Maturation of Mitochondrially Targeted Prx V Involves a Second Cleavage by Mitochondrial Intermediate Peptidase That Is Sensitive to Inhibition by H2O2

Dissecting the molecular mechanisms of mitochondrial import and maturation of peroxiredoxins from yeast and mammalian cells

Peroxiredoxins (Prxs) are cysteine-based peroxidases that play a central role in keeping the H₂O₂ at physiological levels. Eukaryotic cells express different Prxs isoforms, which differ in their subcellular locations and substrate specificities. Mitochondrial Prxs are synthesized in the cytosol as precursor proteins containing N-terminal cleavable presequences that act as mitochondrial targeting signals. Due to the fact that presequence controls the import of the vast majority of mitochondrial matrix proteins, the mitochondrial Prxs were initially predicted to be localized exclusively in the matrix. However, recent studies showed that mitochondrial Prxs are also targeted to the intermembrane space by mechanisms that remain poorly understood. While in yeast the IMP complex can translocate Prx1 to the intermembrane space, the maturation of yeast Prx1 and mammalian Prdx3 and Prdx5 in the matrix has been associated with sequential cleavages of the presequence by MPP and Oct1/MIP proteases. In this review, we describe the state of the art of the molecular mechanisms that control the mitochondrial import and maturation of Prxs of yeast and human cells. Once mitochondria are considered the major intracellular source of H₂O₂, understanding the mitochondrial Prx biogenesis pathways is essential to increase our knowledge about the H₂O₂-dependent cellular signaling, which is relevant to the pathophysiology of some human diseases.

Protein Processing in Plant Mitochondria Compared to Yeast and Mammals

Limited proteolysis, called protein processing, is an essential post-translational mechanism that controls protein localization, activity, and in consequence, function. This process is prevalent for mitochondrial proteins, mainly synthesized as precursor proteins with N-terminal sequences (presequences) that act as targeting signals and are removed upon import into the organelle. Mitochondria have a distinct and highly conserved proteolytic system that includes proteases with sole function in presequence processing and proteases, which show diverse mitochondrial functions with limited proteolysis as an additional one. In virtually all mitochondria, the primary processing of N-terminal signals is catalyzed by the well-characterized mitochondrial processing peptidase (MPP). Subsequently, a second proteolytic cleavage occurs, leading to more stabilized residues at the newly formed N-terminus. Lately, mitochondrial proteases, intermediate cleavage peptidase 55 (ICP55) and octapeptidyl protease 1 (OCT1), involved in proteolytic cleavage after MPP and their substrates have been described in the plant, yeast, and mammalian mitochondria. Mitochondrial proteins can also be processed by removing a peptide from their N- or C-terminus as a maturation step during insertion into the membrane or as a regulatory mechanism in maintaining their function. This type of limited proteolysis is characteristic for processing proteases, such as IMP and rhomboid proteases, or the general mitochondrial quality control proteases ATP23, m-AAA, i-AAA, and OMA1. Identification of processing protease substrates and defining their consensus cleavage motifs is now possible with the help of large-scale quantitative mass spectrometry-based N-terminomics, such as combined fractional diagonal chromatography (COFRADIC), charge-based fractional diagonal chromatography (ChaFRADIC), or terminal amine isotopic labeling of substrates (TAILS). This review summarizes the current knowledge on the characterization of mitochondrial processing peptidases and selected N-terminomics techniques used to uncover protease substrates in the plant, yeast, and mammalian mitochondria.

Peroxiredoxins as Potential Targets for Cardiovascular Disease

Increased oxidative stress (OS) is considered a common etiology in the pathogenesis of cardiovascular disease (CVD). Therefore, the precise regulation of reactive oxygen species (ROS) in cardiovascular cells is essential to maintain normal physiological functions. Numerous regulators of cellular homeostasis are reportedly influenced by ROS. Hydrogen peroxide (H₂O₂), as an endogenous ROS in aerobic cells, is a toxic substance that can induce OS. However, many studies conducted over the past two decades have provided substantial evidence that H₂O₂ acts as a diffusible intracellular signaling messenger. Antioxidant enzymes, including superoxide dismutases, catalase, glutathione peroxidases, and peroxiredoxins (Prdxs), maintain the balance of ROS levels against augmentation of ROS production during the pathogenesis of CVD. Especially, Prdxs are regulatory sensors of transduced intracellular signals. The intracellular abundance of Prdxs that specifically react with H₂O₂ act as regulatory proteins. In this review, we focus on the role of Prdxs in the regulation of ROS-induced pathological changes in the development of CVD.