The COP II adaptor protein TMED7 is required to initiate and mediate the delivery of TLR4 to the plasma membrane

Abstract

Toll-like receptor 4 (TLR4), the receptor for the bacterial product endotoxin, is subject to multiple points of regulation at the levels of signaling, biogenesis, and trafficking. Dysregulation of TLR4 signaling can cause serious inflammatory diseases, such as sepsis. We found that the p24 family protein TMED7 (transmembrane emp24 protein transport domain containing 7) is required for the trafficking of TLR4 from the endoplasmic reticulum to the cell surface through the Golgi. TMED7 formed a stable complex with the ectodomain of TLR4, an interaction that required the coiled-coil and Golgi dynamics (GOLD) domains, but not the cytosolic, coat protein complex II (COP II) sorting motif, of TMED7. Depletion of TMED7 reduced TLR4 signaling mediated by the adaptor protein MyD88 (myeloid differentiation marker 88), but not that mediated by the adaptor proteins TRIF [Toll-interleukin-1 receptor (TIR) domain-containing adaptor protein inducing interferon-β] and TRAM (TRIF-related adaptor molecule). Truncated forms of TMED7 lacking the COP II sorting motif or the transmembrane domain were mislocalized and resulted in ligand-independent signaling that probably arises from receptors accumulated intracellularly. Together, these results support the hypothesis that p24 proteins perform a quality control step by recognizing correctly folded anterograde cargo, such as TLR4, in early secretory compartments and facilitating the translocation of this cargo to the cell surface.

Contributions of Unique Intracellular Domains to Switchlike Biosensing by Toll-like Receptor 4

“…6A). Whereas LPSinduced TRIF signaling via WT TLR4 was comparable with that in prior reports (30), TRIF signaling via all other constructs was reduced or ablated. Notably, only TLR4-H1 (whose hydrophobic ICL differs from that of WT TLR4 in sequence but not composition) retained LPS-inducible TRIF signaling.…”

“…A human TMED7 cDNA plasmid was acquired from the DNASU Plasmid Repository (pLX304-TMED7). pTMED7-Delta was generated by modifying TMED7 from pLX304-TMED7 by PCR as described (30) and inserting this construct into pcDNA 3.1(Ϫ). Primer sequences are listed in supplemental Table S1.…”

“…The TMDs of intracellular TLRs mediate receptor localization via interaction with proteins such as UNC93B1 (36), but to our knowledge, no specific TMD interaction partner has been identified as necessary for surface localization of TLR4 (13). Transmembrane Emp24 protein transport domain-containing protein 7 (TMED7) mediates transport of TLR4 from the endoplasmic reticulum to the Golgi (30,37), and transport of TLR4 from the Golgi to the cell surface involves Rab10 (38). Thus, our TMD modifications could impair one or more of these interactions.…”

“…RP105 (also called CD180) is a TLR4 homolog that is co-expressed with TLR4 and inhibits TLR4 signaling via direct physical interactions mediated by receptor ECDs (41,42). TMED7 is a transmembrane adaptor protein that has been implicated in multiple aspects of TLR4 signaling (30,43). TMED7 appears to be required for trafficking of TLR4 from the endoplasmic reticulum to the Golgi via COPII-coated vesicles, and release of TLR4 from TMED7 in the Golgi precedes trafficking of TLR4 to the cell surface (30,37).…”

“…TMED7 is a transmembrane adaptor protein that has been implicated in multiple aspects of TLR4 signaling (30,43). TMED7 appears to be required for trafficking of TLR4 from the endoplasmic reticulum to the Golgi via COPII-coated vesicles, and release of TLR4 from TMED7 in the Golgi precedes trafficking of TLR4 to the cell surface (30,37). TMED7 may also associate with TLR4 after stimulation with LPS to attenuate LPS-induced signaling (43).…”