Simultaneous induction of the four subunits of the TRAP complex by ER stress accelerates ER degradation

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Abstract
The mammalian translocon‐associated protein (TRAP) complex comprises four transmembrane protein subunits in the endoplasmic reticulum. The complex associates with the Sec61 translocon, although its function in vivo remains unknown. Here, we show the involvement of the TRAP complex in endoplasmic reticulum‐associated degradation (ERAD). All four subunits are induced simultaneously by endoplasmic reticulum stresses from the X‐box‐binding protein 1/inositol‐requiring 1α pathway. RNA interference knockdown of each subunit causes disruption of the native complex and significant delay in the degradation of various ERAD substrates, including the α1‐antitrypsin null Hong Kong variant (NHK). In a pulse‐chase experiment, the TRAP complex associated with NHK at a late stage, indicating its involvement in the ERAD pathway rather than in biosynthesis of nascent polypeptides in the endoplasmic reticulum. In addition, the TRAP complex bound preferentially to misfolded proteins rather than correctly folded wild‐type substrates. Thus, the TRAP complex induced by the unfolded protein response pathway might discriminate ERAD substrates from correctly folded substrates, accelerating degradation.