title: Surprises from an unusual CLC homolog.
abstract: The chloride channel (CLC) family is distinctive in that some members are Cl(-) ion channels and others are Cl(-)/H(+) antiporters. The molecular mechanism that couples H(+) and Cl(-) transport in the antiporters remains unknown. Our characterization of a novel bacterial homolog from Citrobacter koseri, CLC-ck2, has yielded surprising discoveries about the requirements for both Cl(-) and H(+) transport in CLC proteins. First, even though CLC-ck2 lacks conserved amino acids near the Cl(-)-binding sites that are part of the CLC selectivity signature sequence, this protein catalyzes Cl(-) transport, albeit slowly. Ion selectivity in CLC-ck2 is similar to that in CLC-ec1, except that SO(4)(2-) strongly competes with Cl(-) uptake through CLC-ck2 but has no effect on CLC-ec1. Second, and even more surprisingly, CLC-ck2 is a Cl(-)/H(+) antiporter, even though it contains an isoleucine at the Glu(in) position that was previously thought to be a critical part of the H(+) pathway. CLC-ck2 is the first known antiporter that contains a nonpolar residue at this position. Introduction of a glutamate at the Glu(in) site in CLC-ck2 does not increase H(+) flux. Like other CLC antiporters, mutation of the external glutamate gate (Glu(ex)) in CLC-ck2 prevents H(+) flux. Hence, Glu(ex), but not Glu(in), is critical for H(+) permeation in CLC proteins.
authors.affiliation: Department of Chemistry and Biochemistry, University of San Diego, San Diego, California, USA.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Phillips
authors.forename: Ashley E
authors.initials: AE
authors.lastname: Brammer
authors.forename: Luis
authors.initials: L
authors.lastname: Rodriguez
authors.forename: Hyun-Ho
authors.initials: HH
authors.lastname: Lim
authors.forename: Anna
authors.initials: A
authors.lastname: Stary-Weinzinger
authors.forename: Kimberly
authors.initials: K
authors.lastname: Matulef
mesh_terms: D017920:Antiporters / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D018118:Chloride Channels / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism; D041041:Citrobacter koseri / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018698:Glutamic Acid / Q000235:genetics; D017136:Ion Transport; D007532:Isoleucine / Q000235:genetics; D020125:Mutation, Missense; D011522:Protons
chemical_list: D017920:Antiporters; D001426:Bacterial Proteins; D018118:Chloride Channels; D002712:Chlorides; D011522:Protons; D007532:Isoleucine; D018698:Glutamic Acid