The overall goals are to identify and understand the function of molecular chaperones essential for productive folding, retention and degradation of hERG potassium channels during biosynthesis in the endoplasmic reticulum. The hERG gene encodes the rapid component of the cardiac delayed rectifier current IKr that is crucial for cardiac repolarization and critical to the normal duration and propagation of the cardiac action potential. Mutations in hERG produce functionally impaired or trafficking-deficient channels that reduce IKr current and are linked to hereditary long QT syndrome type2 in which delayed repolarization is associated with torsade de pointes and sudden cardiac death in young people.
The specific aims of this study are to: (1) identify the molecular components of the multi-chaperone folding machinery associated with hERG wildtype channels during synthesis, assembly and maturation in the endoplasmic reticulum (ER), (2) probe remodeling of the multi-chaperone folding machinery associated with misprocessed LQT2 hERG mutations retained in the ER, (3) study the relationship of hERG-chaperone complexes with the ubiquitin/proteasome system and determine how triage decisions towards protein degradation are made, and (4) validate in native cardiomyocytes the physiological role for hERG chaperones and components of the ubiquitin/proteasome system identified in heterologous expression systems. An important outcome of this study will be the identification of novel molecular targets that can be exploited to restore trafficking of misfolded LQT2 mutants or increase the folding propensity of wildtype hERG channels to stabilize impaired cardiac action potentials. The research uses pulse-chase labeling, immunoprecipitation, autoradiography and immunoblotting to isolate and characterize the multi-chaperone machinery associated with hERG potassium channels. Mass spectrometry is used to identify novel protein components of the cellular chaperone machinery as well as of the proteasomal degradation machinery associated with hERG potassium channels. Patch-clamp electrophysiology, mutagenesis and adenoviral gene transfer of dominant-negative chaperone constructs are used to manipulate chaperone expression in heterologous expression systems as well as in native cardiomyocytes.
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