Sodium-potassium ATPase β1 subunit is a molecular partner of Wolframin, an endoplasmic reticulum protein involved in ER stress

Abstract

Wolfram syndrome, an autosomal recessive disorder characterized by diabetes mellitus and optic atrophy, is caused by mutations in the WFS1 gene encoding an endoplasmic reticulum (ER) membrane protein, Wolframin. Although its precise functions are unknown, Wolframin deficiency increases ER stress, impairs cell cycle progression and affects calcium homeostasis. To gain further insight into its function and identify molecular partners, we used the WFS1-C-terminal domain as bait in a yeast two-hybrid screen with a human brain cDNA library. Na+/ K+ ATPase β1 subunit was identified as an interacting clone. We mapped the interaction to the WFS1 C-terminal and transmembrane domains, but not the N-terminal domain. Our mapping data suggest that the interaction most likely occurs in the ER. We confirmed the interaction by co-immunoprecipitation in mammalian cells and with endogenous proteins in JEG3 placental cells, neuroblastoma SKNAS and pancreatic MIN6 β cells. Na+K+ ATPase β1 subunit expression was reduced in plasma membrane fractions of human WFS1 mutant fibroblasts and WFS1 knockdown MIN6 pancreatic β-cells compared with wild-type cells; Na+/K+ ATPase α1 subunit expression was also reduced in WFS-depleted MIN6 β cells. Induction of ER stress in wild-type cells only partly accounted for the reduced Na+/K+ ATPase β1 subunit expression observed. We conclude that the interaction may be important for Na+/K+ ATPase β1 subunit maturation; loss of this interaction may contribute to the pathology seen in Wolfram syndrome via reductions in sodium pump α1 and β1 subunit expression in pancreatic β-cells. © The Author 2007. Published by Oxford University Press. All rights reserved.