Transient association of the sarcoplasmic reticulum Ca2+ ATPase with the Na+/K+-ATPase and H+/K+-ATPase beta-subunits during its biogenesis in Xenopus oocytes

Sodium-potassium ATPase 1 subunit is a molecular partner of Wolframin, an endoplasmic reticulum protein involved in ER stress

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 beta1 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 beta cells. Na+/K+ ATPase beta1 subunit expression was reduced in plasma membrane fractions of human WFS1 mutant fibroblasts and WFS1 knockdown MIN6 pancreatic beta-cells compared with wild-type cells; Na+/K+ ATPase alpha1 subunit expression was also reduced in WFS-depleted MIN6 beta cells. Induction of ER stress in wild-type cells only partly accounted for the reduced Na+/K+ ATPase beta1 subunit expression observed. We conclude that the interaction may be important for Na+/K+ ATPase beta1 subunit maturation; loss of this interaction may contribute to the pathology seen in Wolfram syndrome via reductions in sodium pump alpha1 and beta1 subunit expression in pancreatic beta-cells.

Methionine Aminopeptidase II: A Molecular Chaperone for Sarcoplasmic Reticulum Calcium ATPase

The monoclonal antibody to the beta-subunit of H(+)/K(+)-ATPase (mAbHKbeta) cross-reacts with a protein that acts as a molecular chaperone for the structural maturation of sarcoplasmic reticulum (SR) Ca(2+)-ATPase. We partially purified a mAbHKbeta-reactive 65-kDa protein from Xenopus ovary. After in-gel digestion and peptide sequencing, the 65-kDa protein was identified as methionine aminopeptidase II (MetAP2). The effects of MetAP2 on SR Ca(2+)-ATPase expression were examined by injecting the cRNA for MetAP2 into Xenopus oocytes. Immunoprecipitation and pulse-chase experiments showed that MetAP2 was transiently associated with the nascent SR Ca(2+)-ATPase. Synthesis of functional SR Ca(2+)-ATPase was facilitated by MetAP2 and prevented by injecting an antibody specific for MetAP2. These results suggest that MetAP2 acts as a molecular chaperone for SR Ca(2+)-ATPase synthesis.

Phospholipid Translocation and Miltefosine Potency Require Both L. donovani Miltefosine Transporter and the New Protein LdRos3 in Leishmania Parasites

The antitumor drug miltefosine has been recently approved as the first oral drug active against visceral leishmaniasis. We have previously identified the L. donovani miltefosine transporter (LdMT) as a P-type ATPase involved in phospholipid translocation at the plasma membrane of Leishmania parasites. Here we show that this protein is essential but not sufficient for the phospholipid translocation activity and, thus, for the potency of the drug. Based on recent findings in yeast, we have identified the putative beta subunit of LdMT, named LdRos3, as another protein factor required for the translocation activity. LdRos3 belongs to the CDC50/Lem3 family, proposed as likely beta subunits for P4-ATPases. The phenotype of LdRos3-defective parasites was identical to that of the LdMT-/-, including a defect in the uptake of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-amino)-phosphatidylserine, generally considered as not affected in Lem3p-deficient yeast. Both LdMT and LdRos3 normally localized to the plasma membrane but were retained inside the endoplasmic reticulum in the absence of the other protein or when inactivating point mutations were introduced in LdMT. Modulating the expression levels of either protein independently, we show that any one of them could behave as the protein limiting the level of flippase activity. Thus, LdMT and LdRos3 seem to form part of the same translocation machinery that determines flippase activity and miltefosine sensitivity in Leishmania, further supporting the consideration of CDC50/Lem3 proteins as beta subunits required for the normal functioning of P4-ATPases.