The PARL family of mitochondrial rhomboid proteases

Mitochondrial proteases modulate mitochondrial stress signalling and cellular homeostasis in health and disease

Maintenance of mitochondrial homeostasis requires a plethora of coordinated quality control and adaptations' mechanisms in which mitochondrial proteases play a key role. Their activation or loss of function reverberate beyond local mitochondrial biochemical and metabolic remodelling into coordinated cellular pathways and stress responses that feedback onto the mitochondrial functionality and adaptability. Mitochondrial proteolysis modulates molecular and organellar quality control, metabolic adaptations, lipid homeostasis and regulates transcriptional stress responses. Defective mitochondrial proteolysis results in disease conditions most notably, mitochondrial diseases, neurodegeneration and cancer. Here, it will be discussed how mitochondrial proteases and mitochondria stress signalling impact cellular homeostasis and determine the cellular decision to survive or die, how these processes may impact disease etiopathology, and how modulation of proteolysis may offer novel therapeutic strategies.

Human CLPB forms ATP-dependent complexes in the mitochondrial intermembrane space

Human caseinolytic peptidase B protein homolog (CLPB), also known as suppressor of potassium transport defect 3 (SKD3), is a broadly-expressed member of the family of ATPases associated with diverse cellular activities (AAA+). Mutations in the human CLPB gene cause 3-methylglutaconic aciduria type VII. CLPB is upregulated in acute myeloid leukemia (AML), where it contributes to anti-cancer drug resistance. The biological function of CLPB in human cells and mechanistic links to the clinical phenotypes are currently unknown. Herein, subcellular fractionation of human HEK-293 and BT-549 cells showed that a single 57-kDa form of CLPB was present in the mitochondria and not in the cytosolic fraction. Immunofluorescence staining of HEK-293 and BT-549 cells with anti-CLPB antibody co-localized with the mitochondrial staining using a MitoTracker dye. In purified intact mitochondria, CLPB was protected against externally added proteinase K, but it was susceptible to degradation after disruption of the outer membrane, indicating that CLPB resides in the mitochondrial intermembrane space. Overexpressed CLPB, while properly trafficked to the mitochondria, appeared to form large clusters/aggregates that were resistant to extraction with non-ionic detergents and were readily visualized by immunofluorescence microscopy. Importantly, endogenous CLPB formed high molecular weight protein complexes in an ATP-dependent manner that were detected by blue native polyacrylamide gel electrophoresis. These results demonstrate that ATP induces a structural change in CLPB and controls its ability to self-associate or form complexes with other proteins in the intermembrane space of mitochondria.

HRD Complex Self-Remodeling Enables a Novel Route of Membrane Protein Retrotranslocation

ER-associated degradation (ERAD) targets misfolded ER proteins for degradation. Retrotranslocation, a key feature of ERAD, entails removal of ubiquitinated substrates into the cytosol for proteasomal destruction. Recently, it has been shown that the Hrd1 E3 ligase forms a retrotranslocation channel for luminal (ERAD-L) substrates. Conversely, our studies found that integral membrane (ERAD-M) substrates exit the ER through a distinct pathway mediated by the Dfm1 rhomboid protein. Those studies also revealed a second, Hrd1-dependent pathway of ERAD-M retrotranslocation can arise in dfm1Δ null. Here we show that, in the dfm1Δ null, the HRD complex undergoes remodeling to a form that mediates ERAD-M retrotranslocation. Specifically, Hrd1's normally present stochiometric partner Hrd3 is efficiently removed during suppressive remodeling, allowing Hrd1 to function in this novel capacity. Neither Hrd1 autoubiquitination nor its cytosolic domain is required for suppressive ERAD-M retrotranslocation. Thus, the HRD complex displays remarkable functional flexibility in response to ER stress.

Mitochondrial proteases and protein quality control in ageing and longevity

Mitochondria have been implicated in the ageing process and the lifespan modulation of model organisms. Mitochondria are the main providers of energy in eukaryotic cells but also represent both a major source of reactive oxygen species and targets for protein oxidative damage. Since protein damage can impair mitochondrial function, mitochondrial proteases are critically important for protein maintenance and elimination of oxidized protein. In the mitochondrial matrix, protein quality control is mainly achieved by the Lon and Clp proteases which are also key players in damaged mitochondrial proteins degradation. Accumulation of damaged macromolecules resulting from oxidative stress and failure of protein maintenance constitutes a hallmark of cellular and organismal ageing and is believed to participate to the age-related decline of cellular function. Hence, age-related impairment of mitochondrial protein quality control may therefore contribute to the age-associated build-up of oxidized protein and alterations of mitochondrial redox and protein homeostasis.

Mitochondrial protein quality control: the mechanisms guarding mitochondrial health

SIGNIFICANCE

Mitochondria are complex dynamic organelles pivotal for cellular physiology and human health. Failure to maintain mitochondrial health leads to numerous maladies that include late-onset neurodegenerative diseases and cardiovascular disorders. Furthermore, a decline in mitochondrial health is prevalent with aging. A set of evolutionary conserved mechanisms known as mitochondrial quality control (MQC) is involved in recognition and correction of the mitochondrial proteome.

RECENT ADVANCES

Here, we review current knowledge and latest developments in MQC. We particularly focus on the proteolytic aspect of MQC and its impact on health and aging.

CRITICAL ISSUES

While our knowledge about MQC is steadily growing, critical gaps remain in the mechanistic understanding of how MQC modules sense damage and preserve mitochondrial welfare, particularly in higher organisms.

FUTURE DIRECTIONS

Delineating how coordinated action of the MQC modules orchestrates physiological responses on both organellar and cellular levels will further elucidate the current picture of MQC's role and function in health, cellular stress, and degenerative diseases.

Common variants of the PINK1 and PARL genes do not confer genetic susceptibility to schizophrenia in Han Chinese

Schizophrenia is a prevalent psychiatric disorder with a complex etiology. Mitochondrial dysfunction has been frequently reported in schizophrenia. Phosphatase and tension homologue-induced kinase 1 (PINK1) and presenilin-associated rhomboid-like protease (PARL) are mitochondrial proteins, and genetic variants of these two genes may confer genetic susceptibility to schizophrenia by influencing mitochondrial function. In this study, we conducted a two-stage genetic association study to test this hypothesis. We genotyped 4 PINK1 and 5 PARL genetic variants and evaluated the potential association of the 9 SNPs with schizophrenia in two independent case-control cohorts of 2510 Han Chinese individuals. No positive association of common genetic variants of the PINK1 and PARL genes with schizophrenia was identified in our samples after Bonferroni correction. Re-analysis of the newly updated Psychiatric Genetics Consortium (PGC) data sets confirmed our negative result. Intriguingly, one PINK1 SNP (rs10916832), which showed a marginally significant association in only Hunan samples (P = 0.032), is associated with the expression of a schizophrenia susceptible gene KIF17 according to the expression quantitative trait locus (eQTL) analysis. Our study indicated that common genetic variants of the PINK1 and PARL genes are unlikely to be involved in schizophrenia. Further studies are essential to characterize the role of the PINK1 and PARL genes in schizophrenia.

Rhomboid family gene expression profiling in breast normal tissue and tumor samples

Rhomboid is an evolutionary conserved and functionally diversified group of proteins composed of proteolytically active and inactive members that are involved in the modulation of multiple biological processes such as epidermal growth factor receptor signaling pathway, endoplasmic reticulum-associated degradation, cell death, and proliferation. Recently, several human rhomboid genes have been associated with the development of chronic myeloid leukemia and pituitary, colorectal, ovarian, and breast cancers. In this study, we evaluated the mRNA and protein expression profiles of rhomboid genes in cancer cell lines and breast tissue/tumor samples. In silico analysis of publicly available gene expression datasets showed that different rhomboid genes are specifically expressed according to the breast cancer intrinsic subtypes. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis showed a significant RHBDD2 mRNA overexpression in advanced breast cancer compared with normal tissue samples (p = 0.012). In addition, we found that RHBDL2 and PARL mRNA expression was associated with a low/intermediate histologic tumor grade (p = 0.024 and p = 0.015, respectively). Immunohistochemistry analysis showed a significant increase of RHBDD2 protein expression in association with breast cancer samples negative for progesterone receptor (p = 0.015). Moreover, protein expression analysis corroborated the quantitative RT-PCR results, indicating that breast primary tumors belonging to patients with a more disseminated disease expressed significantly increased levels of RHBDD2 protein compared with less disseminated tumors (p = 0.01).

The role of PARL and HtrA2 in striatal neuronal injury after transient global cerebral ischemia

The presenilin-associated rhomboid-like (PARL) protein and high temperature requirement factor A2 (HtrA2) are key regulators of mitochondrial integrity and play pivotal roles in apoptosis. However, their roles after cerebral ischemia have not been thoroughly elucidated. To clarify these roles, mice were subjected to transient global cerebral ischemia, and striatal neuronal injury was assessed. Western blot and coimmunoprecipitation analyses revealed that PARL and processed HtrA2 localized to mitochondria, and that PARL was bound to HtrA2 in sham animals. Expression of PARL and processed HtrA2 in mitochondria significantly decreased 6 to 72 hours after ischemia, and the binding of PARL to HtrA2 disappeared after ischemia. In contrast, expression of processed HtrA2 increased 24 hours after ischemia in the cytosol, where HtrA2 was bound to X chromosome-linked inhibitor-of-apoptosis protein (XIAP). Administration of PARL small interfering RNA inhibited HtrA2 processing and worsened ischemic neuronal injury. Our results show that downregulation of PARL after ischemia is a key step in ischemic neuronal injury, and that it decreases HtrA2 processing and increases neuronal vulnerability. In addition, processed HtrA2 released into the cytosol after ischemia contributes to neuronal injury via inhibition of XIAP.

Structure and mechanism of rhomboid protease

Rhomboid protease was first discovered in Drosophila. Mutation of the fly gene interfered with growth factor signaling and produced a characteristic phenotype of a pointed head skeleton. The name rhomboid has since been widely used to describe a large family of related membrane proteins that have diverse biological functions but share a common catalytic core domain composed of six membrane-spanning segments. Most rhomboid proteases cleave membrane protein substrates near the N terminus of their transmembrane domains. How these proteases function within the confines of the membrane is not completely understood. Recent progress in crystallographic analysis of the Escherichia coli rhomboid protease GlpG in complex with inhibitors has provided new insights into the catalytic mechanism of the protease and its conformational change. Improved biochemical assays have also identified a substrate sequence motif that is specifically recognized by many rhomboid proteases.

Rhomboids of Mycobacteria: characterization using an aarA mutant of Providencia stuartii and gene deletion in Mycobacterium smegmatis

Background

Rhomboids are ubiquitous proteins with unknown roles in mycobacteria. However, bioinformatics suggested putative roles in DNA replication pathways and metabolite transport. Here, mycobacterial rhomboid-encoding genes were characterized; first, using the Providencia stuartii null-rhomboid mutant and then deleted from Mycobacterium smegmatis for additional insight in mycobacteria.

Methodology/Principal Findings

Using in silico analysis we identified in M. tuberculosis genome the genes encoding two putative rhomboid proteins; Rv0110 (referred to as “rhomboid protease 1”) and Rv1337 (“rhomboid protease 2”). Genes encoding orthologs of these proteins are widely represented in all mycobacterial species. When transformed into P. stuartii null-rhomboid mutant (ΔaarA), genes encoding mycobacterial orthologs of “rhomboid protease 2” fully restored AarA activity (AarA is the rhomboid protein of P. stuartii). However, most genes encoding mycobacterial “rhomboid protease 1” orthologs did not. Furthermore, upon gene deletion in M. smegmatis, the ΔMSMEG_4904 single mutant (which lost the gene encoding MSMEG_4904, orthologous to Rv1337, “rhomboid protease 2”) formed the least biofilms and was also more susceptible to ciprofloxacin and novobiocin, antimicrobials that inhibit DNA gyrase. However, the ΔMSMEG_5036 single mutant (which lost the gene encoding MSMEG_5036, orthologous to Rv0110, “rhomboid protease 1”) was not as susceptible. Surprisingly, the double rhomboid mutant ΔMSMEG_4904–ΔMSMEG_5036 (which lost genes encoding both homologs) was also not as susceptible suggesting compensatory effects following deletion of both rhomboid-encoding genes. Indeed, transforming the double mutant with a plasmid encoding MSMEG_5036 produced phenotypes of the ΔMSMEG_4904 single mutant (i.e. susceptibility to ciprofloxacin and novobiocin).

Conclusions/Significance

Mycobacterial rhomboid-encoding genes exhibit differences in complementing aarA whereby it's only genes encoding “rhomboid protease 2” orthologs that fully restore AarA activity. Additionally, gene deletion data suggests inhibition of DNA gyrase by MSMEG_4904; however, the ameliorated effect in the double mutant suggests occurrence of compensatory mechanisms following deletion of genes encoding both rhomboids.

D. melanogaster, mitochondria and neurodegeneration: small model organism, big discoveries

In developed countries, increased life expectancy is accompanied by an increased prevalence of age-related disorders like cancer and neurodegenerative diseases. Albeit the molecular mechanisms behind the clinically, pathologically and etiologically heterogeneous forms of neurodegeneration are often unclear, impairment of mitochondrial fusion-fission and dynamics emerged in recent years as a feature of neuronal dysfunction and death, pinpointing the need for animal models to investigate the relationship between mitochondrial shape and neurodegeneration. While research on mammalian models is slowed down by the complexity of the organisms and their genomes, the long latency of the symptoms and by the difficulty to generate and analyze large cohorts, the lower metazoan Drosophila melanogaster overcomes these problems, proving to be a suitable model to study neurodegenerative diseases and mitochondria-shaping proteins. Here we will summarize our current knowledge on the link between mitochondrial shape and models of neurodegeneration in the fruitfly. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

Rhomboid proteases in mitochondria and plastids: keeping organelles in shape

Rhomboids constitute the most widespread and conserved family of intramembrane cleaving proteases. They are key regulators of critical cellular processes in bacteria and animals, and are poised to play an equally important role also in plants. Among eukaryotes, a distinct subfamily of rhomboids, prototyped by the mammalian mitochondrial protein Parl, ensures the maintenance of the structural and functional integrity of mitochondria and plastids. Here, we discuss the studies that in the past decade have unveiled the role, regulation, and structure of this unique group of rhomboid proteases. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Rhomboid proteases in plants - still in square one?

Rhomboids are ubiquitous intramembrane serine proteases the sequences of which are found in nearly all sequenced genomes, including those of plants. They were molecularly characterized in a number of organisms, and were found to play a role in a variety of biological functions including signaling, development, apoptosis, mitochondrial integrity, parasite invasion and more. Although rhomboid sequences are found in plants, very little is known about their function. Here, we present the current knowledge in the rhomboids field in general, and in plant rhomboids in particular. In addition, we discuss possible physiological roles of different plant rhomboids.

Conformational change in rhomboid protease GlpG induced by inhibitor binding to its S' subsites

Rhomboid protease conducts proteolysis inside the hydrophobic environment of the membrane. The conformational flexibility of the protease is essential for the enzyme mechanism, but the nature of this flexibility is not completely understood. Here we describe the crystal structure of rhomboid protease GlpG in complex with a phosphonofluoridate inhibitor, which is covalently bonded to the catalytic serine and extends into the S' side of the substrate binding cleft. Inhibitor binding causes subtle but extensive changes in the membrane protease. Many transmembrane helices tilt and shift positions, and the gap between S2 and S5 is slightly widened so that the inhibitor can bind between them. The side chain of Phe-245 from a loop (L5) that acts as a cap rotates and uncovers the opening of the substrate binding cleft to the lipid bilayer. A concurrent turn of the polypeptide backbone at Phe-245 moves the rest of the cap and exposes the catalytic serine to the aqueous solution. This study, together with earlier crystallographic investigation of smaller inhibitors, suggests a simple model for explaining substrate binding to rhomboid protease.

Processing peptidases in mitochondria and chloroplasts

Most of the mitochondrial and chloroplastic proteins are nuclear encoded and synthesized in the cytosol as precursor proteins with N-terminal extensions called targeting peptides. Targeting peptides function as organellar import signals, they are recognized by the import receptors and route precursors through the protein translocons across the organellar membranes. After the fulfilled function, targeting peptides are proteolytically cleaved off inside the organelles by different processing peptidases. The processing of mitochondrial precursors is catalyzed in the matrix by the Mitochondrial Processing Peptidase, MPP, the Mitochondrial Intermediate Peptidase, MIP (recently called Octapeptidyl aminopeptidase 1, Oct1) and the Intermediate cleaving peptidase of 55kDa, Icp55. Furthermore, different inner membrane peptidases (Inner Membrane Proteases, IMPs, Atp23, rhomboids and AAA proteases) catalyze additional processing functions, resulting in intra-mitochondrial sorting of proteins, the targeting to the intermembrane space or in the assembly of proteins into inner membrane complexes. Chloroplast targeting peptides are cleaved off in the stroma by the Stromal Processing Peptidase, SPP. If the protein is further translocated to the thylakoid lumen, an additional thylakoid-transfer sequence is removed by the Thylakoidal Processing Peptidase, TPP. Proper function of the D1 protein of Photosystem II reaction center requires its C-terminal processing by Carboxy-terminal processing protease, CtpA. Both in mitochondria and in chloroplasts, the cleaved targeting peptides are finally degraded by the Presequence Protease, PreP. The organellar proteases involved in precursor processing and targeting peptide degradation constitute themselves a quality control system ensuring the correct maturation and localization of proteins as well as assembly of protein complexes, contributing to sustenance of organelle functions. Dysfunctions of several mitochondrial processing proteases have been shown to be associated with human diseases. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

New insights into parasite rhomboid proteases

The rhomboid-like proteins constitute a large family of intramembrane serine proteases that are present in all branches of life. First studied in Drosophila, these enzymes catalyse the release of the active forms of proteins from the membrane and hence trigger signalling events. In protozoan parasites, a limited number of rhomboid-like proteases have been investigated and some of them are associated to pathogenesis. In Apicomplexans, rhomboid-like protease activity is involved in shedding adhesins from the surface of the zoites during motility and host cell entry. Recently, a Toxoplasma gondii rhomboid was also implicated in an intracellular signalling mechanism leading to parasite proliferation. In Entamoeba histolytica, the capacity to adhere to host cells and to phagocytose cells is potentiated by a rhomboid-like protease. Survey of a small number of protozoan parasite genomes has uncovered species-specific rhomboid-like protease genes, many of which are predicted to encode inactive enzymes. Functional investigation of the rhomboid-like proteases in other protozoan parasites will likely uncover novel and unexpected implications for this family of proteases.

Catalytic mechanism of rhomboid protease GlpG probed by 3,4-dichloroisocoumarin and diisopropyl fluorophosphonate

Rhomboid proteases have many important biological functions. Unlike soluble serine proteases such as chymotrypsin, the active site of rhomboid protease, which contains a Ser-His catalytic dyad, is submerged in the membrane and surrounded by membrane-spanning helices. Previous crystallographic analyses of GlpG, a bacterial rhomboid protease, and its complex with isocoumarin have provided insights into the mechanism of the membrane protease. Here, we studied the interaction of GlpG with 3,4-dichloroisocoumarin and diisopropyl fluorophosphonate, both mechanism-based inhibitors for the serine protease, and describe the crystal structure of the covalent adduct between GlpG and diisopropyl fluorophosphonate, which mimics the oxyanion-containing tetrahedral intermediate of the hydrolytic reaction. The crystal structure confirms that the oxyanion is stabilized by the main chain amide of Ser-201 and by the side chains of His-150 and Asn-154. The phosphorylation of the catalytic Ser-201 weakens its interaction with His-254, causing the catalytic histidine to rotate away from the serine. The rotation of His-254 is accompanied by further rearrangement of the side chains of Tyr-205 and Trp-236 within the substrate-binding groove. The formation of the tetrahedral adduct is also accompanied by opening of the L5 cap and movement of transmembrane helix S5 toward S6 in a direction different from that predicted by the lateral gating model. Combining the new structural data with those on the isocoumarin complex sheds further light on the plasticity of the active site of rhomboid membrane protease.

Identification of calcium stress induced genes in amaranth leaves through suppression subtractive hybridization

Calcium (Ca(2+)) is a critical ion for the growth and development of plants and plays an important role in signal transduction pathways in response to biotic and abiotic stresses. We investigated the Ca(2+) stress responsive-genes in amaranth leaves by using the suppression subtractive hybridization technique. Screening of the libraries generated 420 up-regulated transcripts and 199 down-regulated transcripts. The differentially expressed transcripts were associated with general stress response, transcription factors, gene regulation, signal transduction, and some other with unknown function. Selected genes were used to study their differential regulation by sqRT-PCR. Among the up-regulated transcripts, a fragment containing the motif of C3HC4-type RING-Zinc family was further characterized. The ORF of amaranth zinc finger protein (AhZnf) has a closer relationship with its ortholog from Ricinus communis while is distantly related to the Arabidopsis thaliana C3HC4-type ortholog. We have identified a novel putative zinc finger protein along with other novel proteins such as the wall associated kinase, phosphoinositide binding protein, and rhomboid protease involved in response to Ca(2+) stress in amaranth leaves.

Intramembrane proteolysis in regulated protein trafficking

Regulated intramembrane proteolysis is an evolutionarily conserved mechanism by which membrane-anchored bioactive molecules are released from cellular membranes. In eukaryotic cells, intramembrane proteases are found in different cellular organelles ranging from the endosomal system to mitochondria and chloroplasts. These proteases function in diverse processes such as transcription control, regulated growth factor secretion and recently even a role in the control of mitophagy has been suggested. Genomic annotation has predicted 13 different intramembrane proteases in humans. Apart from few studied examples, very little is known about their function. This review describes emerging principles of how intramembrane proteases contribute to the regulation of cellular protein trafficking in eukaryotic cells and raises the important question of how their activity is controlled.

The mitochondrial intramembrane protease PARL cleaves human Pink1 to regulate Pink1 trafficking

Intramembrane proteolysis is a conserved mechanism that regulates a variety of cellular processes ranging from transcription control to signaling. In mitochondria, the inner membrane rhomboid protease PARL has been implicated in the control of life span and apoptosis by a so far uncharacterized mechanism. Here, we show that PARL cleaves human Pink1, which is implicated in Parkinson's disease, within its conserved membrane anchor. Mature Pink1 is then free to be released into the cytosol or the mitochondrial intermembrane space. Upon depolarization of the mitochondrial membrane potential, the canonical import of Pink1 and PARL-catalyzed processing is blocked, leading to accumulation of the Pink1 precursor. As targeting of this precursor to the outer mitochondrial membrane has been shown to trigger mitophagy, we suggest that the PARL-catalyzed removal of the Pink1 signal sequence in the canonical import pathway acts as a cellular checkpoint for mitochondrial integrity. Furthermore, we show that two Parkinson's disease-causing mutations decrease the processing of Pink1 by PARL, with attendant implications for pathogenesis.

Structural and mechanistic basis of Parl activity and regulation

The mitochondrial rhomboid protease Parl governs apoptosis, morphology, metabolism and might be implicated in Parkinson's disease, but the structural basis of its activity and complex regulation remain unknown. We report the discovery of γ-cleavage, a proteolytic event on the loop connecting the first transmembrane helix (TMH) of Parl to the 6-TMH catalytic rhomboid domain of the protease. This cleavage disrupts the '1+6' structure that defines every mitochondrial rhomboid and generates a new form of Parl, PROD (Parl-rhomboid-domain). Structure-function analysis of Parl suggests that γ-cleavage could be implicated in eliminating Parl proteolytic activity, and structural modeling of PROD reveals structural conservation with the bacterial rhomboid GlpG. However, unlike bacterial rhomboids, which employ a diad-based mechanism of catalysis, Parl appears to use a conserved mitochondrial rhomboid-specific Asp residue on TMH-5 in a triad-based mechanism of catalysis. This work provides unexpected insights into the structural determinants regulating Parl stability and activity in vivo, and reveals a complex cascade of proteolytic events controlling the function of the protease in the mitochondrion.

Mitochondrial membrane potential regulates PINK1 import and proteolytic destabilization by PARL

Differential localization to the inner and outer mitochondrial membranes regulates PINK1 stability and function.

PINK1 is a mitochondrial kinase mutated in some familial cases of Parkinson’s disease. It has been found to work in the same pathway as the E3 ligase Parkin in the maintenance of flight muscles and dopaminergic neurons in Drosophila melanogaster and to recruit cytosolic Parkin to mitochondria to mediate mitophagy in mammalian cells. Although PINK1 has a predicted mitochondrial import sequence, its cellular and submitochondrial localization remains unclear in part because it is rapidly degraded. In this study, we report that the mitochondrial inner membrane rhomboid protease presenilin-associated rhomboid-like protein (PARL) mediates cleavage of PINK1 dependent on mitochondrial membrane potential. In the absence of PARL, the constitutive degradation of PINK1 is inhibited, stabilizing a 60-kD form inside mitochondria. When mitochondrial membrane potential is dissipated, PINK1 accumulates as a 63-kD full-length form on the outer mitochondrial membrane, where it can recruit Parkin to impaired mitochondria. Thus, differential localization to the inner and outer mitochondrial membranes appears to regulate PINK1 stability and function.