Myristoylated methionine sulfoxide reductase A is a late endosomal protein

Geumsoo Kim,Rodney L. Levine,Jung Chae Lim,Jung Mi Lim

doi:10.1074/jbc.ra117.000473

Abstract

Methionine residues in proteins provide antioxidant defense by reacting with oxidizing species, which oxidize methionine to methionine sulfoxide. Reduction of the sulfoxide back to methionine is catalyzed by methionine sulfoxide reductases, essential for protection against oxidative stress. The nonmyristoylated form of methionine sulfoxide reductase A (MSRA) is present in mitochondria, whereas the myristoylated form has been previously reported to be cytosolic. Despite the importance of MSRA in antioxidant defense, its in vivo binding partners and substrates have not been identified. Starting with a protein array, and followed by immunoprecipitation experiments, colocalization studies, and subcellular fractionation, we identified the late endosomal protein, StAR-related lipid transfer domain-containing 3 (STARD3), as a binding partner of myristoylated MSRA, but not of nonmyristoylated MSRA. STARD3 is known to have both membrane-binding and cytosolic domains that are important in STARD3-mediated transport of cholesterol from the endoplasmic reticulum to the endosome. We found that the STARD3 cytosolic domain localizes MSRA to the late endosome. We propose that the previous conclusion that myristoylated MSRA is strictly a cytosolic protein is artifactual and likely due to vigorous overexpression of MSRA. We conclude that myristoylated MSRA is a late endosomal protein that may play a role in lipid metabolism or may protect endosomal proteins from oxidative damage.

Highlights

Methionine residues in proteins provide antioxidant defense by reacting with oxidizing species, which oxidize methionine to methionine sulfoxide
Starting with a protein array, and followed by immunoprecipitation experiments, colocalization studies, and subcellular fractionation, we identified the late endosomal protein, StAR-related lipid transfer domain– containing 3 (STARD3), as a binding partner of myristoylated methionine sulfoxide reductase A (MSRA), but not of nonmyristoylated MSRA
We found that the STARD3 cytosolic domain localizes MSRA to the late endosome

Summary

To whom correspondence should be addressed

Despite the importance of the reductase in defense against oxidative stresses, the mechanisms of its action and even its in vivo binding partners and substrates are essentially unknown. Cyclic oxidation and reduction of methionine residues, like any reversible covalent modification, has the potential to function as a regulatory switch. This function has been established for the cytosolic MSRB1. MSRA is a bifunctional enzyme, capable of stereospecifically oxidizing Met and reducing methionine sulfoxide residues, no in vivo substrates have yet been identified [33]. Many proteins with methionine sulfoxide residues have been shown to be substrates of MSRA in vitro, far no in vivo substrates nor binding partners have been confidently identified. The interaction of STARD3 and myristoylated MSRA clarifies the role of myristoylation of MSRA and broadens the possible in vivo functions of MSRA

Results

Discussion

Experimental procedures