Cryo-EM structures of the TTYH family reveal a novel architecture for lipid interactions.

The Tweety homologs (TTYHs) are members of a conserved family of eukaryotic membrane proteins that are abundant in the brain. The three human paralogs were assigned to function as anion channels that are either activated by Ca2+ or cell swelling. To uncover their unknown architecture and its relationship to function, we have determined the structures of human TTYH1-3 by cryo-electron microscopy. All structures display equivalent features of a dimeric membrane protein that contains five transmembrane segments and an extended extracellular domain. As none of the proteins shows attributes reminiscent of an anion channel, we revisited functional experiments and did not find any indication of ion conduction. Instead, we find density in an extended hydrophobic pocket contained in the extracellular domain that emerges from the lipid bilayer, which suggests a role of TTYH proteins in the interaction with lipid-like compounds residing in the membrane.

Localization atomic force microscopy.

Understanding structural dynamics of biomolecules at the single-molecule level is vital to advancing our knowledge of molecular mechanisms. Currently, there are few techniques that can capture dynamics at the sub-nanometre scale and in physiologically relevant conditions. Atomic force microscopy (AFM)1 has the advantage of analysing unlabelled single molecules in physiological buffer and at ambient temperature and pressure, but its resolution limits the assessment of conformational details of biomolecules2. Here we present localization AFM (LAFM), a technique developed to overcome current resolution limitations. By applying localization image reconstruction algorithms3 to peak positions in high-speed AFM and conventional AFM data, we increase the resolution beyond the limits set by the tip radius, and resolve single amino acid residues on soft protein surfaces in native and dynamic conditions. LAFM enables the calculation of high-resolution maps from either images of many molecules or many images of a single molecule acquired over time, facilitating single-molecule structural analysis. LAFM is a post-acquisition image reconstruction method that can be applied to any biomolecular AFM dataset.

Early-stage dynamics of chloride ion-pumping rhodopsin revealed by a femtosecond X-ray laser.

Chloride ion-pumping rhodopsin (ClR) in some marine bacteria utilizes light energy to actively transport Cl- into cells. How the ClR initiates the transport is elusive. Here, we show the dynamics of ion transport observed with time-resolved serial femtosecond (fs) crystallography using the Linac Coherent Light Source. X-ray pulses captured structural changes in ClR upon flash illumination with a 550 nm fs-pumping laser. High-resolution structures for five time points (dark to 100 ps after flashing) reveal complex and coordinated dynamics comprising retinal isomerization, water molecule rearrangement, and conformational changes of various residues. Combining data from time-resolved spectroscopy experiments and molecular dynamics simulations, this study reveals that the chloride ion close to the Schiff base undergoes a dissociation-diffusion process upon light-triggered retinal isomerization.

Glutamate transporters have a chloride channel with two hydrophobic gates.

Glutamate is the most abundant excitatory neurotransmitter in the central nervous system, and its precise control is vital to maintain normal brain function and to prevent excitotoxicity1. The removal of extracellular glutamate is achieved by plasma-membrane-bound transporters, which couple glutamate transport to sodium, potassium and pH gradients using an elevator mechanism2-5. Glutamate transporters also conduct chloride ions by means of a channel-like process that is thermodynamically uncoupled from transport6-8. However, the molecular mechanisms that enable these dual-function transporters to carry out two seemingly contradictory roles are unknown. Here we report the cryo-electron microscopy structure of a glutamate transporter homologue in an open-channel state, which reveals an aqueous cavity that is formed during the glutamate transport cycle. The functional properties of this cavity, combined with molecular dynamics simulations, reveal it to be an aqueous-accessible chloride permeation pathway that is gated by two hydrophobic regions and is conserved across mammalian and archaeal glutamate transporters. Our findings provide insight into the mechanism by which glutamate transporters support their dual function, and add information that will assist in mapping the complete transport cycle shared by the solute carrier 1A transporter family.

Structures and pH-sensing mechanism of the proton-activated chloride channel.

The proton-activated chloride channel (PAC) is active across a wide range of mammalian cells and is involved in acid-induced cell death and tissue injury1-3. PAC has recently been shown to represent a novel and evolutionarily conserved protein family4,5. Here we present two cryo-electron microscopy structures of human PAC in a high-pH resting closed state and a low-pH proton-bound non-conducting state. PAC is a trimer in which each subunit consists of a transmembrane domain (TMD), which is formed of two helices (TM1 and TM2), and an extracellular domain (ECD). Upon a decrease of pH from 8 to 4, we observed marked conformational changes in the ECD-TMD interface and the TMD. The rearrangement of the ECD-TMD interface is characterized by the movement of the histidine 98 residue, which is, after acidification, decoupled from the resting position and inserted into an acidic pocket that is about 5 Å away. Within the TMD, TM1 undergoes a rotational movement, switching its interaction partner from its cognate TM2 to the adjacent TM2. The anion selectivity of PAC is determined by the positively charged lysine 319 residue on TM2, and replacing lysine 319 with a glutamate residue converts PAC to a cation-selective channel. Our data provide a glimpse of the molecular assembly of PAC, and a basis for understanding the mechanism of proton-dependent activation.

Cryo-EM structure of the lysosomal chloride-proton exchanger CLC-7 in complex with OSTM1.

The chloride-proton exchanger CLC-7 plays critical roles in lysosomal homeostasis and bone regeneration and its mutation can lead to osteopetrosis, lysosomal storage disease and neurological disorders. In lysosomes and the ruffled border of osteoclasts, CLC-7 requires a ß-subunit, OSTM1, for stability and activity. Here, we present electron cryomicroscopy structures of CLC-7 in occluded states by itself and in complex with OSTM1, determined at resolutions up to 2.8 Å. In the complex, the luminal surface of CLC-7 is entirely covered by a dimer of the heavily glycosylated and disulfide-bonded OSTM1, which serves to protect CLC-7 from the degradative environment of the lysosomal lumen. OSTM1 binding does not induce large-scale rearrangements of CLC-7, but does have minor effects on the conformation of the ion-conduction pathway, potentially contributing to its regulatory role. These studies provide insights into the role of OSTM1 and serve as a foundation for understanding the mechanisms of CLC-7 regulation.

Inside the cells of mammals, acidic compartments called lysosomes are responsible for breaking down large molecules and worn-out cells parts so their components can be used again. Similar to lysosomes, specialized cells called osteoclasts require an acidic environment to degrade tissues in the bone. Both osteoclasts and lysosomes rely on a two-component protein complex to help them digest molecules. Mutations in the genes for both proteins are directly linked to human diseases including neurodegeneration and osteopetrosis ­ a disease characterized by dense and brittle bones. For the main protein in this complex, called CLC-7, to remain stable and perform its roles, it requires an accessory subunit known as OSTM1. CLC-7 is a transporter that funnels electrically charged particles into and out of the lysosome, which helps to maintain the environment inside the lysosome compartment. However, due to the tight partnership between CLC-7 and OTSM1, how they influence each other is poorly understood. To determine the roles of CLC-7 and OSTM1, Schrecker et al. looked at the structure of the complex using a technique called single particle electron microscopy, which allows proteins to be visualized almost down to the individual atom. The analysis revealed that OSTM1 covers almost the entire inside surface of CLC-7, protecting it from the acidic environment inside the lysosome and contributing to its stability. When the two subunits are bound together, OSTM1 also slightly changes the structure of the pore formed by CLC-7, suggesting that OSTM1 may regulate CLC-7 activity. Schrecker et al. have laid the foundation for understanding more about the activity and regulation of CLC-7 and OSTM1 in lysosomes and osteoclasts. The structures described also help explain previous findings, including why OSTM1 is important for the stability of CLC-7.

Structural and functional characterization of the bestrophin-2 anion channel.

The bestrophin family of calcium (Ca2+)-activated chloride (Cl-) channels, which mediate the influx and efflux of monovalent anions in response to the levels of intracellular Ca2+, comprises four members in mammals (bestrophin 1-4). Here we report cryo-EM structures of bovine bestrophin-2 (bBest2) bound and unbound by Ca2+ at 2.4- and 2.2-Å resolution, respectively. The bBest2 structure highlights four previously underappreciated pore-lining residues specifically conserved in Best2 but not in Best1, illustrating the differences between these paralogs. Structure-inspired electrophysiological analysis reveals that, although the channel is sensitive to Ca2+, it has substantial Ca2+-independent activity for Cl-, reflecting the opening at the cytoplasmic restriction of the ion conducting pathway even when Ca2+ is absent. Moreover, the ion selectivity of bBest2 is controlled by multiple residues, including those involved in gating.

Structure of the human ClC-1 chloride channel.

ClC-1 protein channels facilitate rapid passage of chloride ions across cellular membranes, thereby orchestrating skeletal muscle excitability. Malfunction of ClC-1 is associated with myotonia congenita, a disease impairing muscle relaxation. Here, we present the cryo-electron microscopy (cryo-EM) structure of human ClC-1, uncovering an architecture reminiscent of that of bovine ClC-K and CLC transporters. The chloride conducting pathway exhibits distinct features, including a central glutamate residue ("fast gate") known to confer voltage-dependence (a mechanistic feature not present in ClC-K), linked to a somewhat rearranged central tyrosine and a narrower aperture of the pore toward the extracellular vestibule. These characteristics agree with the lower chloride flux of ClC-1 compared with ClC-K and enable us to propose a model for chloride passage in voltage-dependent CLC channels. Comparison of structures derived from protein studied in different experimental conditions supports the notion that pH and adenine nucleotides regulate ClC-1 through interactions between the so-called cystathionine-ß-synthase (CBS) domains and the intracellular vestibule ("slow gating"). The structure also provides a framework for analysis of mutations causing myotonia congenita and reveals a striking correlation between mutated residues and the phenotypic effect on voltage gating, opening avenues for rational design of therapies against ClC-1-related diseases.

Un-gating and allosteric modulation of a pentameric ligand-gated ion channel captured by molecular dynamics.

Pentameric ligand-gated ion channels (pLGICs) mediate intercellular communication at synapses through the opening of an ion pore in response to the binding of a neurotransmitter. Despite the increasing availability of high-resolution structures of pLGICs, a detailed understanding of the functional isomerization from closed to open (gating) and back is currently missing. Here, we provide the first atomistic description of the transition from open to closed (un-gating) in the glutamate-gated chloride channel (GluCl) from Caenorhabditis Elegans. Starting with the active-state structure solved in complex with the neurotransmitter L-glutamate and the positive allosteric modulator (PAM) ivermectin, we analyze the spontaneous relaxation of the channel upon removal of ivermectin by explicit solvent/membrane Molecular Dynamics (MD) simulations. The µs-long trajectories support the conclusion that ion-channel deactivation is mediated by two distinct quaternary transitions, i.e. a global receptor twisting followed by the radial expansion (or blooming) of the extracellular domain. At variance with previous models, we show that pore closing is exclusively regulated by the global twisting, which controls the position of the ß1-ß2 loop relative to the M2-M3 loop at the EC/TM domain interface. Additional simulations with L-glutamate restrained to the crystallographic binding mode and ivermectin removed indicate that the same twisting isomerization is regulated by agonist binding at the orthosteric site. These results provide a structural model for gating in pLGICs and suggest a plausible mechanism for the pharmacological action of PAMs in this neurotransmitter receptor family. The simulated un-gating converges to the X-ray structure of GluCl resting state both globally and locally, demonstrating the predictive character of state-of-art MD simulations.

Expression, purification, and contaminant detection for structural studies of Ralstonia metallidurance ClC protein rm1.

Single-particle electron cryo-microscopy (cryo-EM) has become a popular method for high-resolution study of the structural and functional properties of proteins. However, sufficient expression and purification of membrane proteins holds many challenges. We describe methods to overcome these obstacles using ClC-rm1, a prokaryotic chloride channel (ClC) family protein from Ralstonia metallidurans, overexpressed in Escherichia coli (E. coli) BL21(DE3) strain. Mass spectrometry and electron microscopy analyses of purified samples revealed multiple contaminants that can obfuscate results of subsequent high-resolution structural analysis. Here we describe the systematic optimization of sample preparation procedures, including expression systems, solubilization techniques, purification protocols, and contamination detection. We found that expressing ClC-rm1 in E. coli BL21(DE3) and using n-dodecyl-ß-D-maltopyranoside as a detergent for solubilization and purification steps resulted in the highest quality samples of those we tested. However, although protein yield, sample stability, and the resolution of structural detail were improved following these changes, we still detected contaminants including Acriflavine resistant protein AcrB. AcrB was particularly difficult to remove as it co-purified with ClC-rm1 due to four intrinsic histidine residues at its C-terminus that bind to affinity resins. We were able to obtain properly folded pure ClC-rm1 by adding eGFP to the C-terminus and overexpressing the protein in the ΔacrB variant of the JW0451-2 E. coli strain.

Structure of a CLC chloride ion channel by cryo-electron microscopy.

CLC proteins transport chloride (Cl-) ions across cellular membranes to regulate muscle excitability, electrolyte movement across epithelia, and acidification of intracellular organelles. Some CLC proteins are channels that conduct Cl- ions passively, whereas others are secondary active transporters that exchange two Cl- ions for one H+. The structural basis underlying these distinctive transport mechanisms is puzzling because CLC channels and transporters are expected to share the same architecture on the basis of sequence homology. Here we determined the structure of a bovine CLC channel (CLC-K) using cryo-electron microscopy. A conserved loop in the Cl- transport pathway shows a structure markedly different from that of CLC transporters. Consequently, the cytosolic constriction for Cl- passage is widened in CLC-K such that the kinetic barrier previously postulated for Cl-/H+ transporter function would be reduced. Thus, reduction of a kinetic barrier in CLC channels enables fast flow of Cl- down its electrochemical gradient.

Brain white matter oedema due to ClC-2 chloride channel deficiency: an observational analytical study.

BACKGROUND: Mutant mouse models suggest that the chloride channel ClC-2 has functions in ion and water homoeostasis, but this has not been confirmed in human beings. We aimed to define novel disorders characterised by distinct patterns of MRI abnormalities in patients with leukoencephalopathies of unknown origin, and to identify the genes mutated in these disorders. We were specifically interested in leukoencephalopathies characterised by white matter oedema, suggesting a defect in ion and water homoeostasis. METHODS: In this observational analytical study, we recruited patients with leukoencephalopathies characterised by MRI signal abnormalities in the posterior limbs of the internal capsules, midbrain cerebral peduncles, and middle cerebellar peduncles from our databases of patients with leukoencephalopathies of unknown origin. We used exome sequencing to identify the gene involved. We screened the candidate gene in additional patients by Sanger sequencing and mRNA analysis, and investigated the functional effects of the mutations. We assessed the localisation of ClC-2 with immunohistochemistry and electron microscopy in post-mortem human brains of individuals without neurological disorders. FINDINGS: Seven patients met our inclusion criteria, three with adult-onset disease and four with childhood-onset disease. We identified homozygous or compound-heterozygous mutations in CLCN2 in three adult and three paediatric patients. We found evidence that the CLCN2 mutations result in loss of function of ClC-2. The remaining paediatric patient had an X-linked family history and a mutation in GJB1, encoding connexin 32. Clinical features were variable and included cerebellar ataxia, spasticity, chorioretinopathy with visual field defects, optic neuropathy, cognitive defects, and headaches. MRI showed restricted diffusion suggesting myelin vacuolation that was confined to the specified white matter structures in adult patients, and more diffusely involved the brain white matter in paediatric patients. We detected ClC-2 in all components of the panglial syncytium, enriched in astrocytic endfeet at the perivascular basal lamina, in the glia limitans, and in ependymal cells. INTERPRETATION: Our observations substantiate the concept that ClC-2 is involved in brain ion and water homoeostasis. Autosomal-recessive CLCN2 mutations cause a leukoencephalopathy that belongs to an emerging group of disorders affecting brain ion and water homoeostasis and characterised by intramyelinic oedema. FUNDING: European Leukodystrophies Association, INSERM and Assistance Publique-Hôpitaux de Paris, Dutch Organisation for Scientific Research (ZonMw), E-Rare, Hersenstichting, Optimix Foundation for Scientific Research, Myelin Disorders Bioregistry Project, National Institute of Neurological Disorders and Stroke, and Genetic and Epigenetic Networks in Cognitive Dysfunction (GENCODYS) Project (funded by the European Union Framework Programme 7).

GlialCAM, a protein defective in a leukodystrophy, serves as a ClC-2 Cl(-) channel auxiliary subunit.

Ion fluxes mediated by glial cells are required for several physiological processes such as fluid homeostasis or the maintenance of low extracellular potassium during high neuronal activity. In mice, the disruption of the Cl(-) channel ClC-2 causes fluid accumulation leading to myelin vacuolation. A similar vacuolation phenotype is detected in humans affected with megalencephalic leukoencephalopathy with subcortical cysts (MLC), a leukodystrophy which is caused by mutations in MLC1 or GLIALCAM. We here identify GlialCAM as a ClC-2 binding partner. GlialCAM and ClC-2 colocalize in Bergmann glia, in astrocyte-astrocyte junctions at astrocytic endfeet around blood vessels, and in myelinated fiber tracts. GlialCAM targets ClC-2 to cell junctions, increases ClC-2 mediated currents, and changes its functional properties. Disease-causing GLIALCAM mutations abolish the targeting of the channel to cell junctions. This work describes the first auxiliary subunit of ClC-2 and suggests that ClC-2 may play a role in the pathology of MLC disease.

Location of release sites and calcium-activated chloride channels relative to calcium channels at the photoreceptor ribbon synapse.

Vesicle release from photoreceptor ribbon synapses is regulated by L-type Ca(2+) channels, which are in turn regulated by Cl(-) moving through calcium-activated chloride [Cl(Ca)] channels. We assessed the proximity of Ca(2+) channels to release sites and Cl(Ca) channels in synaptic terminals of salamander photoreceptors by comparing fast (BAPTA) and slow (EGTA) intracellular Ca(2+) buffers. BAPTA did not fully block synaptic release, indicating some release sites are <100 nm from Ca(2+) channels. Comparing Cl(Ca) currents with predicted Ca(2+) diffusion profiles suggested that Cl(Ca) and Ca(2+) channels average a few hundred nanometers apart, but the inability of BAPTA to block Cl(Ca) currents completely suggested some channels are much closer together. Diffuse immunolabeling of terminals with an antibody to the putative Cl(Ca) channel TMEM16A supports the idea that Cl(Ca) channels are dispersed throughout the presynaptic terminal, in contrast with clustering of Ca(2+) channels near ribbons. Cl(Ca) currents evoked by intracellular calcium ion concentration ([Ca(2+)](i)) elevation through flash photolysis of DM-nitrophen exhibited EC(50) values of 556 and 377 nM with Hill slopes of 1.8 and 2.4 in rods and cones, respectively. These relationships were used to estimate average submembrane [Ca(2+)](i) in photoreceptor terminals. Consistent with control of exocytosis by [Ca(2+)] nanodomains near Ca(2+) channels, average submembrane [Ca(2+)](i) remained below the vesicle release threshold (∼ 400 nM) over much of the physiological voltage range for cones. Positioning Ca(2+) channels near release sites may improve fidelity in converting voltage changes to synaptic release. A diffuse distribution of Cl(Ca) channels may allow Ca(2+) influx at one site to influence relatively distant Ca(2+) channels.

Antiport mechanism for Cl(-)/H(+) in ClC-ec1 from normal-mode analysis.

ClC chloride channels and transporters play major roles in cellular excitability, epithelial salt transport, volume, pH, and blood pressure regulation. One family member, ClC-ec1 from Escherichia coli, has been structurally resolved crystallographically and subjected to intensive mutagenetic, crystallographic, and electrophysiological studies. It functions as a Cl(-)/H(+) antiporter, not a Cl(-) channel; however, the molecular mechanism for Cl(-)/H(+) exchange is largely unknown. Using all-atom normal-mode analysis to explore possible mechanisms for this antiport, we propose that Cl(-)/H(+) exchange involves a conformational cycle of alternating exposure of Cl(-) and H(+) binding sites of both ClC pores to the two sides of the membrane. Both pores switch simultaneously from facing outward to facing inward, reminiscent of the standard alternating-access mechanism, which may have direct implications for eukaryotic Cl(-)/H(+) transporters and Cl(-) channels.

CLIC2-RyR1 interaction and structural characterization by cryo-electron microscopy.

Chloride intracellular channel 2 (CLIC2), a newly discovered small protein distantly related to the glutathione transferase (GST) structural family, is highly expressed in cardiac and skeletal muscle, although its physiological function in these tissues has not been established. In the present study, [3H]ryanodine binding, Ca2+ efflux from skeletal sarcoplasmic reticulum (SR) vesicles, single channel recording, and cryo-electron microscopy were employed to investigate whether CLIC2 can interact with skeletal ryanodine receptor (RyR1) and modulate its channel activity. We found that: (1) CLIC2 facilitated [3H]ryanodine binding to skeletal SR and purified RyR1, by increasing the binding affinity of ryanodine for its receptor without significantly changing the apparent maximal binding capacity; (2) CLIC2 reduced the maximal Ca2+ efflux rate from skeletal SR vesicles; (3) CLIC2 decreased the open probability of RyR1 channel, through increasing the mean closed time of the channel; (4) CLIC2 bound to a region between domains 5 and 6 in the clamp-shaped region of RyR1; (5) and in the same clamp region, domains 9 and 10 became separated after CLIC2 binding, indicating CLIC2 induced a conformational change of RyR1. These data suggest that CLIC2 can interact with RyR1 and modulate its channel activity. We propose that CLIC2 functions as an intrinsic stabilizer of the closed state of RyR channels.

Regulation of bestrophin Cl channels by calcium: role of the C terminus.

Human bestrophin-1 (hBest1), which is genetically linked to several kinds of retinopathy and macular degeneration in both humans and dogs, is the founding member of a family of Cl(-) ion channels that are activated by intracellular Ca(2+). At present, the structures and mechanisms responsible for Ca(2+) sensing remain unknown. Here, we have used a combination of molecular modeling, density functional-binding energy calculations, mutagenesis, and patch clamp to identify the regions of hBest1 involved in Ca(2+) sensing. We identified a cluster of a five contiguous acidic amino acids in the C terminus immediately after the last transmembrane domain, followed by an EF hand and another regulatory domain that are essential for Ca(2+) sensing by hBest1. The cluster of five amino acids (293-308) is crucial for normal channel gating by Ca(2+) because all but two of the 35 mutations we made in this region rendered the channel incapable of being activated by Ca(2+). Using homology models built on the crystal structure of calmodulin (CaM), an EF hand (EF1) was identified in hBest1. EF1 was predicted to bind Ca(2+) with a slightly higher affinity than the third EF hand of CaM and lower affinity than the second EF hand of troponin C. As predicted by the model, the D312G mutation in the putative Ca(2+)-binding loop (312-323) reduced the apparent Ca(2+) affinity by 20-fold. In addition, the D312G and D323N mutations abolished Ca(2+)-dependent rundown of the current. Furthermore, analysis of truncation mutants of hBest1 identified a domain adjacent to EF1 that is rich in acidic amino acids (350-390) that is required for Ca(2+) activation and plays a role in current rundown. These experiments identify a region of hBest1 (312-323) that is involved in the gating of hBest1 by Ca(2+) and suggest a model in which Ca(2+) binding to EF1 activates the channel in a process that requires the acidic domain (293-308) and another regulatory domain (350-390). Many of the approximately 100 disease-causing mutations in hBest1 are located in this region that we have implicated in Ca(2+) sensing, suggesting that these mutations disrupt hBest1 channel gating by Ca(2+).

Volume sensitivity of the bestrophin family of chloride channels.

Bestrophins are a newly identified family of Cl(-) channels. Mutations in the founding member of the family, human bestrophin-1 (hBest1), are responsible for a form of early onset macular degeneration called Best vitelliform macular dystrophy. The link between dysfunction of hBest1 and macular degeneration remains unknown. Because retinal pigmented epithelium (RPE) cells may be subjected to varying osmotic pressure due to light-dependent changes in the ionic composition of the subretinal space and because RPE cells may undergo large volume changes during phagocytosis of shed photoreceptor discs, we investigated whether bestrophin currents were affected by cell volume. When hBest1 and mBest2 were overexpressed in HEK 293, HeLa, and ARPE-19 cells, a new Ca(2+)-activated Cl(-) current appeared. This current was very sensitive to cell volume. A 20% increase in extracellular osmolarity caused cell shrinkage and a approximately 70-80% reduction in bestrophin current. Decreases in extracellular osmolarity increased the bestrophin currents slightly, but this was difficult to quantify due to simultaneous activation of endogenous volume-regulated anion channel (VRAC) current. To determine whether a similar current was present in mouse RPE cells, the effect of hyperosmotic solutions on isolated mouse RPE cells was examined. Mouse RPE cells exhibited an endogenous Cl(-) current that resembled the expressed hBest1 in that it was decreased by hypertonic solution. We conclude that bestrophins are volume sensitive and that they could play a novel role in cell volume regulation of RPE cells.

Intracellular localization of a long alkyl chain tetraphenylporphyrin and chloride channel activation in Psammobatis extenta electrocytes.

The intracellular localization of a tetraphenylporphyrin bearing four long lipophilic alkyl chains in electrocytes from Psammobatis extenta(Rajidae) is described. In contrast to what is usually the case, this porphyrin derivative is localized in the electromotor nerves and the nuclear chromatin of the electrocytes. Both structures exhibited intense fluorescence, whereas, the mitochondria were only slightly fluorescent. These data are discussed in relation to electrocyte death in a weakly electric fish. Additionally, electron probe X-ray microanalysis suggests a migration of chloride and cationic ions, which might be implicated in chloride and cationic channel activation in the electrocyte.

CLIC4 is enriched at cell-cell junctions and colocalizes with AKAP350 at the centrosome and midbody of cultured mammalian cells.

CLIC4 is a member of the chloride intracellular channel (CLIC) protein family whose principal cellular functions are poorly understood. Recently, we demonstrated that several CLIC proteins, including CLIC4, interact with AKAP350. AKAP350 is concentrated at the Golgi apparatus, centrosome, and midbody and acts as a scaffolding protein for several protein kinases and phosphatases. In this report, we show that endogenous CLIC4 and AKAP350 colocalize at the centrosome and midbody of cultured cells by immunofluorescence microscopy. Unlike AKAP350, CLIC4 is not enriched in the Golgi apparatus but is enriched in mitochondria, actin-based structures at the cell cortex, and the nuclear matrix, indicating that CLIC4-AKAP350 interactions are regulated at specific subcellular sites in vivo. In addition to the centrosome and midbody, CLIC4 colocalizes with AKAP350 and the tight junction protein ZO-1 in the apical region of polarized epithelial cells, suggesting that CLIC4 may play a role in maintaining apical-basolateral membrane polarity during mitosis and cytokinesis. Biochemical studies show that CLIC4 behaves mainly as a soluble cytosolic protein and can associate with proteins of the microtubule cytoskeleton. The localization of CLIC4 to the cortical actin cytoskeleton and its association with AKAP350 at the centrosome and midbody suggests that CLIC4 may be important for regulating cytoskeletal organization during the cell cycle. These findings lead to the conclusion that CLIC4 and possibly other CLIC proteins have alternate cellular functions that are distinct from their proposed roles as chloride channels.