The discovery of SycO highlights a new function for type III secretion effector chaperones

Abstract

Bacterial injectisomes deliver effector proteins straight into the cytosol of eukaryotic cells (type III secretion, T3S). Many effectors are associated with a specific chaperone that remains inside the bacterium when the effector is delivered. The structure of such chaperones and the way they interact with their substrate is well characterized but their main function remains elusive. Here, we describe and characterize SycO, a new chaperone for the Yersinia effector kinase YopO. The chaperone-binding domain (CBD) within YopO coincides with the membrane localization domain (MLD) targeting YopO to the host cell membrane. The CBD/MLD causes intrabacterial YopO insolubility and the binding of SycO prevents this insolubility but not folding and activity of the kinase. Similarly, SycE masks the MLD of YopE and SycT covers an aggregation-prone domain of YopT, presumably corresponding to its MLD. Thus, SycO, SycE and most likely SycT mask, inside the bacterium, a domain needed for proper localization of their cognate effector in the host cell. We propose that covering an MLD might be an essential function of T3S effector chaperones.

Secretion and translocation signals and DspB/F-binding domains in the type III effector DspA/E of Erwinia amylovora

“…However, this CBDdetermined specificity does not exist in all bacterial systems. For example, in Yersinia spp., in the absence of the CBD, the N-terminal secretion signal is sufficient for secretion through the T3SS (Letzelter et al, 2006). Thus, although the CBD and its cognate chaperone can both exert considerable influence over whether a protein is secreted and, presumably, translocated into host cells to facilitate disease, the relative influence of the two signals can differ among species of bacteria.…”

Sequestering of Rac by the Yersinia Effector YopO Blocks Fcγ Receptor-mediated Phagocytosis

“…The YopO N-terminal region has a dual function as a chaperone-binding site within bacteria and, once translocated, is responsible for plasma membrane localization in host cells (18). As yet, the mechanism by which it mediates this localization is unknown; in particular, it does not harbor any obvious post-translational modification sites, e.g.…”

“…Expression Vectors-Eukaryotic plasmids encoding wild-type YopO (pEGFP::yopO, encoding YopOwt) and N-terminal truncation mutants (pEGFP::yopO⌬20 -77, encoding YopO ⌬20 -77) were described previously (18). pEGFP::yopO435-729 (encoding YopORBD) was constructed from pEGFP::yopO using Expand Long Template PCR system (Roche Applied Science) and primers 5Ј-GGGGGGCTCGAGAGGCGGATAAC-ACCCAAG-3Ј and 5Ј-GGGGGGGAATTCTTATCACCATT-CCCGCTCCAACCGGTTCAG-3Ј and cloned into pEGFP-C3 (Clontech).…”

“…YopO localizes to the plasma membrane of transfected or infected cells through its N-terminal region (17,18). To test the hypothesis that YopO localization at the plasma membrane is required for anti-phagocytic activity, a GFP-tagged mutant of YopO (YopO⌬20 -77) lacking the region required for plasma membrane localization was used.…”

“…The N-terminal region of YopO functions as the chaperone-binding site prior to translocation through the T3SS and mediates plasma membrane localization after injection into host cells (17,18). It is flanked by a serine/threonine kinase domain recently shown to phosphorylate the heterotrimeric G protein subunit G␣ q (19).…”