Mechanical gating of the auditory transduction channel TMC1 involves the fourth and sixth transmembrane helices

The transmembrane channel-like (TMC) 1 and 2 proteins play a central role in auditory transduction, forming ion channels that convert sound into electrical signals. However, the molecular mechanism of their gating remains unknown. Here, using predicted structural models as a guide, we probed the effects of twelve mutations on the mechanical gating of the transduction currents in native hair cells of Tmc1/2-null mice expressing virally introduced TMC1 variants. Whole-cell electrophysiological recordings revealed that mutations within the pore-lining transmembrane (TM) helices 4 and 6 modified gating, reducing the force sensitivity or shifting the open probability of the channels, or both. For some of the mutants, these changes were accompanied by a change in single-channel conductance. Our observations are in line with a model wherein conformational changes in the TM4 and TM6 helices are involved in the mechanical gating of the transduction channel.

Functionalised Nanopores: Chemical and Biological Modifications

Nanopore technology has established itself as a powerful tool for single-molecule studies. By analysing changes in the ion current flowing through a single transmembrane channel, a wealth of molecular information...

Predicted transmembrane proteins with homology to Mef(A) are not responsible for complementing mef(A) deletion in the mef(A)–msr(D) macrolide efflux system in Streptococcus pneumoniae

Objectives In streptococci, the type M resistance to macrolides is due to the mef(A)–msr(D) efflux transport system of the ATP-Binding cassette (ABC) superfamily, where it is proposed that mef(A) codes for the transmembrane channel and msr(D) for the two ATP-binding domains. Phage ϕ1207.3 of Streptococcus pyogenes, carrying the mef(A)–msr(D) gene pair, is able to transfer the macrolide efflux phenotype to Streptococcus pneumoniae. Deletion of mef(A) in pneumococcal ϕ1207.3-carrying strains did not affect erythromycin efflux. In order to identify candidate genes likely involved in complementation of mef(A) deletion, the Mef(A) amino acid sequence was used as probe for database searching. Results In silico analysis identified 3 putative candidates in the S. pneumoniae R6 genome, namely spr0971, spr1023 and spr1932. Isogenic deletion mutants of each candidate gene were constructed and used in erythromycin sensitivity assays to investigate their contribution to mef(A) complementation. Since no change in erythromycin sensitivity was observed compared to the parental strain, we produced double and triple mutants to assess the potential synergic activity of the selected genes. Also these mutants did not complement the mef(A) function.

A non-transmembrane channel formed by Ca2+-bound calsequestrin-2

Although it is well known that ion channels conduct ions across biomembranes, whether ions are conducted by some non-membrane proteins is not known because of the lack of a detection method. Calsequestrin-2 (CSQ2) is a sarcoplasmic reticulum (SR) Ca2+-binding protein suppling Ca2+ for the ryanodine receptor Ca2+ release during the excitation–contraction coupling in cardiomyocytes. CSQ2 mutations, even in some heterozygous occasions, causes catecholaminergic polymorphic ventricular tachycardia (CPVT2), suggesting that CSQ2 may function beyond a Ca2+ buffer. Here, we identify a non-transmembrane channel in Ca2+-enriched CSQ2 dimers, which facilitates fast Ca2+ mobilization. Using crystallography, we solved the high-resolution structure of Ca2+-bound CSQ2 and discovered that the negatively charged residues at the dimer interface encompassed a tubular channel-like structure, dubbed “tunnel,” in which ∼15 Ca2+ ions aligned across the ∼5 nm tunnel path. To determine the potential tunnel conductance, we developed a graphene-based nanoelectronic technology to connect a CSQ2 dimer into a nanocircuit. In the Tyrode solution containing 1 mM Ca2+, a CSQ2 dimer exhibited a conductance one order of magnitude higher than the background level. This conductance was Ca2+ dependent, and was largely suppressed by the single-residue mutation D309N at the bottleneck region of the tunnel path, indicating that the tunnel conducted Ca2+ fluxes. When the D309N mutant CSQ2 was expressed in wild-type rat cardiomyocytes by adenoviral vectors, isoproterenol treatment induced chaotic Ca2+ waves, delayed after-depolarizations and trigged activities resembling those occurring in CPVT2 models. This dominant negative effect of CSQ2 mutation agreed well with our structural observation that CSQ2 tunnels were interconnected to form a tunnel network. Taken together, these results revealed that CSQ2 builds a nano-highway network for energy-efficient Ca2+ mobilization in the SR. Factors that block the Ca2+ highway may lead to arrhythmogenesis.

Pan-Cancer Analysis Reveals the Signature of TMC Family of Genes as a Promising Biomarker for Prognosis and Immunotherapeutic Response

Transmembrane Channel-like (TMC) genes are critical in the carcinogenesis, proliferation, and cell cycle of human cancers. However, the multi-omics features of TMCs and their role in the prognosis and immunotherapeutic response of human cancer have not been explored. We discovered that TMCs 4-8 were commonly deregulated and correlated with patient survival in a variety of cancers. For example, TMC5 and TMC8 were correlated with the relapse and overall survival rates of breast cancer and skin melanoma, respectively. These results were validated by multiple independent cohorts. TMCs were regulated by DNA methylation and somatic alterations, such as TMC5 amplification in breast cancer (523/1062, 49.2%). Six algorithms concordantly uncovered the critical role of TMCs in the tumor microenvironment, potentially regulating immune cell toxicity and lymphocytes infiltration. Moreover, TMCs 4-8 were correlated with tumor mutation burden and expression of PD-1/PD-L1/CTLA4 in 33 cancers. Thus, we established an immunotherapy response prediction (IRP) score based on the signature of TMCs 4-8. Patients with higher IRP scores showed higher immunotherapeutic responses in five cohorts of skin melanoma (area under curve [AUC] = 0.90 in the training cohort, AUCs range from 0.70 to 0.83 in the validation cohorts). Together, our study highlights the great potential of TMCs as biomarkers for prognosis and immunotherapeutic response, which can pave the way for further investigation of the tumor-infiltrating mechanisms and therapeutic potentials of TMCs in cancer.

Sensory transduction is required for normal development and maturation ofcochlear inner hair cell synapses

Acoustic overexposure and aging can damage auditory synapses in the inner ear by a process known as synaptopathy. These insults may also damage hair bundles and the sensory transduction apparatus in auditory hair cells. However, a connection between sensory transduction and synaptopathy has not been established. To evaluate potential contributions of sensory transduction to synapse formation and development, we assessed inner hair cell synapses in several genetic models of dysfunctional sensory transduction, including mice lacking Transmembrane Channel-like (Tmc) 1, Tmc2 or both, in Beethoven mice which carry a dominant Tmc1 mutation and in Spinner mice which carry a recessive mutation in Transmembrane inner ear (Tmie). Our analyses reveal loss of synapses in the absence of sensory transduction and preservation of synapses in Tmc1-null mice following restoration of sensory transduction via Tmc1 gene therapy. These results provide insight into the requirement of sensory transduction for hair cell synapse development and maturation.

Transmembrane channel-like 4 is involved in pH and temperature-dependent modulation of salty taste

Human susceptibility to NaCl varies depending on temperature and pH, the molecular mechanisms of which remain unclear. The voltage-dependent chloride channel, transmembrane channel-like 4 (TMC4), is activated at approximately 40 °C and is suppressed at pH 5.5. As these are similar in character to human sensory evaluations, hTMC4 may be involved in human salt taste reception.

TMC4 is a novel chloride channel involved in high-concentration salt taste sensation

Abstract“Salty taste” sensation is evoked when sodium and chloride ions are present together in the oral cavity. The presence of an epithelial cation channel that receives Na+ has previously been reported. However, no molecular entity involving Cl− receptors has been elucidated. We report the strong expression of transmembrane channel-like 4 (TMC4) in the circumvallate and foliate papillae projected to the glossopharyngeal nerve, mediating a high-concentration of NaCl. Electrophysiological analysis using HEK293T cells revealed that TMC4 was a voltage-dependent Cl− channel and the consequent currents were completely inhibited by NPPB, an anion channel blocker. TMC4 allowed permeation of organic anions including gluconate, but their current amplitudes at positive potentials were less than that of Cl−. Tmc4-deficient mice showed significantly weaker glossopharyngeal nerve response to high-concentration of NaCl than the wild-type littermates. These results indicated that TMC4 is a novel chloride channel that responds to high-concentration of NaCl.

The structural basis of bacterial manganese import

Metal ions are essential for all forms of life. In prokaryotes, ATP-binding cassette (ABC) permeases serve as the primary import pathway for many micronutrients including the first-row transition metal manganese. However, the structural features of ionic metal transporting ABC permeases have remained undefined. Here, we present the crystal structure of the manganese transporter PsaBC from Streptococcus pneumoniae in an open-inward conformation. The type II transporter has a tightly closed transmembrane channel due to “extracellular gating” residues that prevent water permeation or ion reflux. Below these residues, the channel contains a hitherto unreported metal coordination site, which is essential for manganese translocation. Mutagenesis of the extracellular gate perturbs manganese uptake, while coordination site mutagenesis abolishes import. These structural features are highly conserved in metal-specific ABC transporters and are represented throughout the kingdoms of life. Collectively, our results define the structure of PsaBC and reveal the features required for divalent cation transport.