Role of β-Subunit Domains in the Assembly, Stable Expression, Intracellular Routing, and Functional Properties of Na,K-ATPase

Abstract

The β-subunit of Na,K-ATPase (βNK) interacts with the catalytic α-subunit (αNK) in the ectodomain, the transmembrane, and the cytoplasmic domain. The functional significance of these different interactions was studied by expressing αNK in Xenopus oocytes along with N-terminally modified βNK or with chimeric βNK/βH,K-ATPase (βHK). Complete truncation of the βNK N terminus allows for cell surface-expressed, functional Na,K-pumps that exhibit, however, reduced apparent K+ and Na+ affinities as assessed by electrophysiological measurements. A mutational analysis suggests that these functional effects are not related to a direct interaction of the β N terminus with the αNK but rather that N-terminal truncation induces a conformational change in another functionally relevant β domain. Comparison of the functional properties of αNK·βNK, αNK·βHK, or αNK·βNK/βHK complexes shows that the effect of the βNK on K+ binding is mainly mediated by its ectodomain. Finally, βHK/NK containing the transmembrane domain of βHK produces stable but endoplasmic reticulum-retained αNK·β complexes, while αNK/βHK complexes can leave the ER but exhibit reduced ouabain binding capacity and transport function. Thus, interactions of both the transmembrane and the ectodomain of βNK with αNK are necessary to form correctly folded Na,K-ATPase complexes that can be targeted to the plasma membrane and/or become functionally competent. Furthermore, the β N terminus plays a role in the β-subunit's folding necessary for correct interactions with the α-subunit. The β-subunit of Na,K-ATPase (βNK) interacts with the catalytic α-subunit (αNK) in the ectodomain, the transmembrane, and the cytoplasmic domain. The functional significance of these different interactions was studied by expressing αNK in Xenopus oocytes along with N-terminally modified βNK or with chimeric βNK/βH,K-ATPase (βHK). Complete truncation of the βNK N terminus allows for cell surface-expressed, functional Na,K-pumps that exhibit, however, reduced apparent K+ and Na+ affinities as assessed by electrophysiological measurements. A mutational analysis suggests that these functional effects are not related to a direct interaction of the β N terminus with the αNK but rather that N-terminal truncation induces a conformational change in another functionally relevant β domain. Comparison of the functional properties of αNK·βNK, αNK·βHK, or αNK·βNK/βHK complexes shows that the effect of the βNK on K+ binding is mainly mediated by its ectodomain. Finally, βHK/NK containing the transmembrane domain of βHK produces stable but endoplasmic reticulum-retained αNK·β complexes, while αNK/βHK complexes can leave the ER but exhibit reduced ouabain binding capacity and transport function. Thus, interactions of both the transmembrane and the ectodomain of βNK with αNK are necessary to form correctly folded Na,K-ATPase complexes that can be targeted to the plasma membrane and/or become functionally competent. Furthermore, the β N terminus plays a role in the β-subunit's folding necessary for correct interactions with the α-subunit. endoplasmic reticulum β-subunit of Na,K-ATPase catalytic α-subunit of Na,K-ATPase polymerase chain reaction β-subunit of H,K-ATPase N-methyl-d-glucamine. Maturation of a membrane protein into a functionally active protein is a multistep process starting during co-translational membrane insertion in the endoplasmic reticulum (ER).1 It involves correct orientation in the membrane of transmembrane segments, intermolecular helix-helix interactions in polytopic proteins, transient association with molecular chaperones, and co-translational modifications (for reviews, see Refs. 1High S. Laird V. Trends Cell Biol. 1997; 7: 206-210Abstract Full Text PDF PubMed Scopus (37) Google Scholar and 2Ruddon R.W. Bedows E. J. Biol. Chem. 1997; 272: 3125-3128Abstract Full Text Full Text PDF PubMed Scopus (134) Google Scholar). In addition, in oligomeric membrane proteins, subunit assembly essentially contributes to the structural maturation as well as to a fine control of the protein's function (for a review, see Ref. 3Geering K. von Heijne Membrane Protein Assembly. Springer-Verlag, Heidelberg, Germany1997: 173-182Google Scholar). Still, little is known of the molecular determinants in oligomeric proteins that are involved in subunit assembly and of the particular structural and/or functional importance of different interaction sites. In this study, we have addressed these questions for the oligomeric Na,K-ATPase. Na,K-ATPase is a ubiquitous plasma membrane enzyme that is responsible for the K+ and Na+ homeostasis of animal cells (for a review, see Ref. 4Horisberger J.-D The Na,K-ATPase: Structure-Function Relationship. R. G. Landes Co., Austin, TX1994: 1-130Google Scholar). The functionally active enzyme is a heterodimeric protein that transports K+ and Na+ against their electrochemical gradients by using the energy of the hydrolysis of ATP. The catalytic α-subunit is a polytopic membrane protein with 10 transmembrane segments that contains the binding sites for the cations, for ATP and phosphate, and for drugs such as cardiac glycosides. The β-subunit is a type II membrane protein with a short cytoplasmic N terminus, one transmembrane segment, and an ectodomain that carries three disulfide bridges and several sugar chains. The β-subunit plays an essential role in the maturation of the catalytic α-subunit. Synthesized alone, the α-subunit is rapidly degraded in or close to the ER, and only assembly with β-subunits permits a structural maturation of the α-subunit, which protects it from cellular degradation and renders it functionally active (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google Scholar). In to this structural the β-subunit the transport properties of Na,K-pumps the cell have that the β-subunit is involved in the apparent of Na,K-ATPase (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google J. Biol. Chem. Full Text PDF PubMed Google G. S. S. J. Biol. Chem. Full Text PDF PubMed Google P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google S. PubMed Scopus Google J. Biol. Chem. Full Text PDF PubMed Google P. K. J. PubMed Scopus Google A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). suggests that the β-subunit interacts with the α-subunit which are in the ectodomain, the transmembrane, and the cytoplasmic domain. using the it was that in the ectodomain, the transmembrane and of the α-subunit interacts with a β domain to the transmembrane J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). interaction is in the 10 of the β-subunit P. P. K. PubMed Scopus Google Scholar). that the in the membrane with the β in with of the membrane domain of the α-subunit A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). Finally, that the cytoplasmic N terminus of the β-subunit interacts with the α-subunit (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google Scholar). The functional of the interactions have not that β assembly with the the correct of the membrane domain and in the of the α-subunit against cellular degradation P. Beggah A. K. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). that interaction of the transmembrane domain of the β-subunit be for and stable assembly with the α-subunit P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google J. Biol. Chem. Full Text PDF PubMed Google Scholar). the both ectodomain P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google S. PubMed Scopus Google J. Biol. Chem. Full Text PDF PubMed Google and cytoplasmic (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google interactions of the β-subunit to a role in the K+ binding process of the have that truncation of the cytoplasmic N terminus of the β-subunit not the of the α-subunit but the apparent for K+ of the Na,K-ATPase (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google Scholar). an of cytoplasmic interactions in the control of K+ In this study, we to the functional role of the β N terminus and to the functionally relevant molecular that β N-terminal truncation not only the apparent for K+ but that of Na+ of the mutational analysis suggests that the β N terminus is not involved in K+ and Na+ binding direct interaction with the α-subunit but rather by the effect of this domain on the correct folding of the with the functional properties of chimeric that the cytoplasmic the transmembrane domain but rather the ectodomain of the β-subunit is involved in the β-subunit's effects on the functional properties of Na,K-ATPase. and and using a Xenopus Na,K-ATPase β (βNK) in a by the by and PubMed Scopus Google Scholar). The and (for of see as (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google Scholar). The for the of the the and the a the for the which to the into a type with and the of of by using an of by of and PubMed Scopus Google and containing the The as to the and using a of the and of and PubMed Scopus Google Scholar). The into type using and sites and as for or in which or was as a The of the the for the the for and for or by the for of the The was as for the of and as for the of in which or in which the cytoplasmic and transmembrane from Xenopus Na,K-ATPase (βNK) and the ectodomain from H,K-ATPase and with the orientation as P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google Scholar). of the containing the cytoplasmic domain of βNK and the transmembrane and the ectodomain of a of the βNK N terminus was using a of the and an containing for of βNK and of the of the transmembrane domain of βHK was using an of βHK and a of βNK and of The by and sites to the β into of the containing the cytoplasmic domain of the βHK and the transmembrane and ectodomain of the one the of βHK and was using a of the and an of βHK and of as a another the transmembrane domain of βNK was using an of βNK and a of βHK and of The by and sites to the β into The of by for Na,K-ATPase J. Biol. Chem. Full Text PDF PubMed Google Na,K-ATPase P. E. P. K. J. Google β H,K-ATPase G. S. A. PubMed Scopus Google and β by in K. PubMed Scopus Google Scholar). from of as K. J. PubMed Google Scholar). of and/or of β into in modified containing for and to a of in the of 10 as (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google and the and β-subunits of Na,K-ATPase and the β-subunit of H,K-ATPase or as P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google with a Xenopus β K. PubMed Scopus Google or a K. D. J. Scopus Google Scholar). The complexes by and proteins by of was with a was as the using the as P. K. J. PubMed Scopus Google Scholar). of oocytes with different β the apparent K+ of oocytes with Na+ in a containing of a that the Na,K-pumps but not the Na,K-pumps J. PubMed Scopus Google Scholar). The of by K+ was in a 10 10 or in a was by The by of K+ and 10 and was the and the was to the of the by K+ using a to P. K. J. PubMed Scopus Google the was to a of for in the of Na+ and for in the of of the for Na+ by a K. and in The of this is to the with the and to the to the by of the of the to the with Na+ by a to a in the of In in to Na,K-ATPase and β oocytes with for and of the Na+ G. PubMed Scopus Google Scholar). oocytes for in a modified containing 10 the the the oocytes in a 10 in to the of of and of the on of the oocytes to their Na+ by to a Na+ 10 in the of and a of to from the of the from a of with and in a containing Na+ 10 The was in the by the of of K+ in the of we a of of and of the was and for the and to for the measurements. to a only oocytes with a of and for which a of was the of the by Na+ by the to the and was with the as a to of close to the was using of and The and with these but the of was with The are with a of for oocytes in modified for of as (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google and in a containing and of of protein for with of of protein the of a containing and and of rapidly on three with of an containing and and by the of of binding was in the of a of ouabain and to of the have that the cytoplasmic N terminus of the β-subunit interacts with the α-subunit (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google Scholar). interaction is not necessary for the structural maturation of the α-subunit. the truncation of the β N terminus produces Na,K-pumps the cell that exhibit a apparent K+ with type Na,K-pumps (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google that β N-terminal interaction with the α-subunit have a role in the functional properties of the α-subunit. In to the in the cytoplasmic N terminus of the β-subunit that be for this functional we a of Xenopus Na,K-ATPase β with or in the N The β in Xenopus oocytes with Na,K-ATPase and the cellular and the transport properties of the complexes by of proteins and by electrophysiological for Xenopus Na,K-ATPase (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google the β in which of the N terminus of Xenopus and the the to the transmembrane (for of see was to with and Na,K-ATPase during a to type β-subunits and The complexes containing the type β-subunit or the A and and that are targeted to the plasma with to of the α-subunit, and ER of the complexes with β truncation and as well as with a in which to in the β N terminus and and β active Na,K-pumps the cell as by and and The but not different from of type complexes in different of The apparent affinities for K+ of the type and Na,K-pumps in the or of Na+ and In both the K+ of Na,K-pumps containing the type β-subunit or the β was but in Na,K-pumps containing the or the β A in the with type was in Na,K-pumps containing the β in particular in the of that and in the β N terminus be in the apparent for K+ of the α-subunit. the functional role of and in the β N terminus, we the K+ of Na,K-pumps containing the β in which this domain was and that of the complexes was not different from that of the type complexes both in the and and and of for this is that of for the of in the β this we a of β containing or in the domain in with in the in and the and the apparent K+ of the Na,K-pumps association of these β Na,K-ATPase β with and to type and the complexes the ER as by the of type of the β-subunits a and with complexes well the plasma membrane and In to the in the K+ in the of Na+ for Na,K-pumps containing the β in the type Na,K-pumps and of the Na,K-pumps containing β with in the N terminus The of a functional effect of and in the β N terminus suggests that the effects on the apparent K+ in Na,K-pumps containing the β not be to a of a direct interaction the α-subunit and a in the cytoplasmic β N this we a analysis of the which a type or in the of the β-subunit by or in and the effects of these on the and the functional properties of complexes β in which or by or to form stable complexes and with which β in which by or to form stable these complexes in the ER in their form a which a be the cell with oocytes expressing type complexes and for the β β of this Na,K-pumps with to the of the in the apparent K+ was Na,K-pumps containing type β-subunits or the the or the β and Na,K-pumps containing the or the β a but in the with type but this change was that with Na,K-pumps containing the β these that truncation of the β N terminus the apparent K+ of Na,K-pumps but that a short can or the type the of different produces a it is that and are involved in a interaction the β N terminus and the α-subunit that the effects of the β-subunit on the apparent K+ of rather that of the β N terminus the of the ectodomain and/or the transmembrane domain of the which in functionally interactions the and the α-subunit. truncation of the β N terminus to that the β-subunit to the apparent K+ of we functional effects of the for on the Na+ be with the β The apparent for Na+ of Na,K-pumps containing type or β-subunits was in oocytes the Na+ which to and control the shows that for Na+ was for for type complexes for type for was the for the in the apparent for Na+ of the complexes not to be to a of a interaction of the cytoplasmic β N terminus with the α-subunit. The apparent for Na+ was in type Na,K-pumps and Na,K-pumps containing the β in which the N terminus of only A and The that the effects of the of the β N terminus on the K+ and Na+ of Na,K-pumps be to a change in the of the β ectodomain and/or transmembrane domain. have that Na,K-pumps of and βHK have a apparent Na,K-pumps of αNK and βNK P. K. J. PubMed Scopus Google or of Xenopus αNK and Xenopus βNK P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google Scholar). which of the β-subunit in the Na,K-ATPase for we the and the functional properties of Na,K-pumps containing βNK and βHK with the the cytoplasmic and transmembrane domain from Xenopus βNK and the ectodomain from βHK the cytoplasmic domain from βNK and the transmembrane and ectodomain from βHK the cytoplasmic and transmembrane domain from βHK and the ectodomain from βNK and the cytoplasmic domain from βHK and the transmembrane and ectodomain from βNK and complexes and and complexes complexes the for of these complexes, we the K+ for Na,K-ATPase complexes and P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google that a in with that of complexes and Furthermore, that the αNK·β complexes that the ectodomain of the βHK to that of complexes and with while αNK·β complexes containing the ectodomain of the βNK have to that of complexes and with from αNK·β complexes in Xenopus which an apparent that was that of and complexes P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google Scholar). It is that in this study, the of the apparent K+ by the of The of the in which the Na,K-pumps that the in the apparent for K+ by different β-subunits is to be to a in the ectodomain and not in the transmembrane domain of the A the role of β-subunit interaction with the α-subunit was and complexes as complexes but a was for and this we studied the the and the cellular of these in βNK and βHK and and β to αNK during a and and complexes containing the transmembrane domain of βNK and and leave the ER and in with their cell as by the the complexes that the transmembrane domain of the βHK mainly in the ER in their a that the the cell are relevant in the of the that in the the P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google and not can leave the ER as the type but the P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google and not in the ER as the type that and in the transmembrane domain and that this interaction is to form complexes that can the In to the αNK·β complexes, the in oocytes expressing complexes not be by ER of these complexes during the and that have a that the complexes the plasma membrane and/or that the complexes are functionally have that ouabain binding to oocytes is to P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google we in this the ouabain binding to of oocytes expressing Xenopus and complexes and the ouabain binding with the of the different Na,K-ATPase shows that ouabain binding to of oocytes was that of oocytes but of that in of oocytes expressing the the protein of the complexes of the of the complexes The for ouabain for and that a of the complexes are in a with ouabain binding capacity and function. In this study, we that the β-subunit of Na,K-ATPase the transport properties of Na,K-ATPase. that the β-subunit an effect not only on the apparent for K+ but on that for that direct interaction with the α-subunit of the cytoplasmic β or the β transmembrane domain is not involved in the K+ and Na+ of Na,K-ATPase but that the ectodomain of the β-subunit plays an role in the of the functional properties of have that the cytoplasmic N terminus of the β-subunit of Na,K-ATPase is to the β-subunit is in Xenopus but it the β-subunit is with the α-subunit and is in the of K+ but not in the of Furthermore, we that of the β N terminus the apparent for K+ of the Na,K-pumps (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google Scholar). that a interaction the β N terminus and the α-subunit, which be responsible for the functional in this we that and in the β N terminus not change the apparent K+ of the complexes and that the of a of different to the β N terminus the functional effect of the truncation on the apparent of K+ and Na+ for their binding sites on the Na,K-ATPase. we the that a the N terminus be for the β-subunit to with the α-subunit and to effects on the Na,K-ATPase as an β N terminus, it is that truncation of the β N terminus to conformational in of the β-subunit that be involved in Na,K-ATPase A for the by and A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google that of the N terminus of a β in a membrane by to the of the ectodomain in these the of the β be to of the N that N-terminal the of the β-subunit is by type II proteins, the J. Biol. Chem. Full Text PDF PubMed Google and the chain of II J. Full Text PDF PubMed Scopus Google Scholar). of the N terminus of these proteins the transmembrane to its the a on the of the ER the β-subunit of structural N-terminal truncation to be as well as the against these by with different In β the domain was in to the which is mainly responsible for the orientation of transmembrane segments Heijne G. PubMed Scopus Google Scholar). The of this be a that structural in the β-subunit are as by the that the β-subunit with the α-subunit, and produces functional Na,K-pumps the cell (5Geering K. Beggah A. Good P. Girardet S. Roy S. Schaer D. P. J. Cell Biol. PubMed Scopus Google Scholar). is in but not in or of Na,K-ATPase in β-subunits of the structural for correct membrane of type II proteins, it be to the and functional of N-terminal in or β-subunits this that the functional effects with N-terminally β-subunits be mediated by structural in the ectodomain of the the of the functional properties of chimeric and H,K-ATPase β-subunits shows that it is this domain that is mainly in the apparent K+ the of the ectodomain of βNK or of βHK in the or is necessary and to Na,K-pumps that exhibit apparent K+ affinities to containing type βNK or βHK The role of the β ectodomain in the K+ binding properties of the Na,K-ATPase in this and by several using different P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google S. PubMed Scopus Google J. Biol. Chem. Full Text PDF PubMed Google Scholar). for a role of the β ectodomain for H,K-ATPase (for a review, see Ref. J. Biol. PubMed Google Scholar). The sites of interaction in the and the β-subunits and the of involved in the β-subunit's effects on the K+ binding to be on in the of different and S. PubMed Scopus Google have that binding a conformational change in the the disulfide in the β-subunit and interactions with the α-subunit, which of to its binding from the In of the the and the it is that the K+ transport function is mediated by a conformational change of both rather the β-subunit a functional effect on a reaction of the α-subunit J. Biol. PubMed Google Scholar). In this study, we that the β ectodomain plays a role in the Na+ binding function of Na,K-ATPase. is with that the for disulfide in the β-subunit of Na,K-ATPase can be not only by K+ but by Na+ J. Biol. Chem. Full Text PDF PubMed Google K. K. PubMed Scopus Google and with the that a β with αNK produces Na,K-pumps with a reduced Na+ for from ATP J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). In this it is that in the and be of a membrane that is involved in the control of and/or of Na+ from the cytoplasmic G. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). this is in close to the β interaction J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google it be that the β-subunit the of this domain. The analysis of the β that interaction of the transmembrane domain not a role in the β-subunit's effect on the K+ binding function of Na,K-ATPase. that membrane interactions the and the β-subunits and that these interactions be for The transmembrane domain of the Na,K-ATPase β-subunit be by the domain of the H,K-ATPase β-subunit on the of the complexes these chimeric β-subunits and a of functional Na,K-pumps the cell as assessed by of the complexes are in the ER, which of the β in with Na,K-ATPase of ouabain binding sites in a cell membrane type Na,K-ATPase β-subunits and ouabain binding of these complexes is J. Biol. Chem. Full Text PDF PubMed Google Scholar). S. K. 1997; PubMed Scopus Google to functional of complexes in to the Na,K-ATPase the in ER of complexes is in complexes that the type H,K-ATPase complexes become a that have a that the of the βHK ectodomain and/or cytoplasmic domain can for the of the of the βNK transmembrane complexes are to complexes S. K. 1997; PubMed Scopus Google Scholar). have with oocytes expressing complexes, oocytes expressing complexes a of the and the cell ouabain binding P. Beggah K. P. K. J. Cell Biol. PubMed Scopus Google that the of the Na,K-pumps the cell is to that of In this study, we that this of the Xenopus βHK complexes stable ER and a cellular to that of the ouabain binding capacity of complexes was reduced with that of and the for ouabain of these complexes is not different S. A. PubMed Scopus Google Scholar and this these that of the as S. K. 1997; PubMed Scopus Google αNK with βHK in not to Na,K-ATPase or the and of S. K. 1997; PubMed Scopus Google with with βHK complexes in which a of ouabain binding sites as complexes S. A. PubMed Scopus Google J. 1997; 272: PubMed Google Scholar). in these ouabain binding in the of of ATP and It is that these ouabain to Na,K-ATPase with conformational It was that αNK stable complexes with βHK with with the ouabain binding capacity of complexes J. Biol. Chem. Full Text PDF PubMed Google Scholar). The that complexes are and functionally proteins with complexes be functionally relevant in such as the both βNK and βHK are and the of In the in this that correct assembly of αNK with βNK is mediated by a of transmembrane and ectodomain of one domain but of interaction with the domain to structural and functional of Na,K-ATPase. that the ectodomain and not the transmembrane domain of the β-subunit is involved in the β-subunit's functional effects on both the K+ and Na+ of the the of correct interaction in the transmembrane domain structural that are in ER or R. J. 1997; Google Scholar). Finally, the functional role of the interaction of the β N terminus with the α-subunit to be but that the β N terminus is essential for a correct of the β-subunit and for the functional role of the β-subunit in the transport of the The for the β-subunit of H,K-ATPase and the for the and of the Na+ by G. and