Oxidative folding in the mitochondrial intermembrane space in human health and disease

Abstract

Oxidative folding in the mitochondrial intermembrane space (IMS) is a key cellular event associated with the folding and import of a large and still undetermined number of proteins. This process is catalyzed by an oxidoreductase, Mia40 that is able to recognize substrates with apparently little or no homology. Following substrate oxidation, Mia40 is reduced and must be reoxidized by Erv1/Alr1 that consequently transfers the electrons to the mitochondrial respiratory chain. Although our understanding of the physiological relevance of this process is still limited, an increasing number of pathologies are being associated with the impairment of this pathway; especially because oxidative folding is fundamental for several of the proteins involved in defense against oxidative stress. Here we review these aspects and discuss recent findings suggesting that oxidative folding in the IMS is modulated by the redox state of the cell.

Figure 1

Mia40 pathway is influenced by cytoplasmatic GSH. Reduced and unfolded substrates cross the outer mitochondrial membrane (I) and are recognized by oxidized Mia40 (II). Following the formation of an intermolecular disulfide bond, the substrate is released, oxidized and folded while Mia40 CPC motif is reduced. To allow a new reaction cycle, Mia40 is then reoxidized by Erv1 (III) that can transfer electrons to cytochrome C or directly to oxygen forming H₂O₂. GSH has been shown in vitro to influence the oxidative folding of several substrates by promoting disulfide reshuffling. Despite contradictory results, the free diffusion of GSH:GSSG from the cytoplasm to the IMS, expected considering the presence of porins, was confirmed recently (IV) [26]. The GSH pool in the cytoplasm also influences the redox state of Mia40 but the molecular mechanisms involved remain unclear.

Figure 2

CCS1 and Sod1 import into IMS depend on Mia40. Ccs1 and Sod1 coexist in the cytoplasm and IMS space. Because Ccs1 and Sod1 do not contain mitochondrial targeting signals, their cellular partition is controlled by a folding trap mechanism involving the oxidation of the Ccs1 domain I Cys 27 and 64 residues by Mia40 present in the IMS (S. cerevisiae numbering, in bold) [39]. Cys 27 and 64 are not essential for activity as the protein is fully active in its reduced state present in the cytoplasm. Recognition by Mia40 can also occur independently of cysteine residues as proposed to occur for human Ccs1 where those residues are not conserved [40]. Figure adapted from [39] with authors permission.