Importance for absorption of Na+ from freshwater of lysine, valine and serine substitutions in the alpha1a-isoform of Na,K-ATPase in the gills of rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar).
In the gills of rainbow trout and Atlantic salmon, the alpha1a- and alpha1b-isoforms of Na,K-ATPase are expressed reciprocally during salt acclimation. The alpha1a-isoform is important for Na(+) uptake in freshwater, but the molecular basis for the functional differences between the two isoforms is not known. Here, three amino acid substitutions are identified in transmembrane segment 5 (TM5), TM8 and TM9 of the alpha1a-isoform compared to the alpha1b-isoform, and the functional consequences are examined by mutagenesis and molecular modeling on the crystal structures of Ca-ATPase or porcine kidney Na,K-ATPase. In TM5 of the alpha1a-isoform, a lysine substitution, Asn783 --> Lys, inserts the epsilon-amino group in cation site 1 in the E(1) form to reduce the Na(+)/ATP ratio. In the E(2) form the epsilon-amino group approaches cation site 2 to force ejection of Na(+) to the blood phase and to interfere with binding of K(+). In TM8, a Asp933 --> Val substitution further reduces K(+) binding, while a Glu961 --> Ser substitution in TM9 can prevent interaction of FXYD peptides with TM9 and alter Na(+) or K(+) affinities. Together, the three substitutions in the alpha1a-isoform of Na,K-ATPase act to promote binding of Na(+) over K(+) from the cytoplasm, to reduce the Na(+)/ATP ratio and the work done in one Na,K pump cycle of active Na(+) transport against the steep gradient from freshwater (10-100 microM: Na(+)) to blood (160 mM: Na(+)) and to inhibit binding of K(+) to allow Na(+)/H(+) rather than Na(+)/K(+) exchange.
Jorgensen
Peter Leth
PL
Department of Molecular Biology, University of Copenhagen, Universitetsparken 13, Copenhagen OE, Denmark. pljorgensen@bio.ku.dk
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D005618:Fresh Water / Q000737:chemistry; D005880:Gills / Q000201:enzymology* / Q000378:metabolism; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D008239:Lysine / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D017686:Oncorhynchus mykiss / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D020317:Salmo salar / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D012694:Serine / Q000235:genetics / Q000378:metabolism; D012964:Sodium / Q000493:pharmacokinetics*; D000254:Sodium-Potassium-Exchanging ATPase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014633:Valine / Q000235:genetics / Q000378:metabolism
D007527:Isoenzymes; D012694:Serine; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D014633:Valine; D008239:Lysine; D011188:Potassium