Asymmetric pendrin homodimer reveals its molecular mechanism as anion exchanger

Pendrin (SLC26A4) is an anion exchanger expressed in the apical membranes of selected epithelia. Pendrin ablation causes Pendred syndrome, a genetic disorder associated with sensorineural hearing loss, hypothyroid goiter, and reduced blood pressure. However its molecular structure has remained unknown, limiting our understanding of the structural basis of transport. Here, we determine the cryo-electron microscopy structures of mouse pendrin with symmetric and asymmetric homodimer conformations. The asymmetric homodimer consists of one inward-facing protomer and the other outward-facing protomer, representing coincident uptake and secretion- a unique state of pendrin as an electroneutral exchanger. The multiple conformations presented here provide an inverted alternate-access mechanism for anion exchange. The structural and functional data presented here disclose the properties of an anion exchange cleft and help understand the importance of disease-associated variants, which will shed light on the pendrin exchange mechanism.

Single particle cryo-EM structure of the outer hair cell motor protein prestin

The mammalian outer hair cell (OHC) protein prestin (Slc26a5) differs from other Slc26 family members due to its unique piezoelectric-like property that drives OHC electromotility, the putative mechanism for cochlear amplification. Here, we use cryo-electron microscopy to determine prestin’s structure at 3.6 Å resolution. Prestin is structurally similar to the anion transporter Slc26a9. It is captured in an inward-open state which may reflect prestin’s contracted state. Two well-separated transmembrane (TM) domains and two cytoplasmic sulfate transporter and anti-sigma factor antagonist (STAS) domains form a swapped dimer. The transmembrane domains consist of 14 transmembrane segments organized in two 7+7 inverted repeats, an architecture first observed in the bacterial symporter UraA. Mutation of prestin’s chloride binding site removes salicylate competition with anions while retaining the prestin characteristic displacement currents (Nonlinear Capacitance), undermining the extrinsic voltage sensor hypothesis for prestin function.

Prestin, expressed in outer hair cell (OHC), belongs to the Slc26 transporter family and functions as a voltage-driven motor that drives OHC electromotility. Here, the authors report cryo-EM structure and characterization of gerbil prestin, with insights into its mechanism of action.

Progress in understanding the structural mechanism underlying prestin's electromotile activity

Prestin (SLC26A5), a member of the SLC26 transporter family, is the molecular actuator that drives OHC electromotility (eM). A wealth of biophysical data indicates that eM is mediated by an area motor mechanism, in which prestin molecules act as elementary actuators by changing their area in the membrane in response to changes in membrane potential. The area changes of a large and densely packed population of prestin molecules sum up, resulting in macroscopic cellular movement. At the single protein level, this model implies major voltage-driven conformational rearrangements. However, the nature of these structural dynamics remained unknown. A main obstacle in elucidating the eM mechanism has been the lack of structural information about SLC26 transporters. The recent emergence of several high-resolution cryo-EM structures of prestin as well as other SLC26 transporter family members now provides a reliable picture of prestin's molecular architecture. Thus, SLC26 transporters including prestin generally are dimers, and each protomer is folded according to a 7+7 transmembrane domain inverted repeat (7TMIR) architecture. Here, we review these structural findings and discuss insights into a potential molecular mechanism. Most important, distinct conformations were observed when purifying and imaging prestin bound to either its physiological ligand, chloride, or to competitively inhibitory anions, sulfate or salicylate. Despite differences in detail, these structural snapshots indicate that the conformational landscape of prestin includes rearrangements between the two major domains of prestin's transmembrane region (TMD), core and scaffold ('gate') domains. Notably, distinct conformations differ in the area the TMD occupies in the membrane and in their impact on the immediate lipid environment. Both effects can contribute to generate membrane deformation and thus may underly electromotility. Further functional studies will be necessary to determine whether these or similar structural rearrangements are driven by membrane potential to mediate piezoelectric activity. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

Molecular mechanism of prestin electromotive signal amplification

Hearing involves two fundamental processes: mechano-electrical transduction and signal amplification. Despite decades of studies, the molecular bases for both remain elusive. Here, we show how prestin, the electromotive molecule of outer hair cells (OHCs) that senses both voltage and membrane tension, mediates signal amplification by coupling conformational changes to alterations in membrane surface area. Cryoelectron microscopy (cryo-EM) structures of human prestin bound with chloride or salicylate at a common "anion site" adopt contracted or expanded states, respectively. Prestin is ensconced within a perimeter of well-ordered lipids, through which it induces dramatic deformation in the membrane and couples protein conformational changes to the bulk membrane. Together with computational studies, we illustrate how the anion site is allosterically coupled to changes in the transmembrane domain cross-sectional area and the surrounding membrane. These studies provide insight into OHC electromotility by providing a structure-based mechanism of the membrane motor prestin.

STAS Domain Only Proteins in Bacterial Gene Regulation

Sulfate Transport Anti-Sigma antagonist domains (Pfam01740) are found in all branches of life, from eubacteria to mammals, as a conserved fold encoded by highly divergent amino acid sequences. These domains are present as part of larger SLC26/SulP anion transporters, where the STAS domain is associated with transmembrane anchoring of the larger multidomain protein. Here, we focus on STAS Domain only Proteins (SDoPs) in eubacteria, initially described as part of the Bacillus subtilis Regulation of Sigma B (RSB) regulatory system. Since their description in B. subtilis, SDoPs have been described to be involved in the regulation of sigma factors, through partner-switching mechanisms in various bacteria such as: Mycobacterium. tuberculosis, Listeria. monocytogenes, Vibrio. fischeri, Bordetella bronchiseptica, among others. In addition to playing a canonical role in partner-switching with an anti-sigma factor to affect the availability of a sigma factor, several eubacterial SDoPs show additional regulatory roles compared to the original RSB system of B. subtilis. This is of great interest as these proteins are highly conserved, and often involved in altering gene expression in response to changes in environmental conditions. For many of the bacteria we will examine in this review, the ability to sense environmental changes and alter gene expression accordingly is critical for survival and colonization of susceptible hosts.

Functional Parameters of Prestin Are Not Correlated With the Best Hearing Frequency

Among the vertebrate lineages with different hearing frequency ranges, humans lie between the low-frequency hearing (1 kHz) of fish and amphibians and the high-frequency hearing (100 kHz) of bats and dolphins. Little is known about the mechanism underlying such a striking difference in the limits of hearing frequency. Prestin, responsible for cochlear amplification and frequency selectivity in mammals, seems to be the only candidate to date. Mammalian prestin is densely expressed in the lateral plasma membrane of the outer hair cells (OHCs) and functions as a voltage-dependent motor protein. To explore the molecular basis for the contribution of prestin in hearing frequency detection, we collected audiogram data from humans, dogs, gerbils, bats, and dolphins because their average hearing frequency rises in steps. We generated stable cell lines transfected with human, dog, gerbil, bat, and dolphin prestins (hPres, dPres, gPres, bPres, and nPres, respectively). The non-linear capacitance (NLC) of different prestins was measured using a whole-cell patch clamp. We found that the Q_max/C_lin of bPres and nPres was significantly higher than that of humans. The V_1/2 of hPres was more hyperpolarized than that of nPres. The z values of hPres and bPres were higher than that of nPres. We further analyzed the relationship between the high-frequency hearing limit (F_max) and the functional parameters (V_1/2, z, and Q_max/C_lin) of NLC among five prestins. Interestingly, no significant correlation was found between the functional parameters and F_max. Additionally, a comparative study showed that the amino acid sequences and tertiary structures of five prestins were quite similar. There might be a common fundamental mechanism driving the function of prestins. These findings call for a reconsideration of the leading role of prestin in hearing frequency perception. Other intriguing kinetics underlying the hearing frequency response of auditory organs might exist.

Calmodulin binds to the STAS domain of SLC26A5 prestin with a calcium-dependent, one-lobe, binding mode

SLC26A5 transporter prestin is fundamental for the higher hearing sensitivity and frequency selectivity of mammals. Prestin is a voltage-dependent transporter found in the cochlear outer hair cells responsible for their electromotility. Intracellular chloride binding is considered essential for voltage sensitivity and electromotility. Prestin is composed by a transmembrane domain and by a cytosolic domain called STAS. There is evidence of a calcium/calmodulin regulation of prestin mediated by the STAS domain. Using different biophysical techniques, namely SEC, CD, ITC, MST, NMR and SAXS, here we demonstrate and characterize the direct interaction between calmodulin and prestin STAS. We show that the interaction is calcium-dependent and that involves residues at the N-terminal end of the "variable loop". This is an intrinsically disordered insertion typical of the STAS domains of the SLC26 family of transporters whose function is still unclear. We derive a low-resolution model of the STAS/CaM complex, where only one lobe of calmodulin is engaged in the interaction, and build a model for the entire dimeric prestin in complex with CaM, which can use the unoccupied lobe to interact with other regions of prestin or with other regulatory proteins. We show that also a non-mammalian STAS can interact with calmodulin via the variable loop. These data start to shed light on the regulatory role of the STAS variable loop of prestin.

Structure of MlaFB uncovers novel mechanisms of ABC transporter regulation

ABC transporters facilitate the movement of diverse molecules across cellular membranes, but how their activity is regulated post-translationally is not well understood. Here we report the crystal structure of MlaFB from E. coli, the cytoplasmic portion of the larger MlaFEDB ABC transporter complex, which drives phospholipid trafficking across the bacterial envelope to maintain outer membrane integrity. MlaB, a STAS domain protein, binds the ABC nucleotide binding domain, MlaF, and is required for its stability. Our structure also implicates a unique C-terminal tail of MlaF in self-dimerization. Both the C-terminal tail of MlaF and the interaction with MlaB are required for the proper assembly of the MlaFEDB complex and its function in cells. This work leads to a new model for how an important bacterial lipid transporter may be regulated by small proteins, and raises the possibility that similar regulatory mechanisms may exist more broadly across the ABC transporter family.

Cryo-EM structures and functional characterization of murine Slc26a9 reveal mechanism of uncoupled chloride transport

The epithelial anion transporter SLC26A9 contributes to airway surface hydration and gastric acid production. Colocalizing with CFTR, SLC26A9 has been proposed as a target for the treatment of cystic fibrosis. To provide molecular details of its transport mechanism, we present cryo-EM structures and a functional characterization of murine Slc26a9. These structures define the general architecture of eukaryotic SLC26 family members and reveal an unusual mode of oligomerization which relies predominantly on the cytosolic STAS domain. Our data illustrates conformational transitions of Slc26a9, supporting a rapid alternate-access mechanism which mediates uncoupled chloride transport with negligible bicarbonate or sulfate permeability. The characterization of structure-guided mutants illuminates the properties of the ion transport path, including a selective anion binding site located in the center of a mobile module within the transmembrane domain. This study thus provides a structural foundation for the understanding of the entire SLC26 family and potentially facilitates their therapeutic exploitation.

Many processes in the human body are regulated by chloride and other charged particles (known as ions) moving in and out of cells. Each cell is surrounded by a membrane barrier, which prevents ions from entering or exiting. Therefore, to control the levels of ions inside the cell, specific proteins in the membrane act as channels or transporters to provide routes for the ions to pass through the membrane.

Channel proteins form pores that, when open, allow a steady stream of ions to pass through the membrane. Transporter proteins, on the other hand, generally contain a pocket that is only accessible from one side of the membrane. When individual ions enter this pocket the transporter changes shape. This causes the entrance of the pocket to close and then re-open on the other side of the membrane.

Inside the lung, an ion channel known as CFTR provides a route for chloride ions to move out of cells, which helps clear harmful material from the airways. Mutations affecting this protein cause the mucus lining the airways to become very sticky, leading to a severe disease known as cystic fibrosis. CFTR works together with another protein that is also found in the membrane, called SLC26A9. Previous studies have suggested that SLC26A9 also allows chloride ions to pass through the membrane. It was not clear, however, if SLC26A9 operates as an ion channel or a transporter protein, or how the protein is arranged in the membrane.

Now, Walter, Sawicka and Dutzler combined two techniques known as cryo-electron microscopy and patch-clamp electrophysiology to reveal the detailed three-dimensional structure of the mouse version of SLC26A9, which is highly similar to the human form. The experiments found that mouse SLC26A9 proteins form pairs in the membrane referred to as homodimers, which arranged themselves in an unexpected way. Further investigation into the structure of these homodimers suggests that despite having many channel-like properties, SLC26A9 operates as a fast transporter, rather than a true channel.

These findings help us understand the role of SLC26A9 and other similar proteins in the lung and other parts of the body. In the future it may be possible to develop drugs that target SLC26A9 to treat cystic fibrosis and other severe lung diseases.

Deletion of exons 17 and 18 in prestin's STAS domain results in loss of function

Cochlear outer hair cells (OHC) express the motor protein, prestin, which is required for sensitivity and frequency selectivity. Because our previous work showed that a calmodulin binding site (CBS) was located in prestin's C-terminal, specifically within the intrinsically disordered region, we sought to delete the IDR to study the functional significance of calcium-dependent, calmodulin binding on OHC function. Although the construct lacking the IDR (∆IDR prestin) demonstrated wildtype-like nonlinear capacitance (NLC) in HEK293T cells, the phenotype in ∆IDR prestin knockins (KI) was similar to that in prestin knockouts: thresholds were elevated, NLC was absent and OHCs were missing from basal regions of the cochlea. Although ∆IDR prestin mRNA was measured, no prestin protein was detected. At the mRNA level, both of prestin's exons 17 and 18 were entirely removed, rather than the smaller region encoding the IDR. Our hybrid exon that contained the targeted deletion (17-18 ∆IDR) failed to splice in vitro and prestin protein lacking exons 17 and 18 aggregated and failed to target the cell membrane. Hence, the absence of prestin protein in ∆IDR KI OHCs may be due to the unexpected splicing of the hybrid 17-18 ∆IDR exon followed by rapid degradation of nonfunctional prestin protein.

The E. coli dicarboxylic acid transporters DauA act as a signal transducer by interacting with the DctA uptake system

The Slc26A/SulP family of ions transporter is ubiquitous and widpsread in all kingdon of life. In E. coli, we have demonstrated that the Slc26 protein DauA is a C₄-dicarboxilic acids (C₄-diC) transporter active at acidic pH. The main C₄-diC transporter active at pH7 is DctA and is induced by C₄-diC via the DcuS/R two component system. DctA interacts with DcuS, the membrane embedded histidine kinase, to transfers DcuS to the responsive state, i.e. in the absence of DctA, DcuS is permanently "on", but its activity is C₄-diC-dependent when in complex with DctA. Using phenotypic characterization, transport assays and protein expression studies, we show that at pH7 full DctA production depends on the presence of DauA. A Bacterial Two Hybrid system indicates that DauA and the sensor complex DctA/DcuS physically interact at the membrane. Pull down experiments completed by co-purification study prove that DauA and DctA interact physically at the membrane. These data open a completely new aspect of the C₄-diC metabolism in E. coli and reveals how the bacterial Slc26A uptake systems participate in multiple cellular functions. This constitutes a new example of a bacterial transporter that acts as a processor in a transduction pathway.

Chansporter complexes in cell signaling

Ion channels facilitate diffusion of ions across cell membranes for such diverse purposes as neuronal signaling, muscular contraction, and fluid homeostasis. Solute transporters often utilize ionic gradients to move aqueous solutes up their concentration gradient, also fulfilling a wide variety of tasks. Recently, an increasing number of ion channel-transporter ('chansporter') complexes have been discovered. Chansporter complex formation may overcome what could otherwise be considerable spatial barriers to rapid signal integration and feedback between channels and transporters, the ions and other substrates they transport, and environmental factors to which they must respond. Here, current knowledge in this field is summarized, covering both heterologous expression structure/function findings and potential mechanisms by which chansporter complexes fulfill contrasting roles in cell signaling in vivo.

The novel class of seven transmembrane segment inverted repeat carriers

Solute carriers from the SLC4, SLC23, and SLC26 families are involved in pH regulation, vitamin C transport and ion homeostasis. While these families do not share any obvious sequence relationship, they are united by their unique and novel architecture. Each member of this structural class is organized into two structurally related halves of seven transmembrane segments each. These halves span the membrane with opposite orientations and form an intricately intertwined structure of two inverted repeats. This review highlights the general design principles of this fold and reveals the diversity between the different families. We discuss their domain architecture, structural framework and transport mode and detail an initial transport mechanism for this fold inferred from the recently solved structures of different members.

Prestin and the good vibrations

In a recent paper published in the Biochemical Journal, Lolli et al. presented evidence that the C-terminal STAS (sulfate transporter and anti-sigma factor antagonist) domain of the motor protein prestin possesses an anion-binding site. This discovery might shed light on an aspect of the function of this mysterious and fascinating protein that is crucial for the human hearing system.

The chloride-channel blocker 9-anthracenecarboxylic acid reduces the nonlinear capacitance of prestin-associated charge movement

The basis of the extraordinary sensitivity and frequency selectivity of the cochlea is a chloride-sensitive protein called prestin which can produce an electromechanical response and which resides in the basolateral plasma membrane of outer hair cells (OHCs). The compound 9-anthracenecarboxylic acid (9-AC), an inhibitor of chloride channels, has been found to reduce the electromechanical response of the cochlea and the OHC mechanical impedance. To elucidate these 9-AC effects, the functional electromechanical status of prestin was assayed by measuring the nonlinear capacitance of OHCs from the guinea-pig cochlea and of prestin-transfected human embryonic kidney 293 (HEK 293) cells. Extracellular application of 9-AC caused reversible, dose-dependent and chloride-sensitive reduction in OHC nonlinear charge transfer, Qmax . Prestin-transfected cells also showed reversible reduction in Qmax . For OHCs, intracellular 9-AC application as well as reduced intracellular pH had no detectable effect on the reduction in Qmax by extracellularly applied 9-AC. In the prestin-transfected cells, cytosolic application of 9-AC approximately halved the blocking efficacy of extracellularly applied 9-AC. OHC inside-out patches presented the whole-cell blocking characteristics. Disruption of the cytoskeleton by preventing actin polymerization with latrunculin A or by decoupling of spectrin from actin with diamide did not affect the 9-AC-evoked reduction in Qmax . We conclude that 9-AC acts on the electromechanical transducer principally by interaction with prestin rather than acting via the cytoskeleton, chloride channels or pH. The 9-AC block presents characteristics in common with salicylate, but is almost an order of magnitude faster. 9-AC provides a new tool for elucidating the molecular dynamics of prestin function.