Differences in the large extracellular loop between the K(+)-Cl(-) cotransporters KCC2 and KCC4.
K(+)Cl(-) cotransporters (KCCs) play fundamental physiological roles in processes such as inhibitory neurotransmission and cell volume regulation. Mammalian genomes encode four distinct KCC paralogs, which share basic transport characteristics but differ significantly in ion affinity, pharmacology, and relative sensitivity to cell volume. Studies to identify divergence in functional characteristics have thus far focused on the cytoplasmic termini. Here, we investigated sequence requirements of the large extracellular loop (LEL) for function in KCC2 and KCC4. Mutation of all four evolutionarily conserved cysteines abolished KCC2 transport activity. This behavior differs from that of its closest relative, KCC4, which is insensitive to this mutation. Chimeras supported the differences in the LEL of the two cotransporters, because swapping wild-type LEL resulted in functional KCC2 but rendered KCC4 inactive. Insertion of the quadruple cysteine substitution mutant of the KCC4 loop, which was functional in the parental isoform, abolished transport activity in KCC2. Dose-response curves of wild-type and chimeric KCCs revealed that the LEL contributes to the different sensitivity to loop diuretics; a KCC2 chimera containing the KCC4 LEL displayed an IC(50) of 396.5 mum for furosemide, which was closer to KCC4 (548.8 mum) than to KCC2 (184.4 mum). Cell surface labeling and immunocytochemistry indicated that mutations do not affect trafficking to the plasma membrane. Taken together, our results show a dramatic and unexpected difference in the sequence requirements of the LEL between the closely related KCC2 and KCC4. Furthermore, they demonstrate that evolutionarily highly conserved amino acids can have different functions within KCC members.
Hartmann
Anna-Maria
AM
Department of Neurogenetics, Institute for Biology and Environmental Sciences, Carl von Ossietzky University, Carl von Ossietzky Strasse 9-11, 26129 Oldenburg, Germany.
Wenz
Meike
M
Mercado
Adriana
A
Störger
Christof
C
Mount
David B
DB
Friauf
Eckhard
E
Nothwang
Hans Gerd
HG
D000595:Amino Acid Sequence; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D005665:Furosemide / Q000494:pharmacology; D006801:Humans; D020128:Inhibitory Concentration 50; D051379:Mice; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011993:Recombinant Fusion Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D049994:Sodium Potassium Chloride Symporter Inhibitors / Q000494:pharmacology; D027981:Symporters / Q000378:metabolism*; D000096922:K Cl- Cotransporters
D005665:Furosemide; D011993:Recombinant Fusion Proteins; D049994:Sodium Potassium Chloride Symporter Inhibitors; D027981:Symporters; D000096922:K Cl- Cotransporters; C119651:SLC12A7 protein, human