Mdm35p imports Ups proteins into the mitochondrial intermembrane space by functional complex formation

The role of AAA+ proteases in mitochondrial protein biogenesis, homeostasis and activity control

Mitochondria are specialised organelles that are structurally and functionally integrated into cells in the vast majority of eukaryotes. They are the site of numerous enzymatic reactions, some of which are essential for life. The double lipid membrane of the mitochondrion, that spatially defines the organelle and is necessary for some functions, also creates a physical but semi-permeable barrier to the rest of the cell. Thus to ensure the biogenesis, regulation and maintenance of a functional population of proteins, an autonomous protein handling network within mitochondria is required. This includes resident mitochondrial protein translocation machinery, processing peptidases, molecular chaperones and proteases. This review highlights the contribution of proteases of the AAA+ superfamily to protein quality and activity control within the mitochondrion. Here they are responsible for the degradation of unfolded, unassembled and oxidatively damaged proteins as well as the activity control of some enzymes. Since most knowledge about these proteases has been gained from studies in the eukaryotic microorganism Saccharomyces cerevisiae, much of the discussion here centres on their role in this organism. However, reference is made to mitochondrial AAA+ proteases in other organisms, particularly in cases where they play a unique role such as the mitochondrial unfolded protein response. As these proteases influence mitochondrial function in both health and disease in humans, an understanding of their regulation and diverse activities is necessary.

FtsH protease-mediated regulation of various cellular functions

FtsH, a member of the AAA (ATPases associated with a variety of cellular activities) family of proteins, is an ATP-dependent protease of ∼71 kDa anchored to the inner membrane. It plays crucial roles in a variety of cellular processes. It is responsible for the degradation of both membrane and cytoplasmic substrate proteins. Substrate proteins are unfolded and translocated through the central pore of the ATPase domain into the proteolytic chamber, where the polypeptide chains are processively degraded into short peptides. FtsH is not only involved in the proteolytic elimination of unnecessary proteins, but also in the proteolytic regulation of a number of cellular functions. Its role in proteolytic regulation is achieved by one of two approaches, either the cellular levels of a regulatory protein are controlled by processive degradation of the entire protein, or the activity of a particular substrate protein is modified by processing. In the latter case, protein processing requires the presence of a stable domain within the substrate. Since FtsH does not have a robust unfolding activity, this stable domain is sufficient to abort processive degradation of the protein - resulting in release of a stable protein fragment.

Phosphatidylethanolamine biosynthesis in mitochondria: phosphatidylserine (PS) trafficking is independent of a PS decarboxylase and intermembrane space proteins UPS1P and UPS2P

Phosphatidylethanolamine (PE) plays important roles for the structure and function of mitochondria and other intracellular organelles. In yeast, the majority of PE is produced from phosphatidylserine (PS) by a mitochondrion-located PS decarboxylase, Psd1p. Because PS is synthesized in the endoplasmic reticulum (ER), PS is transported from the ER to mitochondria and converted to PE. After its synthesis, a portion of PE moves back to the ER. Two mitochondrial proteins located in the intermembrane space, Ups1p and Ups2p, have been shown to regulate PE metabolism by controlling the export of PE. It remains to be determined where PS is decarboxylated in mitochondria and whether decarboxylation is coupled to trafficking of PS. Here, using fluorescent PS as a substrate in an in vitro assay for Psd1p-dependent PE production in isolated mitochondria, we show that PS is transferred from the mitochondrial outer membrane to the inner membrane independently of Psd1p, Ups1p, and Ups2p and decarboxylated to PE by Psd1p in the inner membrane. Interestingly, Ups1p is required for the maintenance of Psd1p and therefore PE production. Restoration of Psd1p levels rescued PE production defects in ups1Δ mitochondria. Our data provide novel mechanistic insight into PE biogenesis in mitochondria.

Role for two conserved intermembrane space proteins, Ups1p and Ups2p, [corrected] in intra-mitochondrial phospholipid trafficking

Mitochondrial membranes maintain a specific phospholipid composition. Most phospholipids are synthesized in the endoplasmic reticulum (ER) and transported to mitochondria, but cardiolipin and phosphatidylethanolamine are produced in mitochondria. In the yeast Saccharomyces cerevisiae, phospholipid exchange between the ER and mitochondria relies on the ER-mitochondria encounter structure (ERMES) complex, which physically connects the ER and mitochondrial outer membrane. However, the proteins and mechanisms involved in phospholipid transport within mitochondria remain elusive. Here, we investigated the role of the conserved intermembrane space proteins, Ups1p and Ups2p, and an inner membrane protein, Mdm31p, in phospholipid metabolism. Our data show that loss of the ERMES complex, Ups1p, and Mdm31p causes similar defects in mitochondrial phospholipid metabolism, mitochondrial morphology, and cell growth. Defects in cells lacking the ERMES complex or Ups1p are suppressed by Mdm31p overexpression as well as additional loss of Ups2p, which antagonizes Ups1p. Combined loss of the ERMES complex and Ups1p exacerbates phospholipid defects. Finally, pulse-chase experiments using [(14)C]serine revealed that Ups1p and Ups2p antagonistically regulate conversion of phosphatidylethanolamine to phosphatidylcholine. Our results suggest that Ups proteins and Mdm31p play important roles in phospholipid biosynthesis in mitochondria. Ups proteins may function in phospholipid trafficking between the outer and inner mitochondrial membranes.

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

Distinct functions of evolutionary conserved MSF1 and late embryogenesis abundant (LEA)-like domains in mitochondria

PRELID1, the only late embryogenesis abundant (LEA) domain-containing protein in humans, exerts cytoprotective effects through its LEA domain within the mitochondria. Although PRELID1 homologs in vertebrates contain the LEA domain, homologs in lower eukaryotes are thought to lack this domain. In this study, we identify a novel LEA-like domain in a yeast PRELID1 homolog, Ups2p, which contains sequence conservation with the LEA domain of human PRELID1. PRELID1 homologs, including Ups2p, are known to contain the PRELI/MSF1 domain. Our study reveals that the MSF1 domain of Ups2p maintains proper mitochondrial electron transport chain function, respiratory competency, and mitochondrial phosphatidylethanolamine metabolism. The Ups2p MSF1 domain mediates cardiolipin depletion in the absence of Ups1p. However, the Ups2p LEA-like domain is responsible for cardiolipin depletion resulting from UPS2 overexpression. The regulation of phosphatidylethanolamine levels by the MSF1 domain is antagonized by the Ups2p LEA-like domain. We demonstrate that the yeast LEA-like domain protects cells from oxidative stress and can be functionally replaced by the human LEA domain. Together our studies suggest distinct roles of MSF1 and LEA-like domains in mitochondrial function and resistance to oxidative stress.

FMP30 is required for the maintenance of a normal cardiolipin level and mitochondrial morphology in the absence of mitochondrial phosphatidylethanolamine synthesis

Mitochondria of the yeast Saccharomyces cerevisiae contain enzymes Crd1p and Psd1p, which synthesize cardiolipin (CL) and phosphatidylethanolamine respectively. A previous study indicated that crd1Δ is synthetically lethal with psd1Δ. In this study, to identify novel genes involved in CL metabolism, we searched for genes that genetically interact with Psd1p, and found that deletion of FMP30 encoding a mitochondrial inner membrane protein results in a synthetic growth defect with psd1Δ. Although fmp30Δ cells grew normally and exhibited a slightly decreased CL level, fmp30Δpsd1Δ cells exhibited a severe growth defect and an about 20-fold reduction in the CL level, as compared with the wild-type control. We found also that deletion of FMP30 caused a defect in mitochondrial morphology. Furthermore, FMP30 genetically interacted with seven mitochondrial morphology genes. These results indicated that Fmp30p is involved in the maintenance of mitochondrial morphology and required for the accumulation of a normal level of CL in the absence of mitochondrial phosphatidylethanolamine synthesis.

Making heads or tails of phospholipids in mitochondria

Mitochondria are dynamic organelles whose functional integrity requires a coordinated supply of proteins and phospholipids. Defined functions of specific phospholipids, like the mitochondrial signature lipid cardiolipin, are emerging in diverse processes, ranging from protein biogenesis and energy production to membrane fusion and apoptosis. The accumulation of phospholipids within mitochondria depends on interorganellar lipid transport between the endoplasmic reticulum (ER) and mitochondria as well as intramitochondrial lipid trafficking. The discovery of proteins that regulate mitochondrial membrane lipid composition and of a multiprotein complex tethering ER to mitochondrial membranes has unveiled novel mechanisms of mitochondrial membrane biogenesis.

Cell wall integrity is linked to mitochondria and phospholipid homeostasis in Candida albicans through the activity of the post-transcriptional regulator Ccr4-Pop2

The cell wall is essential for viability of fungi and is an effective drug target in pathogens such as Candida albicans. The contribution of post-transcriptional gene regulators to cell wall integrity in C. albicans is unknown. We show that the C. albicans Ccr4-Pop2 mRNA deadenylase, a regulator of mRNA stability and translation, is required for cell wall integrity. The ccr4/pop2 mutants display reduced wall β-glucans and sensitivity to the echinocandin caspofungin. Moreover, the deadenylase mutants are compromised for filamentation and virulence. We demonstrate that defective cell walls in the ccr4/pop2 mutants are linked to dysfunctional mitochondria and phospholipid imbalance. To further understand mitochondrial function in cell wall integrity, we screened a Saccharomyces cerevisiae collection of mitochondrial mutants. We identify several mitochondrial proteins required for caspofungin tolerance and find a connection between mitochondrial phospholipid homeostasis and caspofungin sensitivity. We focus on the mitochondrial outer membrane SAM complex subunit Sam37, demonstrating that it is required for both trafficking of phospholipids between the ER and mitochondria and cell wall integrity. Moreover, in C. albicans also Sam37 is essential for caspofungin tolerance. Our study provides the basis for an integrative view of mitochondrial function in fungal cell wall biogenesis and resistance to echinocandin antifungal drugs.