Structural and functional properties of two human FXYD3 (Mat-8) isoforms.
Six of 7 FXYD proteins have been shown to be tissue-specific modulators of Na,K-ATPase. In this study, we have identified two splice variants of human FXYD3, or Mat-8, in CaCo-2 cells. Short human FXYD3 has 72% sequence identity with mouse FXYD3, whereas long human FXYD3 is identical to short human FXYD3 but has a 26-amino acid insertion after the transmembrane domain. Short and long human FXYD3 RNAs and proteins are differentially expressed during differentiation of CaCo-2 cells. Long human FXYD3 is mainly expressed in nondifferentiated cells and short human FXYD3 in differentiated cells and both FXYD3 variants can be co-immunoprecipitated with a Na,K-ATPase antibody. In contrast to mouse FXYD3, which has two transmembrane domains for lack of cleavage of the signal peptide, human FXYD3 has a cleavable signal peptide and adopts a type I topology. After co-expression in Xenopus oocytes, both human FXYD3 variants associate stably only with Na,K-ATPase isozymes but not with H,K-ATPase or Ca-ATPase. Similar to mouse FXYD3, short human FXYD3 decreases the apparent K(+) and Na(+) affinity of Na,K-ATPase over a large range of membrane potentials. On the other hand, long human FXYD3 decreases the apparent K(+) affinity only at slightly negative and positive membrane potentials and increases the apparent Na(+) affinity of Na,K-ATPase. Finally, both short and long human FXYD3 induce a hyperpolarization activated current, similar to that induced by mouse FXYD3. Thus, we have characterized two human FXYD3 isoforms that are differentially expressed in differentiated and non-differentiated cells and show different functional properties.
Bibert
Stéphanie
S
Department of Pharmacology and Toxicology, University of Lausanne, rue du Bugnon 27, 1005 Lausanne, Switzerland.
Roy
Sophie
S
Schaer
Danièle
D
Felley-Bosco
Emanuela
E
Geering
Käthi
K
D000818:Animals; D018938:Caco-2 Cells; D002454:Cell Differentiation; D002462:Cell Membrane / Q000378:metabolism; D005786:Gene Expression Regulation; D015971:Gene Expression Regulation, Enzymologic; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry* / Q000502:physiology*; D051379:Mice; D009363:Neoplasm Proteins / Q000737:chemistry* / Q000502:physiology*; D011188:Potassium / Q000737:chemistry; D011487:Protein Conformation; D020033:Protein Isoforms; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D014981:Xenopus
C091777:FXYD3 protein, human; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D020033:Protein Isoforms; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium