Bag1 Co-chaperone Promotes TRC8 E3 Ligase-dependent Degradation of Misfolded Human Ether a Go-Go-related Gene (hERG) Potassium Channels

Abstract

Edited by George N. DeMartinoCardiac long QT syndrome type 2 is caused by mutations in the human ether a go-go-related gene (hERG) potassium channel, many of which cause misfolding and degradation at the endoplasmic reticulum instead of normal trafficking to the cell surface. The Hsc70/Hsp70 chaperones assist the folding of the hERG cytosolic domains. Here, we demonstrate that the Hsp70 nucleotide exchange factor Bag1 promotes hERG degradation by the ubiquitin-proteasome system at the endoplasmic reticulum to regulate hERG levels and channel activity. Dissociation of hERG complexes containing Hsp70 and the E3 ubiquitin ligase CHIP requires the interaction of Bag1 with Hsp70, but this does not involve the Bag1 ubiquitin-like domain. The interaction with Bag1 then shifts hERG degradation to the membrane-anchored E3 ligase TRC8 and its E2-conjugating enzyme Ube2g2, as determined by siRNA screening. TRC8 interacts through the transmembrane region with hERG and decreases hERG functional expression. TRC8 also mediates degradation of the misfolded hERG-G601S disease mutant, but pharmacological stabilization of the mutant structure prevents degradation. Our results identify TRC8 as a previously unknown Hsp70-independent quality control E3 ligase for hERG.

BCL2-ASSOCIATED ATHANOGENE4 Regulates the KAT1 Potassium Channel and Controls Stomatal Movement

“…Our data suggest that BAG4 interacts with KAT1 as it transits through this organelle on its way to the plasma membrane. This model is consistent with that proposed for mammalian BAG proteins that are involved in the regulation of potassium and chloride channels that also act at the ER in cooperation with Hsp70 (Knapp et al, 2014;Hantouche et al, 2017).…”

“…In mammalian systems, although BAG proteins have been related to many cellular processes, a specific role for BAG proteins in the regulation of ion channels has been reported. Both BAG1 and BAG2 have been shown to regulate the cystic fibrosis transmembrane conductance regulator chloride channel, and BAG1 was more recently implicated in human Ether-à-go-go-related gene (hERG) potassium channel regulation (Young, 2014;Hantouche et al, 2017). In both cases, it appears that the BAG proteins mediate the misfolded protein response, thus likely playing a role in channel degradation, not plasma membrane delivery.…”

Co-Chaperones in Targeting and Delivery of Misfolded Proteins to the 26S Proteasome

“…Accordingly, overlapping substrate specificity between different E3s has also been observed in yeast cells [158,187,209], and suggests that BAG-1 may collaborate with other E3s [187]. For example, BAG-1 plays a role in inherited cardiac arrhythmia as it targets misfolded variants of the hERG potassium channel for ER-associated degradation (ERAD) in an indirect and UBL-domain independent manner [192]. While HSP70 assists folding of cytosolic domains of the hERG potassium channel, interaction with BAG-1 releases hERG from HSP70, and thus prevents refolding.…”

Hsp70 molecular chaperones: multifunctional allosteric holding and unfolding machines

“…BAG1, which has a ubiquitin-like domain, is involved in the degradation of misfolded proteins [115]. For example, BAG1 was found to promote binding of the leukemia-associated BCR-ABL protein to the proteasome [116] and, more recently, the Hsp70-BAG1 complex was shown to target mutant misfolded heart-specific potassium channels for proteasomal degradation [117]. BAG2, on the other hand, prevents uncontrolled degradation of proteins by inhibiting the E3 ubiquitin ligase CHIP (the carboxy terminus of Hsc70 interacting protein), which associates with Hsp70s and labels misfolded proteins for degradation [118,119].…”