Proteomic Profiling of the TRAF3 Interactome Network Reveals a New Role for the ER-to-Golgi Transport Compartments in Innate Immunity

van Zuylen, Wendy J.; Doyon, Priscilla; Clément, Jean-François; Khan, Kashif Aziz; D'Ambrosio, Lisa M.; Dô, Florence; St-Amant-Verret, Myriam; Wissanji, Tasheen; Emery, Gregory; Gingras, Anne-Claude; Meloche, Sylvain; Servant, Marc J.

In addition to its role in the RNA sensing pathways, STING is now considered an important effector of innate immune signalling in response to DNA pathogens [60] . Interestingly, STING is an ER-resident protein, which in response to dsDNA treatment, was recently demonstrated to traffic from the ER to the Golgi [61, 62] giving rise to punctate structure formation [62] . It is likely that the use of dsDNA (polydA:dT) used in our study might activate both the RNA-dependent pathway (through RNA polymerase III [63] ) and the recently described DNA-dependent pathway (through IFI16 [2] ), allowing TRAF3-loaded punctae to interact with both MAVS and STING respectively for proper innate immune signalling ( Figure 11 ). Even though this needs to be investigated further, we speculate that the membranous network composed of the ER, Golgi and mitochondria provides a convenient platform on which antiviral cell-signalling complexes are arranged and optimally activated. It is noteworthy that, as a common feature, plus-stranded RNA viruses have the ability to induce cytoplasmic membrane rearrangements that facilitate their replication. Consequently, the formation of these RNA replication complexes results in dramatic reorganization of the secretory pathway of host cells [64] . For example, poliovirus-infected cells accumulate membranous vesicles derived from COPII vesicles [65] whereas Kunjin virus induces ''convoluted membranes'' that contain markers from the ERGIC [66] . The precise role for this internal membrane rearrangement in the virus propagation and virus-host interaction requires further investigation. Nevertheless, localization of TRAF3 and TRADD to these vesicular transport compartments could represent a cellular strategy to increase the rate of RNA detection and the formation of an effective signalling complex at the mitochondrial membrane. The observation that TRADD translocates from the cytoplasm to the mitochondria during Influenza A virus infection supports this model [67] . Additionally, recent observations highlight the fact that viruses have evolved a variety of mechanisms involving the Golgi apparatus to specifically block TRAF3 recruitment into a functional signalling complex. Notably, the SARS Coronavirus M protein, a Golgi localized protein, was recently found to impede the formation of a TRAF3-TANK-TBK1/IKKi complex at the Golgi apparatus [68] . The NY-1 strain Hantavirus glycoprotein (Gn) was also shown to disrupt TRAF3-TBK1 interaction by interacting with TRAF3 through its cytoplasmic tail [69] .