Channel protein trafficking and chaperones

Cystic fibrosis (CF) is a genetic disorder caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator ( CFTR ), a selective anion channel expressed in the epithelium of various organs. The most frequent mutation is F508del. This mutation leads to a misfolded CFTR protein quickly degraded via ubiquitination in the endoplasmic reticulum. Although preventing ubiquitination stabilizes the protein, functionality is not restored due to impaired plasma membrane transport. However, inhibiting the ubiquitination process can improve the effectiveness of correctors which act as chemical chaperones, facilitating F508del CFTR trafficking to the plasma membrane...

INTRODUCTION: The voltage gated potassium ion channel K V 11.1 plays a critical role in cardiac repolarization. Genetic variants that render Kv11.1 dysfunctional cause Long QT Syndrome (LQTS), which is associated with fatal arrhythmias. Approximately 90% of LQTS-associated variants cause intracellular protein transport (trafficking) dysfunction, which can be rescued by pharmacological chaperones like E-4031. Protein folding and trafficking decisions are regulated by chaperones, protein quality control factors, and trafficking machinery, comprising the cellular proteostasis network...

The maintenance of fluid and electrolyte homeostasis by the kidney requires proper folding and trafficking of ion channels and transporters in kidney epithelia. Each of these processes requires a specific subset of a diverse class of proteins termed molecular chaperones. One such chaperone is GRP170, which is an Hsp70-like, endoplasmic reticulum (ER)-localized chaperone that plays roles in protein quality control and protein folding in the ER. We previously determined that loss of GRP170 in the mouse nephron leads to hypovolemia, electrolyte imbalance, and rapid weight loss...

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of pentameric, ligand-gated ion channels that are located on the surface of neurons and non-neuronal cells and have multiple physiological and pathophysiological functions. In order to reach the cell surface, many nAChR subtypes require the help of chaperone and/or auxiliary/accessory proteins for their assembly, trafficking, pharmacological modulation, and normal functioning in vivo. The use of powerful genome-wide cDNA screening has led to the identification and characterisation of the molecules and mechanisms that participate in the assembly and trafficking of receptor subtypes, including chaperone and auxiliary or accessory proteins...

BACKGROUND: Long QT syndrome type 2 (LQT2) is caused by mutations in the KCNH2 /human ether-à-go-go-related gene (hERG). Some hERG genetic mutation-associated diseases are alleviated by hERG-specific drug chaperones (glycerol, dimethyl sulfoxide, trimethylamine N-oxide, thapsigargin), delayed rectifier K+ current (IKr) blockers methanesulfonanilide E4031, the antihistamine astemizole, or the prokinetic drug cisapride, and the anti-arrhythmic drug quinidine. Meanwhile, many in vivo and in vitro studies have reported the efficacy of 4-phenylbutyric acid (4-PBA) in diseases with inherited genetic mutations...

Voltage-gated ion channels (VGICs) comprise multiple structural units whose assembly is required for function1,2 . There is scant structural understanding of how VGIC subunits assemble and whether chaperone proteins are required. High-voltage activated calcium channels (CaV s)3,4 are paradigmatic multi-subunit VGICs whose function and trafficking is powerfully shaped by interactions between pore-forming CaV 1 or CaV 2 CaV α1 3 and auxiliary CaV β5 , and CaV α2 δ subunits6,7 . Here, we present cryo-EM structures of human brain and cardiac CaV 1...

Mammalian UNC119 is a ciliary trafficking chaperone highly expressed in the inner segment of retinal photoreceptors. Previous research has shown that UNC119 can bind to transducin, the synaptic ribbon protein RIBEYE, and the calcium-binding protein CaBP4, suggesting that UNC119 may have a role in synaptic transmission. We made patch-clamp recordings from retinal slices in mice with the UNC119 gene deleted and showed that removal of even one gene of UNC119 has no effect on the rod outer segment photocurrent, but acted on bipolar cells much like background light: it depolarized membrane potential, decreased sensitivity, accelerated response decay, and decreased the Hill coefficient of the response-intensity relationship...

The brine shrimp (Artemia), releases embryos that can remain dormant for up to a decade. Molecular and cellular level controlling factors of dormancy in Artemia are now being recognized or applied as active controllers of dormancy (quiescence) in cancers. Most notably, the epigenetic regulation by SET domain-containing protein 4 (SETD4), is revealed as highly conserved and the primary control factor governing the maintenance of cellular dormancy from Artemia embryonic cells to cancer stem cells (CSCs). Conversely, DEK, has recently emerged as the primary factor in the control of dormancy exit/reactivation, in both cases...

Gram-negative bacteria are surrounded by two protein-rich membranes with a peptidoglycan layer sandwiched between them. Together they form the envelope (or cell wall), crucial for energy production, lipid biosynthesis, structural integrity, and for protection against physical and chemical environmental challenges. To achieve envelope biogenesis, periplasmic and outer-membrane proteins (OMPs) must be transported from the cytosol and through the inner-membrane, via the ubiquitous SecYEG protein-channel. Emergent proteins either fold in the periplasm or cross the peptidoglycan (PG) layer towards the outer-membrane for insertion through the b-barrel assembly machinery (BAM)...

The repolarizing current (Ikr ) produced by the hERG potassium channel forms a major component of the cardiac action potential and blocking this current by small molecule drugs can lead to life-threatening cardiotoxicity. Understanding the mechanisms of drug-mediated hERG inhibition is essential to develop a second generation of safe drugs, with minimal cardiotoxic effects. Although various computational tools and drug design guidelines have been developed to avoid binding of drugs to the hERG pore domain, there are many other aspects that are still open for investigation...

Potassium channels are multi-subunit transmembrane proteins that permit the selective passage of potassium and play fundamental roles in physiological processes, such as action potentials in the nervous system and organismal salt and water homeostasis, which is mediated by the kidney. Like all ion channels, newly translated potassium channels enter the endoplasmic reticulum (ER) and undergo the error-prone process of acquiring post-translational modifications, folding into their native conformations, assembling with other subunits, and trafficking through the secretory pathway to reach their final destinations, most commonly the plasma membrane...

TMEM87s are eukaryotic transmembrane proteins with two members (TMEM87A and TMEM87B) in humans. TMEM87s have proposed roles in protein transport to and from the Golgi, as mechanosensitive ion channels, and in developmental signaling. TMEM87 disruption has been implicated in cancers and developmental disorders. To better understand TMEM87 structure and function, we determined a cryo-EM structure of human TMEM87A in lipid nanodiscs. TMEM87A consists of a Golgi-dynamics (GOLD) domain atop a membrane spanning seven-transmembrane helix domain with a large cavity open to solution and the membrane outer leaflet...

Type A γ-aminobutyric acid receptors (GABAA Rs) represent a family of pentameric GABA-gated Cl- /HCO3 - ion channels which mediate inhibitory transmission in the central nervous system (CNS). Cell surface expression of GABAA Rs, a prerequisite for their function, is dependent on the appropriate assembly of the receptor subunits and their transient interactions with molecular chaperones within the endoplasmic reticulum (ER) and Golgi apparatus. Here we describe a highly conserved amino acid sequence within the extracellular N-terminal domain of the receptor subunits adjoining the first transmembrane domain (TM1) as a region important for GABAA R processing within the ER...

Subcellular organelles in eukaryotes are surrounded by lipid membranes. In an endomembrane system, vesicle trafficking is the primary mechanism for the delivery of organellar proteins to specific organelles. However, organellar proteins for chloroplasts, mitochondria, the nucleus, and peroxisomes that are translated in the cytosol are directly imported into their target organelles. Chloroplasts are a plant-specific organelle with outer and inner envelope membranes, a dual-membrane structure that is similar to mitochondria...

Myotonia congenita (MC) is an inherited rare disease characterized by impaired muscle relaxation after contraction, resulting in muscle stiffness. It is caused by loss-of-function mutations in the skeletal muscle chloride channel ClC-1, important for the stabilization of resting membrane potential and for the repolarization phase of action potentials. Thanks to in vitro functional studies, the molecular mechanisms by which ClC-1 mutations alter chloride ion influx into the cell have been in part clarified, classifying them in "gating-defective" or "expression-defective" mutations...

Gamma-aminobutyric acid type A (GABAA ) receptors are the primary inhibitory neurotransmitter-gated ion channels in the mammalian central nervous system. Maintenance of GABAA receptor protein homeostasis (proteostasis) in cells utilizing its interacting proteins is essential for the function of GABAA receptors. However, how the proteostasis network orchestrates GABAA receptor biogenesis in the endoplasmic reticulum (ER) is not well understood. Here, we employed a proteomics-based approach to systematically identify the interactomes of GABAA receptors...

Brugada syndrome (BrS) is a fatal arrhythmia that causes an estimated 4% of all sudden death in high-incidence areas. SCN5A encodes cardiac sodium channel NaV 1.5 and causes 25 to 30% of BrS cases. Here, we report generation of a knock-in (KI) mouse model of BrS ( Scn5aG1746R/+ ). Heterozygous KI mice recapitulated some of the clinical features of BrS, including an ST segment abnormality (a prominent J wave) on electrocardiograms and development of spontaneous ventricular tachyarrhythmias (VTs), seizures, and sudden death...

GRP170 is an Hsp70-like, molecular chaperone localized to the endoplasmic reticulum (ER). Two separate functions have been described for GRP170. First, GRP170 acts as a nucleotide exchange factor (co-chaperone) for the ER lumenal, Hsp70, BiP. Second, GRP170 possess "holdase" activity, and independently binds to aggregation prone regions of proteins to maintain solubility. We previously demonstrated that GRP170 regulates the quality control of the epithelial sodium channel, ENaC. ENaC is responsible for sodium reabsorption in the distal nephron and regulates salt/water homeostasis, and therefore, blood pressure...

BACKGROUND: Genetic variants in the subunits of the gamma-aminobutyric acid type A (GABAA ) receptors are implicated in the onset of multiple pathologic conditions including genetic epilepsy. Previous work showed that pathogenic GABAA subunits promote misfolding and inefficient assembly of the GABAA receptors, limiting receptor expression and activity at the plasma membrane. However, GABAA receptors containing variant subunits can retain activity, indicating that enhancing the folding, assembly, and trafficking of these variant receptors offers a potential opportunity to mitigate pathology associated with genetic epilepsy...

The stromal interaction molecule 1 (STIM1) is an endoplasmic reticulum (ER) Ca2+ sensor that regulates the activity of Orai plasma membrane Ca2+ channels to mediate the store-operated Ca2+ entry (SOCE) pathway essential for immunity. Unc-93 homologue B1 (UNC93B1) is a multiple membrane-spanning ER protein that acts as a trafficking chaperone by guiding nucleic-acid sensing toll-like receptors (TLRs) to their respective endosomal signaling compartments. We previously showed that UNC93B1 interacts with STIM1 to promote antigen cross-presentation in dendritic cells, but the STIM1 binding site(s) and activation step(s) impacted by this interaction remained unknown...