In a recent paper published in the Biochemical Journal (1), Lolli and collaborators presented evidence that the C-terminal STAS domain of the motor protein prestin possesses an anion-binding site. This discovery might bring light to an aspect of the function of this mysterious and fascinating protein that is crucial for the human hearing system

Birke, Anna

Javelle, Arnaud

PubMed

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryBirke, Anna and Javelle, Arnaud (2016) Prestin and the good vibrations. Biochemical Journal, 473 (15). pp. 2425-2427. ISSN 0264-6021 , http://dx.doi.org/10.1042/BCJ20160015This version is available at http://strathprints.strath.ac.uk/56077/Strathprints is  designed  to  allow  users  to  access  the  research  output  of  the  University  of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may  not  engage  in  further  distribution  of  the  material  for  any  profitmaking  activities  or  any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge. Any  correspondence  concerning  this  service  should  be  sent  to  Strathprints  administrator: strathprints@strath.ac.ukBiochem. J. (2016) 473, 2425–2427 doi:10.1042/BCJ20160015 2425COMMENTARYPrestin and the good vibrationsAnna Sofia Birke* and Arnaud Javelle*1*University of Strathclyde, Strathclyde Institute of Pharmacy and Biomedical Sciences, 161 Cathedral Street, Glasgow, G40RE, United KingdomIn a recent paper published in the Biochemical Journal, Lolliet al. presented evidence that the C-terminal STAS (sulfatetransporter and anti-sigma factor antagonist) domain of the motorprotein prestin possesses an anion-binding site. This discoverymight shed light on an aspect of the function of this mysteriousand fascinating protein that is crucial for the human hearingsystem.Key words: cochlea, hearing system, outer hair cell, prestin,SLC26A5, STAS domain.Prestin [SLC26A5 (solute carrier 26A5)] belongs to theubiquitous SLC26/SulP (sulfate permease) family of anionexchangers and is one of ten SLC26A transporters that havebeen identified in animals, most of which are known totransport anions across membranes either in an electroneutralor electrogenic manner [1]. In humans, genes encoding fourmembers of the SCL26A protein family have been identified asdisease genes: mutations in the genes encoding SLC26A2/DTD,SLC26A3/DRA, SLC26A4/pendrin and SLC26A5/prestin areassociated with diastrophic dysplasia, congenital chloride-losingdiarrhoea, Pendred syndrome and human deafness, respectively[1]. Although prestin is a member of a mammalian anion-exchanger family, it apparently has a very different function incochlear OHCs (outer hair cells), where it is densely packed inthe basolateral membrane. OHCs in mammals have the abilityto alter their cell length in response to changes in membranepotential triggered by incoming sound waves. This form ofvoltage-dependent cell movement, known as electromotility, isrendered possible by prestin. Therefore prestin is often referredto as an OHC motor protein. OHC electromotility is part of themechanical sound amplification system responsible for increasedhearing sensitivity and frequency selectivity in mammals. Incontrast with prestin-based electromotiliy in mammals, non-mammalian prestin orthologues display more ‘traditional’ anion-transporting activity [2,3]. However outstanding mammalianprestin might be, it shares its topology and a highly conservedintracellular C-terminal domain called STAS (sulfate transporterand anti-sigma antagonist) domain with other members of theSLC26 family [2,4]. Despite decades of effort in research tosolve prestin’s structure and to fully understand the underlyingmechanism of action, neither mission has yet been accomplished.However, in early 2016, Lolli et al. [5] presented results inthe Biochemical Journal that suggest the mammalian prestinSTAS domain possesses an anion-binding site, to which thephysiologically relevant Cl− ion binds. The idea that intracellularCl− ions might act as an extrinsic voltage sensor for prestin makesthis discovery all the more exciting [6].Electromotility of cochlear OHCs, which is their ability toelongate or contract actively and quickly (>20 kHz in humans) inresponse to changes in membrane potential, was first observed in1985 [7]. Over the course of the following 15 years, the drivingforce underlying this form of voltage-dependent cellular responsewould remain unknown. With guinea pig OHCs, the changes incell length could be observed after as short a time as 120 µspost-stimulation, which is far more rapid than any actin–myosin-based cell motility functions [8]. This observation suggested thatthe mechanism underlying electromotility was different from anymolecular motor that was known at the time. Furthermore, themechanism was also shown to work independently of ATP [9],and so it became increasingly apparent that OHC electromotility isbased on a novel type of force-generating mechanism. Additionalclues as to what might be driving electromotility included thedescription of IMPs (intermembrane particles) in the 1970s [10],as well as the finding that OHCs whose cell content had beenremoved via tryptic digest were still able to perform electromotilemovements [11]. These results provided enough evidence to cometo the conclusion that the mysterious driving force of OHCelectromotility should manifest itself in a membrane-associatedprotein that is located in the lateral membrane wall of OHCsand that has the ability to undergo rapid conformational changesin a voltage-dependent manner. Fast-forward to the turn of themillennium and Zheng et al. [12] solved that part of the puzzle withthe discovery of prestin, which is named after its most outstandingfeature: the speed at which it operates (musical notation presto =fast).Prestin is the fifth mammalian member of the SLC26A proteins,which in turn belong to the large family of SLC26/SulP anion-transporter-related proteins [2]. Members of the family are widelydistributed among animals, bacteria, plants and fungi. With 700–1000 amino acids, SLC26/SulP proteins are relatively largemembrane proteins. Prestin itself is composed of 744 amino acids.Members of the family appear to have a very similar topology with10 to 14 TM (transmembrane) domains that contain a conservedcytoplasmic C-terminal motif, named the STAS domain. Althoughthe function of the prestin STAS domain has not yet becomequite clear, it was thought to be important for targeting to themembrane in many SLC26/SulP proteins [1,12,13]. However,more recent work using bacterial SLC26A transporters whoseSTAS domains have been replaced with the GFP motif haveshown that the STAS domain does not play a direct role in proteintargeting, but rather in protein stabilization and certainly proteinfunction [14].Abbreviations: cPres, chicken prestin; IHC, inner hair cell; OHC, outer hair cell; rPres, rat prestin; SLC26, solute carrier 26; STAS, sulfate transporterand anti-sigma antagonist; SulP, sulfate permease; SynPres, synthetic prestin; TM, transmembrane.1 To whom correspondence should be addressed (email: arnaud.javelle@strath.ac.uk).c© 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution Licence 4.0 (CC BY).2426 A.S. Birke and A. JavelleDespite belonging to a family of anion-transporter-relatedproteins, mammalian prestin is not an anion transporter assuch. Instead, it is the protein driving electromotiliy of OHCsby undergoing incredibly rapid conformational changes. Thesegenerate a force great enough to result in alteration of OHClength. The resulting mechanical amplification of sound wavesis a mechanism unique to mammals. OHCs and IHCs (inner haircells) are two distinct types of mechanosensory cells found in theorgan of Corti, where IHCs function as primary auditory signalreceivers. Sound amplification by OHCs occurs in a cycle-by-cycle manner and relies on the ability of prestin to sense changesin membrane potential, either depolarization or hyperpolarization.In response to that, it must be able to change its conformationvery quickly, causing the cell to either contract or elongate. Thus,presumably, prestin has at least two functional domains, namelythe voltage-sensor and the actuator.As mentioned above, prestin in mammals and non-mammaliananimals does not have the same function despite amino acidsimilarity. Although only prestin in mammals acts as a molecularmotor that drives electromotility in OHCs, the non-mammalianorthologue functions as a divalent/chloride anion exchanger [3].Interestingly, the construction of a synthetic prestin (SynPres),composed of zebrafish prestin for most parts and some TMdomains from rat prestin, appears to have the best of both worlds.SynPres has anion-exchanging abilities in the way that zebrafishprestin does and SynPres-transfected cells feature electromotility.The generation and assessment of SynPres function deliveredresults indicating that the actuator domain, that is the proteinregion responsible for inducing fast conformational changes,might be located in the TM part of prestin [15]. But how doesprestin ‘measure’ changes in membrane potential? It has beenproposed that binding of Cl− from the cytoplasmic side may actas an extrinsic voltage sensor since the removal of intracellularCl− resulted in the absence of electromotility of rat OHCs [6].It was not until 2016, when Lolli et al. [5] published their paper‘The STAS domain of mammalian SLC26A5 prestin harboursan anion-binding site’, that the idea of intracellular Cl− anionspotentially playing a part in voltage sensing was confirmed bystructural analysis of the rat prestin (rPres) STAS domain. Lolliet al. [5] examined several versions of both rat and chicken STASdomains, of which the former was bound to a range of differentanions, among them Cl− , in order to assess conformationalchanges that may occur within the STAS domains. To obtaincrystal structures of the STAS domain, it was necessary to deletea region of the domain called the variable loop, which has alength of 74 amino acids in rPres STAS and 83 amino acidsin the chicken prestin (cPres) STAS domain. By comparing thestructures of rPres STAS in the presence of Cl− in the allegedanion-binding site and in the absence thereof, it does not appearthat the crystallized part of the protein undergoes any majorconformational rearrangements as a result of Cl− binding. Thisindicates that the anion-binding site maintains its conformationthroughout the process of anions binding and dissociating, whichunderpins the concept of the binding site to be pre-formed andready to accommodate anions that could actuate prestin function.In fact, all other anions, besides Cl− , that were also shownto bind to the crystallized rPres STAS domain trigger prestinactivity, which indicates a specificity for these anions. In additionto solving the structure of the rPres STAS domain, Lolli et al.[5] obtained the crystal structure of the cPres STAS domain.They found that the anion-binding site present in the mammalianSTAS domain was, indeed, absent from the bird version of theprotein, which suggests that anion binding to a pre-formed cavityis likely to be a unique property of the mammalian orthologueand possibly linked to the difference in biological function. Theauthors propose that the anion-binding site in the mammalianprestin STAS domain might function as a low-affinity ‘ready-to-use’ reservoir of actuator anions [5]. Clearly, the crystal structuresobtained suggest that no conformational rearrangements in theSTAS domain occur upon Cl− binding. However, this may be acrystallization artefact or the result of the missing variable loop,as well as the absence of the rest of the prestin protein [5].Despite the progress brought about by revelations such as theidentification of the C-terminal anion-binding site within theSTAS domain, unanswered questions remain. Although Cl− isclearly a key component of prestin’s voltage-sensing ability, itposes a chicken-and-egg problem. Does the binding of Cl− anionsto prestin act as an extrinsic voltage sensor or is it the consequenceof a preliminary intrinsic voltage-sensing mechanism? Althoughanions might be binding to the site in the STAS domain, themystery of its actual function and how it fits in with the rest ofthe prestin protein has not yet been solved. Recently, the crystalstructure of a SLC26 protein from Deinococcus geothermalis,SLC26Dg, was obtained and published as the first ever completestructure of a SLC26 protein [16]. Unfortunately, the structuregives no clue about the functional relationship of the TMdomain and the STAS domain within SLC26Dg [16]. Besidesthe questions concerning the interaction between the TM domainand the STAS domain and the general function of the latter,examination of the functional actuator domain, once identified,should be particularly interesting from an evolutionary pointof view. How and why did prestin evolve in a way that it lostits anion-exchanging capabilities and simultaneously gained thepower to generate electromotility-driving forces in mammalianOHCs? Last, but not least, how is prestin able to function at suchhigh speed; so very presto?FUNDINGThis work was supported by the Medical Research Counicl (G1000054/1). I thank themfor their financial support.REFERENCES1 Alper, S.L. and Sharma, A.K. (2013) The SLC26 gene family of anion transporters andchannels. Mol. Aspects Med. 34, 494–515 CrossRef PubMed2 Dallos, P. and Fakler, B. (2002) Prestin, a new type of motor protein. Nat. Rev. Mol. CellBiol. 3, 104–111 CrossRef PubMed3 Schaechinger, T.J. and Oliver, D. (2007) Nonmammalian orthologs of prestin (SLC26A5)are electrogenic divalent/chloride anion exchangers. Proc. Natl. Acad. Sci. U.S.A. 104,7693–7698 CrossRef PubMed4 Aravind, L. and Koonin, E.V. (2000) The STAS domain: a link between anion transportersand antisigma-factor antagonists. Curr. Biol. 10, R53–R55 CrossRef PubMed5 Lolli, G., Pasqualetto, E., Costanzi, E., Bonetto, G. and Battistutta, R. (2016) The STASdomain of mammalian SLC26A5 prestin harbours an anion-binding site. Biochem. J.473, 365–370 CrossRef PubMed6 Oliver, D., He, D.Z.Z., Klo¨cker, N., Ludwig, J., Schulte, U., Waldegger, S., Ruppersberg,J.P., Dallos, P. and Fakler, B. (2001) Intracellular anions as the voltage sensor of prestin,the outer hair cell motor protein. Science 292, 2340–2343 CrossRef PubMed7 Brownell, W., Bader, C., Bertrand, D. and de Ribaupierre, Y. (1985) Evoked mechanicalresponses of isolated cochlear outer hair cells. Science 227, 194–196 CrossRef PubMed8 Ashmore, J.F. (1987) A fast motile response in guinea-pig outer hair cells: the cellularbasis of the cochlear amplifier. J. Physiol. 388, 323–347 CrossRef PubMed9 Kachar, B., Brownell, W.E., Altschuler, R. and Fex, J. (1985) Electrokinetic shape changesof cochlear outer hair cells. Nature 322, 365–368 CrossRef10 Gulley, R. and Reese, T. (1977) Regional specialization of the hair cell plasmalemma in theorgan of Corti. Anat. Rec. 189, 109–123 CrossRef PubMed11 Kalinec, F., Holley, M.C., Iwasa, K.H., Lim, D.J. and Kachar, B. (1992) A membrane-basedforce generation mechanism in auditory sensory cells. Proc. Natl. Acad. Sci. U.S.A. 89,8671–8675 CrossRef PubMed12 Zheng, J., Shen, W., He, D.Z.Z., Long, K.B., Madison, L.D. and Dallos, P. (2000) Prestin isthe motor protein of cochlear outer hair cells. Nature 405, 149–155 CrossRef PubMedc© 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution Licence 4.0 (CC BY).Commentary 242713 Sharma, A.K., Rigby, A.C. and Alper, S.L. (2011) STAS domain structure and function.Cell. Physiol. Biochem. 28, 407–422 CrossRef PubMed14 Compton, E.L., Page, K., Findlay, H.E., Haertlein, M., Moulin, M., Zachariae, U., Norman,D.G., Gabel, F. and Javelle, A. (2014) Conserved structure and domain organizationamong bacterial Slc26 transporters. Biochem. J. 463, 297–307CrossRef PubMed15 Schaechinger, T.J., Gorbunov, D., Halaszovich, C.R., Moser, T., Ku¨gler, S., Fakler, B. andOliver, D. (2011) A synthetic prestin reveals protein domains and molecular operation ofouter hair cell piezoelectricity. EMBO J. 30, 2793–2804 CrossRef PubMed16 Geertsma, E.R., Chang, Y.-N., Shaik, F.R., Neldner, Y., Pardon, E., Steyaert, J. and Dutzler,R. (2015) Structure of a prokaryotic fumarate transporter reveals the architecture of theSLC26 family. Nat. Struct. Mol. Biol. 22, 803–808 CrossRef PubMedReceived 22 January 2016/4 April 2016; accepted 5 April 2016Version of Record published 28 July 2016, doi:10.1042/BCJ20160015c© 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution Licence 4.0 (CC BY).

Prestin and the good vibrations

Name not available

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.</jats:p

Anna Sofia Birke

Arnaud Javelle

Crossref

Biochem. J. (2016) 473, 2425–2427 doi:10.1042/BCJ20160015 2425COMMENTARYPrestin and the good vibrationsAnna Sofia Birke* and Arnaud Javelle*1*University of Strathclyde, Strathclyde Institute of Pharmacy and Biomedical Sciences, 161 Cathedral Street, Glasgow, G40RE, United KingdomIn a recent paper published in the Biochemical Journal, Lolliet al. presented evidence that the C-terminal STAS (sulfatetransporter and anti-sigma factor antagonist) domain of the motorprotein prestin possesses an anion-binding site. This discoverymight shed light on an aspect of the function of this mysteriousand fascinating protein that is crucial for the human hearingsystem.Key words: cochlea, hearing system, outer hair cell, prestin,SLC26A5, STAS domain.Prestin [SLC26A5 (solute carrier 26A5)] belongs to theubiquitous SLC26/SulP (sulfate permease) family of anionexchangers and is one of ten SLC26A transporters that havebeen identified in animals, most of which are known totransport anions across membranes either in an electroneutralor electrogenic manner [1]. In humans, genes encoding fourmembers of the SCL26A protein family have been identified asdisease genes: mutations in the genes encoding SLC26A2/DTD,SLC26A3/DRA, SLC26A4/pendrin and SLC26A5/prestin areassociated with diastrophic dysplasia, congenital chloride-losingdiarrhoea, Pendred syndrome and human deafness, respectively[1]. Although prestin is a member of a mammalian anion-exchanger family, it apparently has a very different function incochlear OHCs (outer hair cells), where it is densely packed inthe basolateral membrane. OHCs in mammals have the abilityto alter their cell length in response to changes in membranepotential triggered by incoming sound waves. This form ofvoltage-dependent cell movement, known as electromotility, isrendered possible by prestin. Therefore prestin is often referredto as an OHC motor protein. OHC electromotility is part of themechanical sound amplification system responsible for increasedhearing sensitivity and frequency selectivity in mammals. Incontrast with prestin-based electromotiliy in mammals, non-mammalian prestin orthologues display more ‘traditional’ anion-transporting activity [2,3]. However outstanding mammalianprestin might be, it shares its topology and a highly conservedintracellular C-terminal domain called STAS (sulfate transporterand anti-sigma antagonist) domain with other members of theSLC26 family [2,4]. Despite decades of effort in research tosolve prestin’s structure and to fully understand the underlyingmechanism of action, neither mission has yet been accomplished.However, in early 2016, Lolli et al. [5] presented results inthe Biochemical Journal that suggest the mammalian prestinSTAS domain possesses an anion-binding site, to which thephysiologically relevant Cl− ion binds. The idea that intracellularCl− ions might act as an extrinsic voltage sensor for prestin makesthis discovery all the more exciting [6].Electromotility of cochlear OHCs, which is their ability toelongate or contract actively and quickly (>20 kHz in humans) inresponse to changes in membrane potential, was first observed in1985 [7]. Over the course of the following 15 years, the drivingforce underlying this form of voltage-dependent cellular responsewould remain unknown. With guinea pig OHCs, the changes incell length could be observed after as short a time as 120 μspost-stimulation, which is far more rapid than any actin–myosin-based cell motility functions [8]. This observation suggested thatthe mechanism underlying electromotility was different from anymolecular motor that was known at the time. Furthermore, themechanism was also shown to work independently of ATP [9],and so it became increasingly apparent that OHC electromotility isbased on a novel type of force-generating mechanism. Additionalclues as to what might be driving electromotility included thedescription of IMPs (intermembrane particles) in the 1970s [10],as well as the finding that OHCs whose cell content had beenremoved via tryptic digest were still able to perform electromotilemovements [11]. These results provided enough evidence to cometo the conclusion that the mysterious driving force of OHCelectromotility should manifest itself in a membrane-associatedprotein that is located in the lateral membrane wall of OHCsand that has the ability to undergo rapid conformational changesin a voltage-dependent manner. Fast-forward to the turn of themillennium and Zheng et al. [12] solved that part of the puzzle withthe discovery of prestin, which is named after its most outstandingfeature: the speed at which it operates (musical notation presto =fast).Prestin is the fifth mammalian member of the SLC26A proteins,which in turn belong to the large family of SLC26/SulP anion-transporter-related proteins [2]. Members of the family are widelydistributed among animals, bacteria, plants and fungi. With 700–1000 amino acids, SLC26/SulP proteins are relatively largemembrane proteins. Prestin itself is composed of 744 amino acids.Members of the family appear to have a very similar topology with10 to 14 TM (transmembrane) domains that contain a conservedcytoplasmic C-terminal motif, named the STAS domain. Althoughthe function of the prestin STAS domain has not yet becomequite clear, it was thought to be important for targeting to themembrane in many SLC26/SulP proteins [1,12,13]. However,more recent work using bacterial SLC26A transporters whoseSTAS domains have been replaced with the GFP motif haveshown that the STAS domain does not play a direct role in proteintargeting, but rather in protein stabilization and certainly proteinfunction [14].Abbreviations: cPres, chicken prestin; IHC, inner hair cell; OHC, outer hair cell; rPres, rat prestin; SLC26, solute carrier 26; STAS, sulfate transporterand anti-sigma antagonist; SulP, sulfate permease; SynPres, synthetic prestin; TM, transmembrane.1 To whom correspondence should be addressed (email: arnaud.javelle@strath.ac.uk).c© 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution Licence 4.0 (CC BY).2426 A.S. Birke and A. JavelleDespite belonging to a family of anion-transporter-relatedproteins, mammalian prestin is not an anion transporter assuch. Instead, it is the protein driving electromotiliy of OHCsby undergoing incredibly rapid conformational changes. Thesegenerate a force great enough to result in alteration of OHClength. The resulting mechanical amplification of sound wavesis a mechanism unique to mammals. OHCs and IHCs (inner haircells) are two distinct types of mechanosensory cells found in theorgan of Corti, where IHCs function as primary auditory signalreceivers. Sound amplification by OHCs occurs in a cycle-by-cycle manner and relies on the ability of prestin to sense changesin membrane potential, either depolarization or hyperpolarization.In response to that, it must be able to change its conformationvery quickly, causing the cell to either contract or elongate. Thus,presumably, prestin has at least two functional domains, namelythe voltage-sensor and the actuator.As mentioned above, prestin in mammals and non-mammaliananimals does not have the same function despite amino acidsimilarity. Although only prestin in mammals acts as a molecularmotor that drives electromotility in OHCs, the non-mammalianorthologue functions as a divalent/chloride anion exchanger [3].Interestingly, the construction of a synthetic prestin (SynPres),composed of zebrafish prestin for most parts and some TMdomains from rat prestin, appears to have the best of both worlds.SynPres has anion-exchanging abilities in the way that zebrafishprestin does and SynPres-transfected cells feature electromotility.The generation and assessment of SynPres function deliveredresults indicating that the actuator domain, that is the proteinregion responsible for inducing fast conformational changes,might be located in the TM part of prestin [15]. But how doesprestin ‘measure’ changes in membrane potential? It has beenproposed that binding of Cl− from the cytoplasmic side may actas an extrinsic voltage sensor since the removal of intracellularCl− resulted in the absence of electromotility of rat OHCs [6].It was not until 2016, when Lolli et al. [5] published their paper‘The STAS domain of mammalian SLC26A5 prestin harboursan anion-binding site’, that the idea of intracellular Cl− anionspotentially playing a part in voltage sensing was confirmed bystructural analysis of the rat prestin (rPres) STAS domain. Lolliet al. [5] examined several versions of both rat and chicken STASdomains, of which the former was bound to a range of differentanions, among them Cl− , in order to assess conformationalchanges that may occur within the STAS domains. To obtaincrystal structures of the STAS domain, it was necessary to deletea region of the domain called the variable loop, which has alength of 74 amino acids in rPres STAS and 83 amino acidsin the chicken prestin (cPres) STAS domain. By comparing thestructures of rPres STAS in the presence of Cl− in the allegedanion-binding site and in the absence thereof, it does not appearthat the crystallized part of the protein undergoes any majorconformational rearrangements as a result of Cl− binding. Thisindicates that the anion-binding site maintains its conformationthroughout the process of anions binding and dissociating, whichunderpins the concept of the binding site to be pre-formed andready to accommodate anions that could actuate prestin function.In fact, all other anions, besides Cl− , that were also shownto bind to the crystallized rPres STAS domain trigger prestinactivity, which indicates a specificity for these anions. In additionto solving the structure of the rPres STAS domain, Lolli et al.[5] obtained the crystal structure of the cPres STAS domain.They found that the anion-binding site present in the mammalianSTAS domain was, indeed, absent from the bird version of theprotein, which suggests that anion binding to a pre-formed cavityis likely to be a unique property of the mammalian orthologueand possibly linked to the difference in biological function. Theauthors propose that the anion-binding site in the mammalianprestin STAS domain might function as a low-affinity ‘ready-to-use’ reservoir of actuator anions [5]. Clearly, the crystal structuresobtained suggest that no conformational rearrangements in theSTAS domain occur upon Cl− binding. However, this may be acrystallization artefact or the result of the missing variable loop,as well as the absence of the rest of the prestin protein [5].Despite the progress brought about by revelations such as theidentification of the C-terminal anion-binding site within theSTAS domain, unanswered questions remain. Although Cl− isclearly a key component of prestin’s voltage-sensing ability, itposes a chicken-and-egg problem. Does the binding of Cl− anionsto prestin act as an extrinsic voltage sensor or is it the consequenceof a preliminary intrinsic voltage-sensing mechanism? Althoughanions might be binding to the site in the STAS domain, themystery of its actual function and how it fits in with the rest ofthe prestin protein has not yet been solved. Recently, the crystalstructure of a SLC26 protein from Deinococcus geothermalis,SLC26Dg, was obtained and published as the first ever completestructure of a SLC26 protein [16]. Unfortunately, the structuregives no clue about the functional relationship of the TMdomain and the STAS domain within SLC26Dg [16]. Besidesthe questions concerning the interaction between the TM domainand the STAS domain and the general function of the latter,examination of the functional actuator domain, once identified,should be particularly interesting from an evolutionary pointof view. How and why did prestin evolve in a way that it lostits anion-exchanging capabilities and simultaneously gained thepower to generate electromotility-driving forces in mammalianOHCs? Last, but not least, how is prestin able to function at suchhigh speed; so very presto?FUNDINGThis work was supported by the Medical Research Counicl (G1000054/1). I thank themfor their financial support.REFERENCES1 Alper, S.L. and Sharma, A.K. (2013) The SLC26 gene family of anion transporters andchannels. Mol. Aspects Med. 34, 494–515 CrossRef PubMed2 Dallos, P. and Fakler, B. (2002) Prestin, a new type of motor protein. Nat. Rev. Mol. CellBiol. 3, 104–111 CrossRef PubMed3 Schaechinger, T.J. and Oliver, D. (2007) Nonmammalian orthologs of prestin (SLC26A5)are electrogenic divalent/chloride anion exchangers. Proc. Natl. Acad. Sci. U.S.A. 104,7693–7698 CrossRef PubMed4 Aravind, L. and Koonin, E.V. (2000) The STAS domain: a link between anion transportersand antisigma-factor antagonists. Curr. Biol. 10, R53–R55 CrossRef PubMed5 Lolli, G., Pasqualetto, E., Costanzi, E., Bonetto, G. and Battistutta, R. (2016) The STASdomain of mammalian SLC26A5 prestin harbours an anion-binding site. Biochem. J.473, 365–370 CrossRef PubMed6 Oliver, D., He, D.Z.Z., Klo¨cker, N., Ludwig, J., Schulte, U., Waldegger, S., Ruppersberg,J.P., Dallos, P. and Fakler, B. (2001) Intracellular anions as the voltage sensor of prestin,the outer hair cell motor protein. Science 292, 2340–2343 CrossRef PubMed7 Brownell, W., Bader, C., Bertrand, D. and de Ribaupierre, Y. (1985) Evoked mechanicalresponses of isolated cochlear outer hair cells. Science 227, 194–196 CrossRef PubMed8 Ashmore, J.F. (1987) A fast motile response in guinea-pig outer hair cells: the cellularbasis of the cochlear amplifier. J. Physiol. 388, 323–347 CrossRef PubMed9 Kachar, B., Brownell, W.E., Altschuler, R. and Fex, J. (1985) Electrokinetic shape changesof cochlear outer hair cells. Nature 322, 365–368 CrossRef10 Gulley, R. and Reese, T. (1977) Regional specialization of the hair cell plasmalemma in theorgan of Corti. Anat. Rec. 189, 109–123 CrossRef PubMed11 Kalinec, F., Holley, M.C., Iwasa, K.H., Lim, D.J. and Kachar, B. (1992) A membrane-basedforce generation mechanism in auditory sensory cells. Proc. Natl. Acad. Sci. U.S.A. 89,8671–8675 CrossRef PubMed12 Zheng, J., Shen, W., He, D.Z.Z., Long, K.B., Madison, L.D. and Dallos, P. (2000) Prestin isthe motor protein of cochlear outer hair cells. Nature 405, 149–155 CrossRef PubMedc© 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution Licence 4.0 (CC BY).Commentary 242713 Sharma, A.K., Rigby, A.C. and Alper, S.L. (2011) STAS domain structure and function.Cell. Physiol. Biochem. 28, 407–422 CrossRef PubMed14 Compton, E.L., Page, K., Findlay, H.E., Haertlein, M., Moulin, M., Zachariae, U., Norman,D.G., Gabel, F. and Javelle, A. (2014) Conserved structure and domain organizationamong bacterial Slc26 transporters. Biochem. J. 463, 297–307CrossRef PubMed15 Schaechinger, T.J., Gorbunov, D., Halaszovich, C.R., Moser, T., Ku¨gler, S., Fakler, B. andOliver, D. (2011) A synthetic prestin reveals protein domains and molecular operation ofouter hair cell piezoelectricity. EMBO J. 30, 2793–2804 CrossRef PubMed16 Geertsma, E.R., Chang, Y.-N., Shaik, F.R., Neldner, Y., Pardon, E., Steyaert, J. and Dutzler,R. (2015) Structure of a prokaryotic fumarate transporter reveals the architecture of theSLC26 family. Nat. Struct. Mol. Biol. 22, 803–808 CrossRef PubMedReceived 22 January 2016/4 April 2016; accepted 5 April 2016Version of Record published 28 July 2016, doi:10.1042/BCJ20160015c© 2016 The Author(s). This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution Licence 4.0 (CC BY).

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Prestin and the good vibrations

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