Depletion of the 26S proteasome adaptor Ecm29 increases Toll-like receptor 3 signaling.

Toll-like receptor 3 (TLR3) recognizes viral double-stranded RNA and stimulates the innate immune response. We found that depletion of extracellular mutant 29 (Ecm29), an adaptor protein that binds to a subset of 26S proteasomes (Ecm proteasomes), increased the abundance of TLR3 in human embryonic kidney-293 and HeLa cells. Loss of Ecm29 also increased the amounts of LC3 beta and p62, two proteins that mediate autophagy. The absence of Ecm29 enhanced TLR3 signaling, which was characterized by the increased abundance of the adaptor protein and E3 ubiquitin ligase tumor necrosis factor receptor-associated factor 3, increased phosphorylation and activation of effector kinases downstream of TLR3, increased nuclear localization of the transcription factor interferon regulatory factor 3, and the accumulation of signaling molecules at juxtanuclear recycling endosomes. We conclude that Ecm proteasomes play a previously uncharacterized role in mediating autophagy, trafficking of TLR3, and attenuation of TLR3-dependent signaling.