IQGAP1 Is Important for Activation of Caspase-1 in Macrophages and Is Targeted by Yersinia pestis Type III Effector YopM

Abstract

YopM is a leucine-rich repeat (LRR)-containing effector in several Yersinia species, including Yersinia pestis and Y. pseudotuberculosis. Different Yersinia strains encode distinct YopM isoforms with variable numbers of LRRs but conserved C-terminal tails. A 15-LRR isoform in Y. pseudotuberculosis YPIII was recently shown to bind and inhibit caspase-1 via a YLTD motif in LRR 10, and attenuation of YopM− YPIII was reversed in mice lacking caspase-1, indicating that caspase-1 inhibition is a major virulence function of YopMYPIII. To determine if other YopM proteins inhibit caspase-1, we utilized Y. pseudotuberculosis strains natively expressing a 21-LRR isoform lacking the YLTD motif (YopM32777) or ectopically expressing a Y. pestis 15-LRR version with a functional (YopMKIM) or inactivated (YopMKIM D271A) YLTD motif. Results of mouse and macrophage infections with these strains showed that YopM32777, YopMKIM, and YopMKIM D271A inhibit caspase-1 activation, indicating that the YLTD motif is dispensable for this activity. Analysis of YopMKIM deletion variants revealed that LRRs 6 to 15 and the C-terminal tail are required to inhibit caspase-1 activation. YopM32777, YopMKIM, and YopMKIM deletion variants were purified, and binding partners in macrophage lysates were identified. Caspase-1 bound to YopMKIM but not YopM32777. Additionally, YopMKIM bound IQGAP1 and the use of Iqgap1−/− macrophages revealed that this scaffolding protein is important for caspase-1 activation upon infection with YopM− Y. pseudotuberculosis. Thus, while multiple YopM isoforms inhibit caspase-1 activation, their variable LRR domains bind different host proteins to perform this function and the LRRs of YopMKIM target IQGAP1, a novel regulator of caspase-1, in macrophages.

Manipulation of Interleukin-1β and Interleukin-18 Production by Yersinia pestis Effectors YopJ and YopM and Redundant Impact on Virulence

“…Although it is unclear which mechanisms YopM uses for inhibition of inflammasome activation, direct interaction with caspase-1 and the scaffold protein Iqgap1 have been proposed (15,16). Interactions between YopM and kinases such as RSK1/2 and PKN1/2 (55-57), known for binding to the effector, may also play a role in caspase-1 regulation.…”

“…It was recently demonstrated that YopM inhibits caspase-1 (15,16). Caspase-1 mediates maturation of IL-1␤ and IL-18, cytokines that promote a robust early immune response that enhances host resistance against Y. pestis infection (17,18).…”

Guanylate Binding Proteins Regulate Inflammasome Activation in Response to Hyperinjected Yersinia Translocon Components

“…The activity of the T3SS itself, as well as the activity of translocated effectors, triggers inflammasome activation, which promotes Yersinia clearance in vivo (36,80). However, Yersinia also inhibits inflammasome activation through the activities of YopK and YopM (36,38,39). Recent studies have revealed that YopJ can cooperate with YopM in activated macrophages to limit inflammasome activation (78,81), although YopJ blockade of NF-B and MAPK signaling triggers the activation of caspase-8 and caspase-1 in naive macrophages (82,83).…”

“…YopM interferes with YopE-and YopT-mediated activation of the pyrin inflammasome by binding to protein kinase C-related kinases (PRKs) and promoting PRK-dependent phosphorylation and inactivation of pyrin (37). Interestingly, Yersinia pseudotuberculosis and Yersinia pestis YopK mutants are unable to prevent inflammasome activation despite the presence of YopM, indicating that YopM and YopK block the inflammasome via distinct, nonredundant mechanisms (36,38,40).…”

“…Pathogenic Yersinia bacteria inject Yop proteins that modulate a number of fundamental cellular processes, which collectively promote Yersinia virulence. Notably, two Yersinia proteins, YopM and YopK, prevent inflammasome activation in Yersinia-infected cells (36)(37)(38)(39). YopM interferes with YopE-and YopT-mediated activation of the pyrin inflammasome by binding to protein kinase C-related kinases (PRKs) and promoting PRK-dependent phosphorylation and inactivation of pyrin (37).…”