Characterization of exsC and exsD self-association and heterocomplex formation

The flagellum and injectisome of gram-negative bacteria are large multiprotein organelles spanning the inner and outer membranes (12). Whereas flagella are involved in motility, injectisomes function by injecting toxins into eukaryotic host cells. The biogenesis of both organelles requires a dedicated protein export system referred to as a type III secretion system (T3SS). Many of the individual protein constituents of flagella and injectisomes are first secreted via the T3SS before assembly into the complete organelles (10). The flagellar T3SS is located at the base of the basal body in the inner membrane, where it secretes proteins required for assembly of the flagellar rod, hook, and filament (12). The T3SS associated with the injectisome is also located at the base of a basal body-like structure and secretes proteins required for injectisome assembly as well as the toxins destined for delivery to host cells (10). An interesting regulatory feature of both the flagellum and injectisome systems is the direct coupling of transcription with secretion (16). The prototypical example of this regulatory mechanism is seen in Salmonella spp., where genes required for assembly of the flagellar basal body, hook, and filament are expressed in a temporal hierarchy (1). The late genes, required for the final stage of flagellar biogenesis, are not expressed until the basal body is completely assembled. Prior to completion of the basal body complex, the FlgM anti-σ factor accumulates in the cytoplasm, where it binds to and inhibits the FliA σ factor required for late gene transcription. Upon completion of the basal body complex, however, there is a switch in the substrate specificity of the T3SS, FlgM is secreted from cells, and FliA-dependent transcription of late genes ensues. The coupling of transcription to secretion provides an elegant means of coordinating gene expression with the different stages of the flagellar assembly process. Secretion competence also serves as an inducing signal for transcription in the injectisome system (16). Unlike the case in the flagellar system, however, where FlgM secretion is linked to basal body assembly, secretion competence in many injectisome systems is controlled by environmental signals (2, 14, 15, 18, 19, 27). Through a poorly understood mechanism, these environmental signals convert the type III secretion machinery from a closed (secretion incompetent) conformation to an open (secretion competent) conformation. Conversion to the open state triggers a regulatory cascade resulting in transcriptional activation. Mechanistically, these regulatory cascades fall into one of the following three general categories: (i) secretion of a negative regulatory factor, seen in Yersinia spp.; (ii) sequestration of a coactivator, as reported for Salmonella and Shigella spp.; and (iii) sequestration of a negative regulatory factor, as recently described for Pseudomonas aeruginosa (6, 7, 13, 14, 19-21). Transcription of the P. aeruginosa T3SS is induced under Ca2+-limiting growth conditions or following contact of the bacterium with a host cell (9, 22). Although the mechanism of activation by host cell contact is unclear, the low-Ca2+ signal converts the T3SS machinery from a secretion-incompetent to a secretion-competent state (14). Recent studies have demonstrated that transcription of the T3SS is intimately linked to the activity of the type III secretion machinery. Transcription is repressed when the T3SS machinery is secretion incompetent and is derepressed when the machinery is secretion competent (7, 14, 21). The mechanism of coupling transcription to secretion involves a cascade of four interacting regulatory proteins (ExsA, ExsD, ExsC, and ExsE). ExsA is a positive activator of T3SS transcription, while ExsD functions as an antiactivator by binding to and inhibiting ExsA activity (11, 14, 25, 26). ExsC functions as an anti-antiactivator by binding to and inhibiting the negative regulatory activity of ExsD and also as a type III-specific chaperone for ExsE (7, 21). Finally, ExsE binds to ExsC and inhibits its activity (20, 21). A key feature of the system is that ExsE is secreted from cells under low-Ca2+ conditions. Based on these findings, the following model has been proposed to account for the coupling of transcription with secretion (7, 21). Under high-Ca2+ conditions, intracellular ExsE binds to and sequesters ExsC, and ExsD is preferentially bound to ExsA, resulting in the inhibition of ExsA-dependent transcription. Under low-Ca2+ conditions, however, ExsE is secreted from cells. This results in a shift in the binding equilibrium whereby ExsD is preferentially bound to ExsC, and liberated ExsA is made available to activate transcription of the T3SS. In the present study, we characterize the ExsC-ExsD binding interaction. Both ExsC and ExsD were found to form self-associated multimeric complexes. ExsC and ExsD readily form a complex when coexpressed in vivo or when individually purified and mixed in vitro. Biochemical analyses suggest that formation of the complex is thermodynamically favorable and consists of multiple copies of ExsC and ExsD. Finally, an ExsD mutant unable to interact with ExsC was isolated and found to have a hyperrepressive phenotype.