Chaperone-Templated Folding in Type III Secretion

Loren Rodgers, UC-San Diego
Biomedical and Clinical Sciences

Background: One of the hallmarks of advanced acquired immune deficiency syndrome (AIDS) is an increased susceptibility to bacterial pathogens, many of which utilize a type III secretion system (TTSS) to deliver toxin proteins (effectors) directly into the cytosol of host cells. Our long-term goal is to understand the steps required for protein transport by the TTSS. Here, we investigate the function of TTSS chaperone proteins. Inside the bacterium, TTSS chaperones bind one (or sometimes two) specific effector proteins. Although chaperones are not translocated into host cells, they are required for the translocation of their cognate effectors, and thus are central to virulence. Two alternative hypotheses have been proposed to describe the role of chaperones in type III secretion. The first hypothesis is that chaperones unfold their bound effectors, thereby maintaining them in a conformation that is competent for secretion. The alternative hypothesis suggests that chaperones mediate effector recognition by templating the formation of a three-dimensional motif that serves to target the effector to a component of the TTSS. Our specific aim is to determine how association of the Yersinia chaperone SycE affects the structure and dynamics of its corresponding effector YopE. If the chaperone werefound to locallyunfold the effector, this would provide direct evidence for the localized unfolding hypothesis. If instead the chaperone were found to locally fold the effector, this would support the hypothesis that structuring of the effector, as induced by the chaperone, may be required for interactions with bacterial components involved in TTSS transport.

Methods: Here, we use Nuclear Magnetic Resonance (NMR) Spectroscopy to compare an intact effector (YopE) in its free and chaperone-bound states, thereby determining how association with SycE affects the structure and dynamics of YopE.
Results: By using NMR spectroscopy we found that the chaperone-binding domain ofYopE is flexible and lacks secondary structure in the "free" form, but becomes well-structured upon association with SycE.

Conclusions: These data demonstrate a "disorder to order" mode of chaperone action, thereby supporting the translocation-targeting hypothesis.