Dynamic Simulations of Interaction of the PEG-DPPE Micelle-Encapsulated Short-Chain Ceramides with the Raft-Included Membrane.

The lipid raft subdomains in cancer cell membranes play a key role in signal transduction, biomolecule recruitment, and drug transmembrane transport. Augmented membrane rigidity due to the formation of a lipid raft is unfavorable for the entry of drugs, a limiting factor in clinical oncology. The short-chain ceramide (CER) has been reported to promote drug entry into membranes and disrupt lipid raft formation, but the underlying mechanism is not well understood. We recently explored the carrier-membrane fusion dynamics of PEG-DPPE micelles in delivering doxorubicin (DOX). Based on the phase-segregated membrane model composed of DPPC/DIPC/CHOL/GM1/PIP2, we aim to explore the dynamic mechanism of the PEG-DPPE micelle-encapsulating DOXs in association with the raft-included cell membrane modulated by C8 acyl tail CERs. The results show that the lipid raft remains integrated and DOX-resistant subjected to free DOXs and the micelle-encapsulating ones. Addition of CERs disorganizes the lipid raft by pushing CHOL aside from DPPC. It subsequently allows for a good permeability for PEG-DPPE micelle-encapsulated DOXs, which penetrate deeper as CER concentration increases. GM1 is significant in guiding drugs' redistributing between bilayer phases, and the anionic PIP2 further helps DOXs attain the inner bilayer surface. These results elaborate on the perturbing effect of CERs on lipid raft stability, which provides a new comprehensive approach for further design of drug delivery systems.

Molecular basis of PIP2-dependent conformational switching of phosphorylated CD44 in binding FERM.

Association of the cellular adhesive protein CD44 and the N-terminal (FERM) domain of cytoskeleton adaptors is critical for cell proliferation, migration, and signaling. Phosphorylation of the cytoplasmic domain (CTD) of CD44 acts as an important regulator of the protein association, but the structural transformation and dynamics mechanism remain enigmatic. In this study, extensive coarse-grained simulations were employed to explore the molecular details in the formation of CD44-FERM complex under S291 and S325 phosphorylation, a modification path known to exert reciprocal effects on the protein association. We find that phosphorylation of S291 inhibits complexation by causing the CTD of CD44 to adopt a more closed structure. In contrast, S325 phosphorylation liberates the CD44-CTD from the membrane surface and promotes the linkage with FERM. The phosphorylation-driven transformation is found to occur in a PIP2-dependent manner, with PIP2 effecting the relative stability of the closed and open conformation, and a replacement of PIP2 by POPS greatly abrogates this effect. The revealed interdependent regulation mechanism by phosphorylation and PIP2 in the association of CD44 and FERM further strengthens our understanding of the molecular basis of cellular signaling and migration.

Zafirlukast inhibits the growth of lung adenocarcinoma via inhibiting TMEM16A channel activity.

Lung cancer has the highest mortality among cancers worldwide due to its high incidence and lack of the effective cures. We have previously demonstrated that the membrane ion channel TMEM16A is a potential drug target for the treatment of lung adenocarcinoma and have identified a pocket of inhibitor binding that provides the basis for screening promising new inhibitors. However, conventional drug discovery strategies are lengthy and costly, and the unpredictable side effects lead to a high failure rate in drug development. Therefore, finding new therapeutic directions for already marketed drugs may be a feasible strategy to obtain safe and effective therapeutic drugs. Here, we screened a library of over 1400 Food and Drug Administration-approved drugs through virtual screening and activity testing. We identified a drug candidate, Zafirlukast (ZAF), clinically approved for the treatment of asthma, that could inhibit the TMEM16A channel in a concentration-dependent manner. Molecular dynamics simulations and site-directed mutagenesis experiments showed that ZAF can bind to S387/N533/R535 in the nonselective inhibitor binding pocket, thereby blocking the channel pore. Furthermore, we demonstrate ZAF can target TMEM16A channel to inhibit the proliferation and migration of lung adenocarcinoma LA795 cells. In vivo experiments showed that ZAF can significantly inhibit lung adenocarcinoma tumor growth in mice. Taken together, we identified ZAF as a novel TMEM16A channel inhibitor with excellent anticancer activity, and as such, it represents a promising candidate for future preclinical and clinical studies.

Self-Association of ACE-2 with Different RBD Amounts: A Dynamic Simulation Perspective on SARS-CoV-2 Infection.

Transmissibility of SARS-CoV-2 initially relies on its trimeric Spike-RBDs to tether the ACE-2 on host cells, and enhanced self-association of ACE-2 engaged with Spike facilitates the viral infection. Two primary packing modes of Spike-ACE2 heteroproteins exist potentially due to discrepant amounts of RBDs loading on ACE-2, but the resultant self-association difference is inherently unclear. We used extensive coarse-grained dynamic simulations to characterize the self-association efficiency, the conformation relevance, and the molecular mechanism of ACE-2 with different RBD amounts. It was revealed that the ACE-2 hanging two/full RBDs (Mode-A) rapidly dimerized into the heteroprotein complex in a compact "linear" conformation, while the bare ACE-2 showed weakened self-association and a protein complex. The RBD-tethered ectodomains of ACE-2 presented a more upright conformation relative to the membrane, and the intermolecular ectodomains were predominantly packed by the neck domains, which was obligated to the rapid protein self-association in a compact pattern. Noted is the fact that the ACE-2 tethered by a single RBD (Mode-B) retained considerable self-association efficiency and clustering capability, which unravels the interrelation of ACE-2 colocalization and protein cross-linkage. The molecular perspectives in this study expound the self-association potency of ACE-2 with different RBD amounts and the viral activity implications, which can greatly enhance our comprehension of SARS-CoV-2 infection details.

Correction: Delivery mechanism of doxorubicin by PEG-DPPE micelles on membrane invasion by dynamic simulations.

Correction for 'Delivery mechanism of doxorubicin by PEG-DPPE micelles on membrane invasion by dynamic simulations' by Lina Zhao et al., Phys. Chem. Chem. Phys., 2023, 25, 16114-16125, https://doi.org/10.1039/D2CP05946K.

Delivery mechanism of doxorubicin by PEG-DPPE micelles on membrane invasion by dynamic simulations.

Exploiting micelles of polyethylene glycol-dipalmitoylglycerophosphoethanolamine (PEG-DPPE) as a drug delivery approach is of great promise for improving therapeutic targeting and the half-lives of drugs. To optimize the micelle carriers, pending issues concerning the kinetics underlying the carrier-membrane interplay and the specific contributions of the micelle hydrophobic/hydrophilic components remain to be addressed. Relying on MARTINI coarse-grain (CG) molecular dynamics simulations, we explored the carrier-membrane fusion dynamics of PEG-DPPE micelles with different PEG repetitions in delivering doxorubicin (DOX). A bilayer model composed of 20% phosphatidylglycerol (POPG) and 80% phosphatidylcholine (POPC) was constructed to mimic anionic cancer cell membranes. The CG model of DOX was pioneeringly constructed herein, and it was found to distribute at the hydrophilic/hydrophobic interface of the PEGylated micelles, in agreement with experimental results. The free DOXs cause insignificant disorder of the membrane organization, whereas the PEG-DPPE micelles encapsulating DOX lead to a remarkable membrane invasion supported by the order parameter of the lipid acyl carbon tails and the membrane permeation free energy of DOX. The carrier-bilayer interaction shows a stepwise form attributed to the rearrangement of the zwitterionic/anionic lipids upon the absorption of the DOX-micelle complex on a membrane locality, which initiates the rapid release of DOX to the bilayer interior. Benefiting from the enhanced micelle-membrane interplay, the PEG1250-DPPE micelles result in severe bilayer breakage and deeper membrane insertion of DOX compared to the PEG2000-DPPE micelles. This study provides new theoretical insights into the mechanism of PEG-DPPE micelles in delivering drugs through membranes, which is of benefit for further optimization of PEGylated delivery systems.

Molecular mechanism of CD44 homodimerization modulated by palmitoylation and membrane environments.

The homodimerization of CD44 plays a key role in an intercellular-to-extracellular signal transduction and tumor progression. Acylated modification and specific membrane environments have been reported to mediate translocation and oligomerization of CD44; however, the underlying molecular mechanism remains elusive. In this study, extensive molecular dynamics simulations are performed to characterize the dimerization of palmitoylated CD44 variants in different bilayer environments. CD44 forms homodimer depending on the cysteines on the juxta-membrane domains, and the dimerization efficiency and packing configurations are defected by their palmitoylated modifications. In the phase-segregated (raft included) membrane, homodimerization of the palmitoylated CD44 is hardly observed, whereas PIP2 addition compensates to realize dimerization. However, the structure of CD44 homodimer formed in the phase-segregated bilayer turns susceptive and PIP2 addition allows for an extensive conformation of the cytoplasmic domain, a proposal prerequisite to access the cytoskeleton linker proteins. The results unravel a delicate competitive relationship between PIP2, lipid raft, and palmitoylation in mediating protein homodimerization, which helps to clarify the dynamic dimer conformations and involved cellular signaling of the CD44 likewise proteins.

Theaflavin binds to a druggable pocket of TMEM16A channel and inhibits lung adenocarcinoma cell viability.

As a calcium-activated chloride channel regulated by the intracellular Ca2+ concentration and membrane potential, TMEM16A has attracted considerable attention and has been proposed as a novel anticancer drug target. We have previously reported that the pocket above the ion conductance pore could be a nonselective inhibitor-binding pocket. However, whether this pocket is druggable remains unexplored. In this study, we performed virtual screening to target the presumed inhibitor-binding pocket and identified a highly effective TMEM16A inhibitor, theaflavin (TF: a tea polyphenol in black tea). Molecular dynamics simulations revealed that theaflavin adopts a "wedge insertion mode" to block the ion conduction pore and induces pore closure. Moreover, the binding mode showed that the TF pedestal plays an important role in pore blockade, and R515, R535, T539, K603, E623, and E633 were determined to be most likely to interact directly with the pedestal. Mutagenesis experiment results corroborated the mechanism through which TF binds to this pocket. Combined with the quantitative calculation results, our data indicated that the three hydroxyl groups on the pedestal may be the most crucial pharmacophores for TMEM16A inhibition by TF. Finally, antitumor experiments revealed that TF could target TMEM16A to inhibit the proliferation and migration of LA795 cells, indicating the potential therapeutic effect of TF on the growth of lung adenocarcinoma with high TMEM16A expression. The successful application of drug screening strategies based on this binding pocket highlights new directions for discovering superior modulators and contributes to the development of novel therapeutics for lung adenocarcinoma.

Molecular mechanism for bidirectional regulation of CD44 for lipid raft affiliation by palmitoylations and PIP2.

The co-localization of Cluster-of-Differentiation-44 protein (CD44) and cytoplasmic adaptors in specific membrane environments is crucial for cell adhesion and migration. The process is controlled by two different pathways: On the one hand palmitoylation keeps CD44 in lipid raft domains and disables the linking to the cytoplasmic adaptor, whereas on the other hand, the presence of phosphatidylinositol-4,5-biphosphate (PIP2) lipids accelerates the formation of the CD44-adaptor complex. The molecular mechanism explaining how CD44 is migrating into and out of the lipid raft domains and its dependence on both palmitoylations and the presence of PIP2 remains, however, elusive. In this study, we performed extensive molecular dynamics simulations to study the raft affinity and translocation of CD44 in phase separated model membranes as well as more realistic plasma membrane environments. We observe a delicate balance between the influence of the palmitoylations and the presence of PIP2 lipids: whereas the palmitoylations of CD44 increases the affinity for raft domains, PIP2 lipids have the opposite effect. Additionally, we studied the association between CD44 and the membrane adaptor FERM in dependence of these factors. We find that the presence of PIP2 lipids allows CD44 and FERM to associate in an experimentally observed binding mode whereas the highly palmitoylated species shows no binding affinity. Together, our results shed light on the sophisticated mechanism on how membrane translocation and peripheral protein association can be controlled by both protein modifications and membrane composition.

Molecular Mechanisms and Structural Basis of Retigabine Analogues in Regulating KCNQ2 Channel.

KCNQ2 channel is one of the important members of potassium voltage-gated channel. KCNQ2 is closely related to neuronal excitatory diseases including epilepsy and neuropathic pain, and also acts as a drug target of the anti-epileptic drug, retigabine (RTG). In the past few decades, RTG has shown strong efficacy in the treatment of refractory epilepsy but has been withdrawn from clinical use due to its multiple adverse effects in clinical phase III trials. To overcome the drawbacks of RTG, several RTG analogues have been developed with different activation potency to KCNQ2. However, the detailed molecular mechanism by which these RTG analogues regulate KCNQ2 channel remains obscure. In this study, we used molecular simulations to analyse the interaction mode between the RTG analogues and KCNQ2, and to determine their molecular mechanism of action. Our data show that the van der Waals interactions, hydrophobic interactions, hydrogen bond, halogen bond, and π-π stacking work together to maintain the binding stability of the drugs in the binding pocket. On an atomic scale, the amide group in the carbamate and the amino group in the 2-aminophenyl moiety of RTG and RL648_81 are identified as key interaction sites. Our finding provides insight into the molecular mechanism by which KCNQ2 channels are regulated by RTG analogues. It also provides direct theoretical support for optimizing design of the KCNQ2 channel openers in the future, which will help treat refractory epilepsy caused by nerve excitability.

Molecular mechanism of CaCCinh-A01 inhibiting TMEM16A channel.

TMEM16A is a calcium-activated chloride channel that is associate with several diseases, including pulmonary diseases, hypertension, diarrhea and cancer. The CaCCinh-A01 (A01) is widely recognized as an efficient blocker of TMEM16A and has been used as a tool drug to inhibit TMEM16A currents in the laboratory. A01 also has excellent pharmacokinetic properties and can be developed as a drug to target TMEM16A. However, the molecular mechanism how A01 inhibits TMEM16A is still elusive, which slows down its drug development process. Here, calculations identified that the binding pocket of A01 was located above the pore, and it was also discovered that the binding of A01 to TMEM16A not only blocked the pore but also led to its collapse. The interaction model analysis predicted that R515/K603/E623 were crucial residues for the binding between TMEM16A and A01, and the site-directed mutagenesis studies confirmed the above results. The binding mode and quantum chemical calculations showed that the carboxyl and the amide oxygen atom of A01 were the key interaction sites between TMEM16A and A01. Therefore, our study proposed the inhibitory mechanism of TMEM16A current by A01 and revealed how A01 inhibits TMEM16A at the molecular level. These findings will shed light on both the development of A01 as a potential drug for TMEM16A dysfunction-related disorders and drug screening targeting the pocket.

Molecular Dynamics of the Association of L-Selectin and FERM Regulated by PIP2.

Phosphatidylinositol 4,5-bisphosphate (PIP2) acts as a signaling lipid, mediating membrane trafficking and recruitment of proteins to membranes. A key example is the PIP2-dependent regulation of the adhesion of L-selectin to the cytoskeleton adaptors of the N-terminal subdomain of ezrin-radixin-moesin (FERM). The molecular details of the mediating behavior of multivalent anionic PIP2 lipids in this process, however, remain unclear. Here, we use coarse-grained molecular dynamics simulation to explore the mechanistic details of PIP2 in the transformation, translocation, and association of the FERM/L-selectin complex. We compare membranes of different compositions and find that anionic phospholipids are necessary for both FERM and the cytoplasmic domain of L-selectin to absorb on the membrane surface. The subsequent formation of the FERM/L-selectin complex is strongly favored by the presence of PIP2, which clusters around both proteins and triggers a conformational transition in the cytoplasmic domain of L-selectin. We are able to quantify the effect of PIP2 on the association free energy of the complex by means of a potential of mean force. We conclude that PIP2 behaves as an adhesive agent to enhance the stability of the FERM/L-selectin complex and identify key residues involved. The molecular information revealed in this study highlights the specific role of membrane lipids such as PIP2 in protein translocation and potential signaling.

Critical residues and motifs for homodimerization of the first transmembrane domain of the plasma membrane glycoprotein CD36.

The plasma transmembrane (TM) glycoprotein CD36 is critically involved in many essential signaling processes, especially the binding/uptake of long-chain fatty acids and oxidized low-density lipoproteins. The association of CD36 potentially activates cytosolic protein tyrosine kinases that are thought to associate with the C-terminal cytoplasmic tail of CD36. To understand the mechanisms by which CD36 mediates ligand binding and signal transduction, we have characterized the homo-oligomeric interaction of CD36 TM domains in membrane environments and with molecular dynamics (MD) simulations. Analysis of pyrene- and coumarin-labeled TM1 peptides in SDS by FRET confirmed the homodimerization of the CD36 TM1 peptide. Homodimerization assays of CD36 TM domains with the TOXCAT technique showed that its first TM (TM1) domain, but not the second TM (TM2) domain, could homodimerize in a cell membrane. Small-residue, site-specific mutation scanning revealed that the CD36 TM1 dimerization is mediated by the conserved small residues Gly12, Gly16, Ala20, and Gly23 Furthermore, molecular dynamics (MD) simulation studies demonstrated that CD36 TM1 exhibited a switching dimerization with two right-handed packing modes driven by the 12GXXXGXXXA20 and 20AXXG23 motifs, and the mutational effect of G16I and G23I revealed these representative conformations of CD36 TM1. This packing switch pattern of CD36 TM1 homodimer was further examined and confirmed by FRET analysis of monobromobimane (mBBr)-labeled CD36 TM1 peptides. Overall, this work provides a structural basis for understanding the role of TM association in regulating signal transduction via CD36.

The dimerization of PSGL-1 is driven by the transmembrane domain via a leucine zipper motif.

P-selectin glycoprotein ligand-1 (PSGL-1) is a homodimeric mucin ligand that is important to mediate the earliest adhesive event during an inflammatory response by rapidly forming and dissociating the selectin-ligand adhesive bonds. Recent research indicates that the noncovalent associations between the PSGL-1 transmembrane domains (TMDs) can substitute for the C320-dependent covalent bond to mediate the dimerization of PSGL-1. In this article, we combined TOXCAT assays and molecular dynamics (MD) simulations to probe the mechanism of PSGL-1 dimerization. The results of TOXCAT assays and Martini coarse-grained molecular dynamics (CG MD) simulations demonstrated that PSGL-1 TMDs strongly dimerized in a natural membrane and a leucine zipper motif was responsible for the noncovalent dimerization of PSGL-1 TMD since mutations of the residues that occupied a or d positions in an (abcdefg)n leucine heptad repeat motif significantly reduced the dimer activity. Furthermore, we studied the effects of the disulfide bond on the PSGL-1 dimer using MD simulations. The disulfide bond was critical to form the leucine zipper structure, by which the disulfide bond further improved the stability of the PSGL-1 dimer. These findings provide insights to understand the transmembrane association of PSGL-1 that is an important structural basis for PSGL-1 preferentially binding to P-selectin to achieve its biochemical and biophysical functions.

Insights into the transmembrane helix associations of kit ligand by molecular dynamics simulation and TOXCAT.

Kit ligand (KITL) plays important roles in cell proliferation, differentiation, and survival via interaction with its receptor Kit. The previous studies demonstrated that KITL formed a noncovalent homodimer through transmembrane (TM) domain; however, the undergoing mechanism of transmembrane association that determines KITL TM dimerization is still not clear. Herein, molecular dynamics (MD) simulation strategy and TOXCAT assay were combined to characterize the dimerization interface and structure of KITL TM in details. KITL TM formed a more energetically favorable noncovalent dimer through a conserved SxxxGxxxG motif in the MD simulation. Furthermore, the TOXCAT results demonstrated that KITL TM self-associated strongly in the bilayer membrane environment. Mutating any one of the small residues Ser11, Gly15 or Gly19 to Ile disrupted KITL TM dimerization dramatically, which further validated our MD simulation results. In addition, our results showed that Tyr22 could help to stabilize the TM interactions via interacting with the phosphoric group in the bilayer membrane. Pro7 did not induce helix kinks or swivel angles in KITL TM, but it was related with the pitch of the turn around this residue so as to affect the dimer formation. Combining the results of computer modeling and experimental mutagenesis studies on the KITL TM provide new insights for the transmembrane helix association of KITL dimerization. Proteins 2017; 85:1362-1370. © 2017 Wiley Periodicals, Inc.

Dimerization and Structural Stability of Amyloid Precursor Proteins Affected by the Membrane Microenvironments.

The lipid raft microenvironment is implicated in the generation of the pathological amyloid-ß (Aß) species in amyloid precursor protein (APP) that is associated with neurodegenerative diseases. Evidence shows that APP forms a transmembrane homodimer with changeable structures as a function of the membrane compositions. However, the molecular responsibility of the dimerization and structural alteration for the amyloidogenic process in segregated membranes remains largely unclear. Here, we performed multiple coarse grained (CG) simulations to explore the behavioral preference of the transmembrane domain of APP (called C99) that is affected by the lipid raft microenvironment. The results showed that C99 was anchored at the boundary of the lipid raft relying on the conserved hydrophobic motif of V710xxA713xxxV717xxxV721. Moreover, the dimerization of C99 was greatly destabilized by the lipid raft, which led to a susceptible switching packing conformation. The molecular driving forces were derived from the combined regulation of the saturated lipids and cholesterols rather than from the simple binding competition of cholesterol in the C99 dimerization. The molecular details of the differential dimerization in the raft-forming and bulk fluid bilayer environments were compared, and the structural information was helpful for further understanding the enzymolysis responsiveness of APP.

Role of peptide self-assembly in antimicrobial peptides.

Antimicrobial peptides (AMPs) are considered as potential antibiotic substitutes because of their potent activities. Previous studies mainly focused on the effects of peptide charges and secondary structures, but the self-assembly of AMPs was neglected. As more and more researchers notice the roles of peptide self-assembly in AMPs, it has been considered as another important property. In this review, we will discuss the influences of peptide self-assembly on the activity and mode of action, and some specific features it introduces to the AMPs, such as particular responsiveness, improved cell selectivity and stability and sustained release. In addition, some methods to design self-assembling AMPs are primarily discussed. With further understanding about the self-assembling regularity, design of particular self-assembling AMPs will be very helpful for their applications, especially in the fields of drug delivery and biomedical engineering.

UNC-45A: A potential therapeutic target for malignant tumors.

Uncoordinated mutant number-45 myosin chaperone A (UNC-45A), a protein highly conserved throughout evolution, is ubiquitously expressed in somatic cells. It is correlated with tumorigenesis, proliferation, metastasis, and invasion of multiple malignant tumors. The current understanding of the role of UNC-45A in tumor progression is mainly related to the regulation of non-muscle myosin II (NM-II). However, many studies have suggested that the mechanisms by which UNC-45A is involved in tumor progression are far greater than those of NM-II regulation. UNC-45A can also promote tumor cell proliferation by regulating checkpoint kinase 1 (ChK1) phosphorylation or the transcriptional activity of nuclear receptors, and induces chemoresistance to paclitaxel in tumor cells by destabilizing microtubule activity. In this review, we discuss the recent advances illuminating the role of UNC-45A in tumor progression. We also put forward therapeutic strategies targeting UNC-45A, in the hope of paving the way the development of UNC-45A-targeted therapies for patients with malignant tumors.

Small-molecule fulvic acid with strong hydration ability for non-vitreous cellular cryopreservation.

The exploitation of biocompatible ice-control materials especially the small molecules for non-vitreous cryopreservation remains challenging. Here, we report a small molecule of fulvic acid (FA) with strong hydration ability, which enables non-vitreous cellular cryopreservation by reducing ice growth during freezing and reducing ice recrystallization/promoting ice melting during thawing. Without adding any other cryoprotectants, FA can enhance the recovery of sheep red blood cells (RBCs) by three times as compared with a commercial cryoprotectant (hydroxyethyl starch) under a stringent test condition. Investigation of water mobility reveals that the ice-control properties of FA can be ascribed to its strong bondage to water molecules. Furthermore, we found that FA can be absorbed by RBCs and mainly locates on membranes, suggesting the possible contribution of FA to cell protection through stabilizing membranes. This work bespeaks a bright future for small-molecule cryoprotectants in non-vitreous cryopreservation application.

Molecular dynamics simulation of TMEM16A channel: Linking structure with gating.

TMEM16A, the calcium-activated chloride channel, is broadly expressed and plays pivotal roles in diverse physiological processes. To understand the structural and functional relationships of TMEM16A, it is necessary to fully clarify the structural basis of the gating of the TMEM16A channel. Herein, we performed the protein electrostatic analysis and molecular dynamics simulation on the TMEM16A in the presence and absence of Ca2+. Data showed that the separation of TM4 and TM6 causes pore expansion, and Q646 may be a key residue for the formation of π-helix in the middle segment of TM6. Moreover, E705 was found to form a group of H-bond interactions with D554/K588/K645 below the hydrophobic gate to stabilize the closed conformation of the pore in the Ca2+-free state. Interestingly, in the Ca2+ bound state, the E705 side chain swings 100o to serve as Ca2+-binding coordination and released K645. K645 is closer to the hydrophobic gate in the calcium-bound state, which facilitates the provision of electrostatic forces for chloride ions as the ions pass through the hydrophobic gate. Our findings provide the structural-based insights to understanding the mechanisms of gating of TMEM16A.