Posttranslational regulation and effector specificity of the type VI secretion system
Silverman, Julie Michelle
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Bacteria mediate interactions with their surroundings by exporting a variety of proteins into the extracellular environment. Gram-negative bacteria have evolved at least six dedicated secretory pathways to accomplish this task, each exporting a discrete set of proteins through complex and genetically divergent systems. One such system is the type VI secretion system (T6SS), which is a contact-dependent protein export pathway that delivers toxic effectors into target bacterial and eukaryotic cells. The export of effectors is controlled by sophisticated regulatory networks that can be triggered by specific environmental cues. The characterization of these regulatory pathways has yielded new insight into the physiologically relevant conditions in which these systems are active. In this thesis work, the Hcp secretion island I (HSI-I)-encoded T6SS (H1-T6SS) of the opportunistic pathogen, <italic>Pseudomonas aeruginosa<italic>, was used as a model system to investigate the factors that govern T6S activity. Specifically, this work describes two distinct posttranslational regulatory pathways — mediated by H1-T6SS associated proteins — that coordinate T6S apparatus assembly and effector export. One of these regulatory pathways, the threonine phosphorylation pathway (TPP), is stimulated when P. aeruginosa is subjected to surface-associated growth conditions. In contrast, the second pathway, which is mediated by a negative regulator, TagF, does not respond to surface or planktonic growth conditions and, instead, is likely stimulated by an unknown cue. As productive H1-T6S-dependent toxin delivery requires close cell contact, the presence of these regulatory pathways may provide a means to efficiently initiate H1-T6S activity under appropriate environmental conditions. Another important aspect of the T6SS and secretion systems in general, is the mechanisms used to specifically select substrates for export. This thesis work has uncovered a mechanism for substrate recognition by the T6SS. I found that specific interactions between T6S-effectors and a secreted T6S component, Hcp (haemolysin co-regulated protein), are essential for effector export. Thus, Hcp plays a central role in T6S substrate discrimination. Together, these findings have advanced our understanding of the T6SS and have shed light on the diverse mechanisms by which proteins can be exported by bacteria.
- Microbiology