ENaC at the Cutting Edge: Regulation of Epithelial Sodium Channels by Proteases

Epithelial Na+ channels facilitate the transport of Na+ across high resistance epithelia. Proteolytic cleavage has an important role in regulating the activity of these channels by increasing their open probability. Specific proteases have been shown to activate epithelial Na+ channels by cleaving channel subunits at defined sites within their extracellular domains. This minireview addresses the mechanisms by which proteases activate this channel and the question of why proteolysis has evolved as a mechanism of channel activation. Epithelial Na+ channels facilitate the transport of Na+ across high resistance epithelia. Proteolytic cleavage has an important role in regulating the activity of these channels by increasing their open probability. Specific proteases have been shown to activate epithelial Na+ channels by cleaving channel subunits at defined sites within their extracellular domains. This minireview addresses the mechanisms by which proteases activate this channel and the question of why proteolysis has evolved as a mechanism of channel activation. Many ion channels are silent at rest and are activated in response to a variety of factors, including membrane potential, external ligands, and intracellular signaling processes. The ENaC 2The abbreviations used are: ENaCepithelial Na+ channelPoopen probabilityCAPchannel-activating proteaseASICacid-sensing ion channel. 2The abbreviations used are: ENaCepithelial Na+ channelPoopen probabilityCAPchannel-activating proteaseASICacid-sensing ion channel. has evolved as a channel that is thought to reside primarily in an active state, facilitating the bulk movement of Na+ out of renal tubular or airway lumens. The regulated insertion and retrieval of channels at the plasma membrane have important roles in modulating ENaC-dependent Na+ transport (1Snyder P.M. Endocrinology. 2005; 146: 5079-5085Crossref PubMed Scopus (194) Google Scholar). A number of factors also have a role in regulating ENaC activity via changes in channel Po or gating. In this regard, it has become increasingly apparent that proteolysis of ENaC subunits has a key role in this process (2Hughey R.P. Carattino M.D. Kleyman T.R. Curr. Opin. Nephrol. Hypertens. 2007; 16: 444-450Crossref PubMed Scopus (67) Google Scholar). This minireview addresses several questions regarding the role of ENaC subunit proteolysis in regulating channel gating. (i) Where are ENaC subunits cleaved? (ii) Which proteases mediate ENaC cleavage? (iii) Why are channels activated by proteolysis? (iv) Is proteolysis responsible, in part, for the highly variable channel Po that has been noted for ENaC? (v) Why have ENaCs evolved as channels that require proteolysis for activation? epithelial Na+ channel open probability channel-activating protease acid-sensing ion channel. epithelial Na+ channel open probability channel-activating protease acid-sensing ion channel. Reports in the early 1980s that serine protease inhibitors reduced transepithelial Na+ transport across toad urinary bladder suggested that proteases have a role in activating ENaC (3Orce G.G. Castillo G.A. Margolius H.S. Am. J. Physiol. Renal Physiol. 1980; 239: F459-F465Crossref PubMed Google Scholar). A series of studies over the past decade have confirmed that proteases activate ENaC and have begun to elucidate the mechanism by which this occurs. Following the observation that ENaC activity was significantly reduced in epithelial cells treated with aprotinin and that low concentrations of external trypsin rapidly activated ENaC in aprotinin-pretreated cells, a series of CAPs were identified that activated ENaC when coexpressed in heterologous expression systems (4Vallet V. Chraibi A. Gaeggeler H.P. Horisberger J.D. Rossier B.C. Nature. 1997; 389: 607-610Crossref PubMed Scopus (451) Google Scholar, 5Chraïbi A. Vallet V. Firsov D. Hess S.K. Horisberger J.D. J. Gen. Physiol. 1998; 111: 127-138Crossref PubMed Scopus (168) Google Scholar, 6Vuagniaux G. Vallet V. Jaeger N.F. Hummler E. Rossier B.C. J. Gen. Physiol. 2002; 120: 191-201Crossref PubMed Scopus (199) Google Scholar). Furthermore, channels with a very low Po responded to external trypsin with a dramatic increase in Po (7Caldwell R.A. Boucher R.C. Stutts M.J. Am. J. Physiol. Cell Physiol. 2004; 286: C190-C194Crossref PubMed Scopus (169) Google Scholar). What is the target of these proteases? ENaC is composed of three structurally related subunits (α, β, and γ) that have two membrane-spanning domains connected by a large extracellular loop composed of ∼450 residues. Early reports suggested that ENaC subunits or closely associated proteins were the protease target (5Chraïbi A. Vallet V. Firsov D. Hess S.K. Horisberger J.D. J. Gen. Physiol. 1998; 111: 127-138Crossref PubMed Scopus (168) Google Scholar). Subsequent studies demonstrated that the α and γ subunits of ENaC were processed by proteases (8Hughey R.P. Mueller G.M. Bruns J.B. Kinlough C.L. Poland P.A. Harkleroad K.L. Carattino M.D. Kleyman T.R. J. Biol. Chem. 2003; 278: 37073-37082Abstract Full Text Full Text PDF PubMed Scopus (235) Google Scholar, 9Hughey R.P. Bruns J.B. Kinlough C.L. Kleyman T.R. J. Biol. Chem. 2004; 279: 48491-48494Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar, 10Masilamani S. Kim G.H. Mitchell C. Wade J.B. Knepper M.A. J. Clin. Invest. 1999; 104: R19-R23Crossref PubMed Scopus (623) Google Scholar, 11Ergonul Z. Frindt G. Palmer L.G. Am. J. Physiol. Renal Physiol. 2006; 291: F683-F693Crossref PubMed Scopus (131) Google Scholar). The presence of full-length forms as well as faster migrating forms of the α and γ subunits on SDS-polyacrylamide gels, both in cell lysates and at the plasma membrane, provided the first clue that channel subunits were processed by proteases. Furthermore, the size of the cleaved fragments helped to define the sites of proteolysis (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). Functionally relevant cleavage sites were identified within the proximal regions of the extracellular domains of the α and γ subunits, as mutations of putative protease consensus cleavage sites prevented both subunit cleavage and channel activation (Fig. 1) (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar, 13Bruns J.B. Carattino M.D. Sheng S. Maarouf A.B. Weisz O.A. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: 6153-6160Abstract Full Text Full Text PDF PubMed Scopus (265) Google Scholar, 14Passero C.J. Mueller G.M. Rondon-Berrios H. Tofovic S.P. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2008; 283: 36586-36591Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar). Subsequent studies also showed that proteolytic processing of subunits within a channel complex was an all-or-none event (9Hughey R.P. Bruns J.B. Kinlough C.L. Kleyman T.R. J. Biol. Chem. 2004; 279: 48491-48494Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar). Jasti et al. (15Jasti J. Furukawa H. Gonzales E.B. Gouaux E. Nature. 2007; 449: 316-323Crossref PubMed Scopus (849) Google Scholar) recently resolved the crystal structure of the acid-sensing ion channel ASIC1, a member of the ENaC/degenerin ion channel family. This structure has provided important insights into the structural organization of ASIC and related family members, such as ENaC. The extracellular domain of ASIC1 has a highly ordered structure that resembles an outstretched hand containing a ball and has defined subdomains termed wrist, finger, thumb, palm, β-ball, and knuckle (Fig. 2) (15Jasti J. Furukawa H. Gonzales E.B. Gouaux E. Nature. 2007; 449: 316-323Crossref PubMed Scopus (849) Google Scholar). ASIC1 proton-dependent gating has been proposed to occur in conjunction with conformational changes within the thumb and finger domains, which are transmitted to the wrist region and eventually to the transmembrane domains, where the channel's gate likely resides (15Jasti J. Furukawa H. Gonzales E.B. Gouaux E. Nature. 2007; 449: 316-323Crossref PubMed Scopus (849) Google Scholar). Sites of ENaC subunit proteolysis that have been shown to be functionally relevant are within the “finger” domains and are likely located at peripheral sites that would be expected to be accessible to proteases (Fig. 2). As the finger domains are not conserved among members of the ENaC/degenerin family (15Jasti J. Furukawa H. Gonzales E.B. Gouaux E. Nature. 2007; 449: 316-323Crossref PubMed Scopus (849) Google Scholar), the structures of the finger domains of ENaC subunits are likely to differ significantly from the resolved structure of the ASIC1 finger domain. Even within the three ENaC subunits, there are notable differences within the finger domains. For example, the finger domain of the β subunit lacks protease cleavage sites and instead exhibits three consensus sites for N-linked glycosylation and a unique pair of Cys residues (Fig. 1).FIGURE 2Structure of an ASIC1 subunit. The extracellular domain of ASIC1 is a highly ordered structure that resembles an outstretched hand containing a ball (15Jasti J. Furukawa H. Gonzales E.B. Gouaux E. Nature. 2007; 449: 316-323Crossref PubMed Scopus (849) Google Scholar). Defined subdomains are highlighted. Sites of proteolysis are primarily within the corresponding finger domain of ENaC. TM1 and TM2, first and second transmembrane domains.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Additional cleavage sites within the distal regions of the extracellular domains of ENaC subunits have been described (11Ergonul Z. Frindt G. Palmer L.G. Am. J. Physiol. Renal Physiol. 2006; 291: F683-F693Crossref PubMed Scopus (131) Google Scholar, 16Michlig S. Harris M. Loffing J. Rossier B.C. Firsov D. J. Biol. Chem. 2005; 280: 38264-38270Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar, 17Myerburg M.M. Butterworth M.B. McKenna E.E. Peters K.W. Frizzell R.A. Kleyman T.R. Pilewski J.M. J. Biol. Chem. 2006; 281: 27942-27949Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar). Recent studies examining the regulation and processing of ENaC subunits by the protease CAP2 (or TMPRSS4) have located cleavage sites within the “palm” domains of all three subunits (18García-Caballero A. Dang H. Stutts M.J. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar). mutations that cleavage in the domains not ENaC that cleavage at these sites is not in regulating channel the structure of the domain in ASIC1 that is by it is that cleavage at sites within these domains in ENaC subunits not The of the cleaved α and γ subunit fragments suggested that proteolysis within a region of the extracellular loop that is in residues and that consensus cleavage sites (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). is a member of the family of serine proteases that resides primarily within the and the consensus of where is G. Cell Biol. 2002; PubMed Scopus Google Scholar). of ENaC subunits with mutations of key residues within the consensus sites that α subunits were cleaved at two sites and that γ subunits were cleaved at a (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). Na+ in were reduced by to when these sites were to that proteolysis was for channels to be active (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). mutations in the α subunit ENaC activity by to a in was when the γ subunit was (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). channel activity was when these channels were to the protease that the channels were at the plasma membrane in a functionally (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). cell Na+ were also reduced by when ENaC was in cells with cells and channel activity was by of ENaC and (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). Furthermore, inhibitors reduced Na+ in cells ENaC (12Hughey R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar). to have a role in the processing of ENaC subunits, it is likely that members of the family and activate ENaC. Additional proteases have been shown to process the γ subunit and activate the channel. (or is a serine protease likely on the of renal and airway (4Vallet V. Chraibi A. Gaeggeler H.P. Horisberger J.D. Rossier B.C. Nature. 1997; 389: 607-610Crossref PubMed Scopus (451) Google Scholar, A. G. J. Boucher R.C. J. Biol. Chem. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). cleavage of the γ subunit at a defined distal to the identified cleavage ENaC J.B. Carattino M.D. Sheng S. Maarouf A.B. Weisz O.A. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: 6153-6160Abstract Full Text Full Text PDF PubMed Scopus (265) Google Scholar) and is to activate ENaC (18García-Caballero A. Dang H. Stutts M.J. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar). proteases have been shown to both activate ENaC and the γ subunit at sites distal to the including CAP2 and C.J. Mueller G.M. Rondon-Berrios H. Tofovic S.P. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2008; 283: 36586-36591Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar, A. Dang H. Stutts M.J. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar, A. H. J. Gen. Physiol. 2007; PubMed Scopus Google Scholar, M. Firsov D. G. Stutts M.J. Rossier B.C. J. Biol. Chem. 2007; 282: Full Text Full Text PDF PubMed Scopus Google Scholar). CAP2 was also shown to cleavage at the consensus of the γ subunit (18García-Caballero A. Dang H. Stutts M.J. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar). channel activation was when this was with CAP2 cleavage of the γ subunit at sites (18García-Caballero A. Dang H. Stutts M.J. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar). of a in the cleavage of the γ subunit in channel activation as well as cleavage at this by an A. M. C. J. Physiol. 2008; PubMed Scopus Google Scholar). (or the it has not been shown that this protease ENaC G. Vallet V. Jaeger N.F. Hummler E. Rossier B.C. J. Gen. Physiol. 2002; 120: 191-201Crossref PubMed Scopus (199) Google Scholar). ENaC and subunit cleavage at sites that are in the of the cleavage sites P.M. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar). with that expression that this protease have a role in processing the γ subunit D. S. C. S. Q. M. G. Knepper M.A. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar), proteases also have a role in processing the γ subunit and activating ENaC in the of S. Kim G.H. Mitchell C. Wade J.B. Knepper M.A. J. Clin. Invest. 1999; 104: R19-R23Crossref PubMed Scopus (623) Google Scholar, 11Ergonul Z. Frindt G. Palmer L.G. Am. J. Physiol. Renal Physiol. 2006; 291: F683-F693Crossref PubMed Scopus (131) Google Scholar, G. Z. Palmer L.G. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar). a number of proteases and activate ENaC. is likely that and not the of proteases that and activate ENaC. In proteases are likely to activate ENaC by cleaving and activating proteases that the channel. For example, is as a that be cleaved to be active not cleavage A. H. Harris 2004; PubMed Scopus Google Scholar). is of the proteases that and S. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar). et al. A. J. J. Biol. Chem. 2007; 282: Full Text Full Text PDF PubMed Scopus Google Scholar) have suggested that cleavage of also have a role in regulating ENaC proteases have an important role in cleaving and activating a number of important questions to be (i) Which are the key proteases that and activate ENaC in (ii) there proteases that and activate ENaC? (iii) Is ENaC proteolysis a regulated (iv) Is there expression of proteases that and activate ENaC and studies have begun to the question of ENaC proteolysis is a regulated The expression of as well as protease an of and serine be regulated by M. H. J. J. Clin. Invest. 2002; PubMed Scopus Google Scholar, A. M. V. H. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar). ENaC at the plasma membrane the of α and γ subunit cleavage P.M. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar, P.M. 2006; PubMed Scopus Google Scholar). were associated with a of subunit factors, including signaling increase ENaC at the plasma membrane (1Snyder P.M. Endocrinology. 2005; 146: 5079-5085Crossref PubMed Scopus (194) Google Scholar, S. Physiol. 2002; PubMed Scopus Google Scholar). Furthermore, of Na+ or changes in intracellular proteolytic processing of the α and γ subunits P.M. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar). A reduced of Na+ to cleavage of channel by which of Na+ channel cleavage to be have also begun to the question of the of proteases that and activate ENaC are in studies that ENaC activation by to the renal Na+ and that are in C.J. Mueller G.M. Rondon-Berrios H. Tofovic S.P. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2008; 283: 36586-36591Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar, C. M. S. J. T.R. C. J. Am. Nephrol. PubMed Scopus Google Scholar). ENaC by cleaving the γ subunit at a distal to the Furthermore, as well as is in the of both and with C.J. Mueller G.M. Rondon-Berrios H. Tofovic S.P. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2008; 283: 36586-36591Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar, C. M. S. J. T.R. C. J. Am. Nephrol. PubMed Scopus Google Scholar). studies that when the is is and to activate by that is in the of renal studies have suggested that there is cleavage of ENaC subunits in airway in the of M.M. Butterworth M.B. McKenna E.E. Peters K.W. Frizzell R.A. Kleyman T.R. Pilewski J.M. J. Biol. Chem. 2006; 281: 27942-27949Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar, Boucher R.C. J. Gen. Physiol. 2006; PubMed Scopus Google Scholar) as well as in the distal in the of and S. Kim G.H. Mitchell C. Wade J.B. Knepper M.A. J. Clin. Invest. 1999; 104: R19-R23Crossref PubMed Scopus (623) Google Scholar, G. Z. Palmer L.G. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar). ENaC activation in airway to the in airway and the in M. Boucher R.C. 2004; PubMed Scopus Google Scholar). The proteases for the cleavage of ENaC subunits in these to be it that proteases activate ENaC by cleaving subunits, the question to was channel activation occurs. there a where an increase in subunit cleavage would facilitate from a to an open cleavage at a be to activate the channel. cleavage at a in the α subunit was not to activate ENaC P.M. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar, M.D. Sheng S. Bruns J.B. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar). α subunits to be cleaved at both sites for channels to that the the cleavage sites in the α subunit as an that the channel in the M.D. Sheng S. Bruns J.B. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar). In of this channels both α subunit cleavage sites and the these sites were to be the α subunit was not cleaved M.D. Sheng S. Bruns J.B. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar). Furthermore, a corresponding to the that is from the α subunit by cleavage ENaC by channel Po M.D. Sheng S. Bruns J.B. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar). The key region within these residues was identified as an that is highly conserved among M.D. C.J. Maarouf A.B. Pilewski J.M. M.M. Hughey R.P. Kleyman T.R. Am. J. Physiol. Renal Physiol. 2008; PubMed Scopus Google Scholar). the γ subunit also to be cleaved to activate the to this with a γ subunit the and cleavage sites and the activity to a very high the γ subunit was not cleaved J.B. Carattino M.D. Sheng S. Maarouf A.B. Weisz O.A. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: 6153-6160Abstract Full Text Full Text PDF PubMed Scopus (265) Google Scholar). A corresponding to the from the γ subunit by and cleavage was also a ENaC J.B. Carattino M.D. Sheng S. Maarouf A.B. Weisz O.A. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: 6153-6160Abstract Full Text Full Text PDF PubMed Scopus (265) Google Scholar). As proteases to activate ENaC by cleaving the γ subunit at sites distal to the cleavage J.B. Carattino M.D. Sheng S. Maarouf A.B. Weisz O.A. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: 6153-6160Abstract Full Text Full Text PDF PubMed Scopus (265) Google Scholar, A. H. J. Gen. Physiol. 2007; PubMed Scopus Google Scholar, M. Firsov D. G. Stutts M.J. Rossier B.C. J. Biol. Chem. 2007; 282: Full Text Full Text PDF PubMed Scopus Google Scholar, D. S. C. S. Q. M. G. Knepper M.A. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar). has that the mechanism by which CAP2 the channel is by γ subunit cleavage at the their also a role for cleavage of the γ subunit at sites distal to the in activating ENaC (18García-Caballero A. Dang H. Stutts M.J. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar). proteolysis channels by from the α and γ subunits, these the As these are located within the finger regions of the extracellular domains (Fig. 2). Jasti et al. (15Jasti J. Furukawa H. Gonzales E.B. Gouaux E. Nature. 2007; 449: 316-323Crossref PubMed Scopus (849) Google Scholar) have proposed that conformational changes within the thumb and finger domains are for ASIC1 gating. this is also for the movement of the thumb and finger domains. ENaC by γ subunit cleavage at a defined that have or proteolytic activity also activate the channel J.B. Carattino M.D. Sheng S. Maarouf A.B. Weisz O.A. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: 6153-6160Abstract Full Text Full Text PDF PubMed Scopus (265) Google Scholar, D. G. Hummler E. Rossier B.C. J. Am. Nephrol. 2006; PubMed Scopus Google Scholar). proteolysis is to activate why these activate the very proteolytic activity for cleavage to activate the channel J.B. Carattino M.D. Sheng S. Maarouf A.B. Weisz O.A. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: 6153-6160Abstract Full Text Full Text PDF PubMed Scopus (265) Google to channels where γ subunits have been processed by the from within the ENaC complex and facilitating a to an open at a channel in ENaCs a of Po from to J. Frindt G. Palmer L.G. J. Gen. Physiol. PubMed Scopus Google Scholar). Why is ENaC Po What are the mechanisms that this variable A number of factors, such as and channel Po V. Stockand J.D. Curr. Opin. Nephrol. Hypertens. 2008; PubMed Scopus Google Scholar, C. Biol. 2004; PubMed Scopus Google Scholar). subunits cleavage have been at the plasma membrane in as well as in heterologous expression systems (9Hughey R.P. Bruns J.B. Kinlough C.L. Kleyman T.R. J. Biol. Chem. 2004; 279: 48491-48494Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar, M. Firsov D. G. Stutts M.J. Rossier B.C. J. Biol. Chem. 2007; 282: Full Text Full Text PDF PubMed Scopus Google Scholar, P.M. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar, G. Z. Palmer L.G. J. Gen. Physiol. 2008; PubMed Scopus Google Scholar). with subunits have a very low Po and a of channels that be activated by proteases in (7Caldwell R.A. Boucher R.C. Stutts M.J. Am. J. Physiol. Cell Physiol. 2004; 286: C190-C194Crossref PubMed Scopus (169) Google Scholar, S. Carattino M.D. Bruns J.B. Hughey R.P. Kleyman T.R. Am. J. Physiol. Renal Physiol. 2006; PubMed Scopus Google Scholar). cleavage by the α subunit and the channel to an Po (2Hughey R.P. Carattino M.D. Kleyman T.R. Curr. Opin. Nephrol. Hypertens. 2007; 16: 444-450Crossref PubMed Scopus (67) Google Scholar, R.P. Bruns J.B. Kinlough C.L. Harkleroad K.L. Tong Q. Carattino M.D. Johnson J.P. Stockand J.D. Kleyman T.R. J. Biol. Chem. 2004; 279: 18111-18114Abstract Full Text Full Text PDF PubMed Scopus (318) Google Scholar, M.D. Sheng S. Bruns J.B. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2006; 281: Full Text Full Text PDF PubMed Scopus Google Scholar). also the channel for activation by cleaving the γ subunit Subsequent cleavage of the γ subunit at sites distal to the the channel to a high Po (2Hughey R.P. Carattino M.D. Kleyman T.R. Curr. Opin. Nephrol. Hypertens. 2007; 16: 444-450Crossref PubMed Scopus (67) Google Scholar, 13Bruns J.B. Carattino M.D. Sheng S. Maarouf A.B. Weisz O.A. Pilewski J.M. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2007; 282: 6153-6160Abstract Full Text Full Text PDF PubMed Scopus (265) Google Scholar). from the α and γ subunits the the processing of subunit have a role in activating ENaC? Recent that the γ subunit be to the channel to a high Po state, in the of α subunit cleavage M.D. Hughey R.P. Kleyman T.R. J. Biol. Chem. 2008; 283: Full Text Full Text PDF PubMed Scopus Google Scholar). ENaCs evolved from a family of channels that are activated in response to factors within their external For example, channels in are activated by a family of channels in are activated by and are activated by external M. 1999; PubMed Scopus Google Scholar, E. E. M. 2006; PubMed Scopus Google Scholar). channels reside primarily in the and to an open in response to external the ENaC the bulk movement of Na+ across an epithelial For this process to it is for ENaC to be that proteolytic processing of ENaC subunits provided a mechanism that has ENaCs to from channels that reside primarily in the to active channels that facilitate transepithelial Na+