Hitchhiking of Cu/Zn Superoxide Dismutase to Peroxisomes – Evidence for a Natural Piggyback Import Mechanism in Mammals

Abstract

Most newly synthesized peroxisomal proteins are imported in a receptor-mediated fashion, depending on the interaction of a peroxisomal targeting signal (PTS) with its cognate targeting receptor Pex5 or Pex7 located in the cytoplasm. Apart from this classic mechanism, heterologous protein complexes that have been proposed more than a decade ago are also to be imported into peroxisomes. However, it remains still unclear if this so-called piggyback import is of physiological relevance in mammals. Here, we show that Cu/Zn superoxide dismutase 1 (SOD1), an enzyme without an endogenous PTS, is targeted to peroxisomes using its physiological interaction partner 'copper chaperone of SOD1' (CCS) as a shuttle. Both proteins have been identified as peroxisomal constituents by 2D-liquid chromatography mass spectrometry of isolated rat liver peroxisomes. Yet, while a major fraction of CCS was imported into peroxisomes in a PTS1-dependent fashion in CHO cells, overexpressed SOD1 remained in the cytoplasm. However, increasing the concentrations of both CCS and SOD1 led to an enrichment of SOD1 in peroxisomes. In contrast, CCS-mediated SOD1 import into peroxisomes was abolished by deletion of the SOD domain of CCS, which is required for heterodimer formation. SOD1/CCS co-import is the first demonstration of a physiologically relevant piggyback import into mammalian peroxisomes.

Combined zebrafish-yeast chemical-genetic screens reveal gene–copper-nutrition interactions that modulate melanocyte pigmentation

“…We speculate this dependency arises because the copper-zinc SOD is a constituent enzyme of the peroxisome matrix, which is essential for its dismutase and peroxidase function (Titorenko and Mullen, 2006;Islinger et al, 2009). Deletions in other peroxisomal genes have also recently been shown to sensitize to low-iron conditions, possibly because of an elevated requirement for peroxisomal -oxidation in energy production when iron-dependent mitochondrial energy production is impaired (Jo et al, 2009).…”

Classification of Subcellular Location by Comparative Proteomic Analysis of Native and Density-shifted Lysosomes

“…Although SOD1 is generally regarded as a cytosolic protein, some reports have shown that it can be found in the mitochondrial intermembrane space (37,38). A recent report has identified SOD1 in a peroxisomeenriched subcellular fraction and, based on immunofluorescence microscopy studies on cells overexpressing both SOD1 and "copper chaperone of SOD1," concluded that copper chaperone of SOD1 mediates import of SOD1 into peroxisomes (39).…”

Molecular Mechanisms and Physiological Significance of Organelle Interactions and Cooperation

“…This has originally been attributed to a potential third type of peroxisomal targeting signals (PTS3), but this signal has never been characterized and the import either depends on a PTS-independent interaction with a receptor protein (Klein et al, 2002) or on co-import of proteins (piggy-back), which is a specific property of peroxisomal import (Yang et al, 2001;Subramani, 2002;van der Klei and Veenhuis, 2006;Islinger et al, 2009). …”