HERG Trafficking is Dependent on a Cytosolic Chaperone Network

Abstract

The KCNH2 or human ether-a-go-go related gene (hERG1) encodes the Kv11.1 α-subunit of the hERG potassium channel that underlies the rapidly activating delayed rectifier current IKr. In the heart, mutations in KCNH2 cause a reduction in IKr resulting in the proarrhythmic type 2 long-QT syndrome (LQT2). While multiple factors can cause the loss of the functional phenotype, the dominant mechanism is a trafficking deficiency due to abnormalities in protein folding that result in endoplasmic reticulum (ER) retention. To identify chaperones or co-chaperones potentially involved in the folding and ER retention of hERG we performed a proteomics analysis to identify hERG-interacting proteins. In addition to Hsc70 and Hsp90, key members of the cytosolic chaperone system, we found the co-chaperones Dja1, Bag2, Hop, Tpr2, FKBP38, and the lumenal chaperone calnexin. We recently reported on the putative ER-resident hERG chaperone FKBP38. FKBP38 co-precipitates with hERG both in vivo and in vitro. Functionally, siRNA knockdown of FKBP38 reduces wild type (WT) hERG trafficking. In vitro experiments have also confirmed that translated hERG C-terminus (CT) and cyclic nucleotide binding domain (CNBD) bind to Hsc70 and Hsp90 as well as Dja1 and related family members Dja2 and Dja4. In vivo results indicate that overexpression of the Dja proteins result in differential reduction of hERG trafficking. This reduction in hERG trafficking appears to be related to the proteasomal degradation system as inhibiting the proteasome with lactacystin diminishes this effect and re-establishes hERG trafficking to control levels. Another protein involved in degradation, Chip, was also tested and preliminary work indicates that its overexpression leads to decreased hERG expression. Taken together, these data allow us to outline a model of chaperone-mediated hERG maturation and quality control.