Intracellular Trafficking of FXYD1 (Phospholemman) and FXYD7 Proteins in Xenopus Oocytes and Mammalian Cells

Shiri Moshitzky,Carol Asher,Haim Garty

doi:10.1074/jbc.m112.347807

Shiri Moshitzky, Carol Asher + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m112.347807

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Abstract

FXYD proteins are a group of short single-span transmembrane proteins that interact with the Na(+)/K(+) ATPase and modulate its kinetic properties. This study characterizes intracellular trafficking of two FXYD family members, FXYD1 (phospholemman (PLM)) and FXYD7. Surface expression of PLM in Xenopus oocytes requires coexpression with the Na(+)/K(+) ATPase. On the other hand, the Na(+)/Ca(2+) exchanger, another PLM-interacting protein could not drive it to the cell surface. The Na(+)/K(+) ATPase-dependent surface expression of PLM could be facilitated by either a phosphorylation-mimicking mutation at Thr-69 or a truncation of three terminal arginine residues. Unlike PLM, FXYD7 could translocate to the cell surface of Xenopus oocytes independently of the coexpression of α1β1 Na(+)/K(+) ATPase. The Na(+)/K(+) ATPase-independent membrane translocation of FXYD7 requires O-glycosylation of at least two of three conserved threonines in its ectodomain. Subsequent experiments in mammalian cells confirmed the role of conserved extracellular threonine residues and demonstrated that FXYD7 protein, in which these have been mutated to alanine, is trapped in the endoplasmic reticulum and Golgi apparatus.

Highlights

FXYD proteins are auxiliary subunits of the Naϩ/Kϩ ATPase that modulate its kinetic properties
Surface expression of FXYD proteins was measured in Xenopus oocytes injected with cRNA coding for PLM or FXYD7 tagged in their extracellular domain by the HA epitope
In contrast to the large effect of the Naϩ/Kϩ ATPase on PLM surface expression, FXYD7 was expressed on the oocyte surface at similar levels irrespective of the coexpression of exogenous Naϩ/Kϩ ATPase

Summary

Introduction

FXYD proteins are auxiliary subunits of the Naϩ/Kϩ ATPase that modulate its kinetic properties. Conclusion: O-glycosylation of FXYD7 in the Golgi is required for its surface expression. FXYD proteins are a group of short single-span transmembrane proteins that interact with the Na؉/K؉ ATPase and modulate its kinetic properties. Surface expression of PLM in Xenopus oocytes requires coexpression with the Na؉/K؉ ATPase. Unlike PLM, FXYD7 could translocate to the cell surface of Xenopus oocytes independently of the coexpression of ␣1␤1 Na؉/K؉ ATPase. The Na؉/K؉ ATPase-independent membrane translocation of FXYD7 requires O-glycosylation of at least two of three conserved threonines in its ectodomain. Subsequent experiments in mammalian cells confirmed the role of conserved extracellular threonine residues and demonstrated that FXYD7 protein, in which these have been mutated to alanine, is trapped in the endoplasmic reticulum and Golgi apparatus

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