS to control translocation.

Keywords: YopE; YopK; YopN; injectisome; substrate specificity; type III secretion.

Figure 1

Model of the injectisome. Shown is a cartoon depicting the structural components of the Yersinia injectisome. Purple, scaffold proteins: YscC, YscD, YscJ; Orange, export apparatus proteins: YscR, YscS, YscT, YscU, YscV; Blue, cytoplasmic components: YscQ (C-ring) and YscN, YscL, YscK (ATPase complex); Green, YscI (rod) and YscF (needle); Red, pore complex: LcrV (needle tip complex) and YopB/YopD (translocation pore).

Figure 2

Progression of injectisome assembly and activation. In the early stage, the basal body recognizes early substrates (green) for secretion. These substrates are required for needle formation. Upon completing needle assembly, YscU (orange) undergoes autocleavage, which triggers a substrate specificity switch and transition to the middle stage. During this phase, YopN associates with the basal body to allow middle substrates (red) to be secreted, while rejecting late substrates. The middle substrates are required to form the tip complex and translocation pores. Upon cell contact, YopN is released from the basal body and secreted, triggering transition to the late stage. Two models are presented to depict the late stage. In the One-step model, the pore complex assembles at the tip of the needle to create a continuous channel, through which late substrates (yellow) are injected. In the Two-step model, late substrates are secreted into the extracellular space and then interact with pore proteins. The late substrate-pore complexes diffuse across the space and interact with the host membrane to deliver the late substrates.

Figure 3

Models for YopK functions. (A) Controlling fidelity. During the middle stage, YopN is associated with the basal body to prevent premature release of late substrates. This blockade is released upon cell contact by translocating YopN into host cells. YopK now associates with the basal body to prevent aberrant injection of middle substrates. (B) An OFF switch. During the late stage, YopK is injected into the host cell and acts to down-regulate injection of the other late substrates. Two models are shown to depict how this may happen. In the Signal transduction model, YopK would interact with the pore complex and cause a conformational change in the pore, which then triggers structural changes along the length of the injectisome to provide a signal to the basal body. Further transport of late substrates is then inhibited. In the Plug model, such structural changes are not necessarily induced. Rather, YopK binding to the pore causes a physical blockade to the channel such that substrates cannot pass through the pore.