Ca2+ Sensitivity of Anoctamin 6/TMEM16F Is Regulated by the Putative Ca2+-Binding Reservoir at the N-Terminal Domain

Anoctamin 6/TMEM16F (ANO6) is a dual-function protein with Ca2+-activated ion channel and Ca2+-activated phospholipid scramblase activities, requiring a high intracellular Ca2+ concentration (e.g., half-maximal effective Ca2+ concentration [EC50] of [Ca2+]i > 10 μM), and strong and sustained depolarization above 0 mV. Structural comparison with Anoctamin 1/TMEM16A (ANO1), a canonical Ca2+- activated chloride channel exhibiting higher Ca2+ sensitivity (EC50 of 1 μM) than ANO6, suggested that a homologous Ca2+-transferring site in the N-terminal domain (Nt) might be responsible for the differential Ca2+ sensitivity and kinetics of activation between ANO6 and ANO1. To elucidate the role of the putative Ca2+-transferring reservoir in the Nt (Nt-CaRes), we constructed an ANO6-1-6 chimera in which Nt-CaRes was replaced with the corresponding domain of ANO1. ANO6- 1-6 showed higher sensitivity to Ca2+ than ANO6. However, neither the speed of activation nor the voltage-dependence differed between ANO6 and ANO6-1-6. Molecular dynamics simulation revealed a reduced Ca2+ interaction with Nt- CaRes in ANO6 than ANO6-1-6. Moreover, mutations on potentially Ca2+-interacting acidic amino acids in ANO6 Nt- CaRes resulted in reduced Ca2+ sensitivity, implying direct interactions of Ca2+ with these residues. Based on these results, we cautiously suggest that the net charge of Nt- CaRes is responsible for the difference in Ca2+ sensitivity between ANO1 and ANO6.

Understanding the mammalian TRAP complex function(s)

In eukaryotic cells, about one-third of the synthesized proteins are translocated into the endoplasmic reticulum; they are membrane or lumen resident proteins and proteins direct to the Golgi apparatus. The co-translational translocation takes place through the heterotrimeric protein-conducting channel Sec61 which is associated with the ribosome and many accessory components, such as the heterotetrameric translocon-associated protein (TRAP) complex. Recently, microscopic techniques, such as cryo-electron microscopy and cryo-electron tomography, have enabled the determination of the translocation machinery structure. However, at present, there is a lack of understanding regarding the roles of some of its components; indeed, the TRAP complex function during co-translational translocation needs to be established. In addition, TRAP may play a role during unfolded protein response, endoplasmic-reticulum-associated protein degradation and congenital disorder of glycosylation (ssr4 CDG). In this article, I describe the current understanding of the TRAP complex in the light of its possible function(s).

Spectrins: a structural platform for stabilization and activation of membrane channels, receptors and transporters

This review focuses on structure and functions of spectrin as a major component of the membrane skeleton. Recent advances on spectrin function as an interface for signal transduction mediation and a number of data concerning interaction of spectrin with membrane channels, adhesion molecules, receptors and transporters draw a picture of multifaceted protein. Here, we attempted to show the current depiction of multitask role of spectrin in cell physiology. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Mapping the functional domains of TCblR/CD320, the receptor for cellular uptake of transcobalamin-bound cobalamin

The membrane receptor TCblR/CD320 binds transcobalamin (TC) saturated with vitamin B12 [cobalamin (Cbl)] and mediates cellular uptake of the vitamin. The specificity of TC for Cbl and of the receptor for TC-Cbl ensures efficient uptake of Cbl into cells. The high-affinity interaction of TCblR with TC-Cbl (Ka=10 nM(-1)) was investigated using deletions and mutations of amino acid sequences in TCblR. Only the extracellular region (aa 32-229) is needed for TC-Cbl binding, but the N-glycosylation sites (N126, N195, and N213) are of no importance for this function. Deleting the cysteine-rich region (aa 95-141) that separates the two low-density lipoprotein receptor type A (LDLR-A) domains does not affect TC-Cbl binding (Ka = 19-24 nM(-1)). The two LDLR-A domains (aa 54-89 and 132-167) with the negatively charged acidic residues involved in Ca(2+) binding are critical determinants of ligand binding. The cytoplasmic tail is apparently crucial for internalization of the ligand. Within this region, the RPLGLL motif and the PDZ binding motifs (QERL/KESL) appear to be involved in initiating and completing the process of ligand internalization. Mutations and deletions of these regions involved in binding and internalization of TC-Cbl are likely to produce the biochemical and clinical phenotype of Cbl deficiency.

C-terminal acidic cluster is involved in Ca2+-induced regulation of human transient receptor potential ankyrin 1 channel

The transient receptor potential ankyrin 1 (TRPA1) channel is a Ca(2+)-permeable cation channel whose activation results from a complex synergy between distinct activation sites, one of which is especially important for determining its sensitivity to chemical, voltage and cold stimuli. From the cytoplasmic side, TRPA1 is critically regulated by Ca(2+) ions, and this mechanism represents a self-modulating feedback loop that first augments and then inhibits the initial activation. We investigated the contribution of the cluster of acidic residues in the distal C terminus of TRPA1 in these processes using mutagenesis, whole cell electrophysiology, and molecular dynamics simulations and found that the neutralization of four conserved residues, namely Glu(1077) and Asp(1080)-Asp(1082) in human TRPA1, had strong effects on the Ca(2+)- and voltage-dependent potentiation and/or inactivation of agonist-induced responses. The surprising finding was that truncation of the C terminus by only 20 residues selectively slowed down the Ca(2+)-dependent inactivation 2.9-fold without affecting other functional parameters. Our findings identify the conserved acidic motif in the C terminus that is actively involved in TRPA1 regulation by Ca(2+).