Mitochondrial Lon of Saccharomyces cerevisiae is a ring-shaped protease with seven flexible subunits

Knockout Mouse Studies Show That Mitochondrial CLPP Peptidase and CLPX Unfoldase Act in Matrix Condensates near IMM, as Fast Stress Response in Protein Assemblies for Transcript Processing, Translation, and Heme Production

LONP1 is the principal AAA+ unfoldase and bulk protease in the mitochondrial matrix, so its deletion causes embryonic lethality. The AAA+ unfoldase CLPX and the peptidase CLPP also act in the matrix, especially during stress periods, but their substrates are poorly defined. Mammalian CLPP deletion triggers infertility, deafness, growth retardation, and cGAS-STING-activated cytosolic innate immunity. CLPX mutations impair heme biosynthesis and heavy metal homeostasis. CLPP and CLPX are conserved from bacteria to humans, despite their secondary role in proteolysis. Based on recent proteomic-metabolomic evidence from knockout mice and patient cells, we propose that CLPP acts on phase-separated ribonucleoprotein granules and CLPX on multi-enzyme condensates as first-aid systems near the inner mitochondrial membrane. Trimming within assemblies, CLPP rescues stalled processes in mitoribosomes, mitochondrial RNA granules and nucleoids, and the D-foci-mediated degradation of toxic double-stranded mtRNA/mtDNA. Unfolding multi-enzyme condensates, CLPX maximizes PLP-dependent delta-transamination and rescues malformed nascent peptides. Overall, their actions occur in granules with multivalent or hydrophobic interactions, separated from the aqueous phase. Thus, the role of CLPXP in the matrix is compartment-selective, as other mitochondrial peptidases: MPPs at precursor import pores, m-AAA and i-AAA at either IMM face, PARL within the IMM, and OMA1/HTRA2 in the intermembrane space.

WITHDRAWN: Dual targeting of mitochondrial Lon peptidase 1 and chymotrypsin-like protease by small molecule BT317, as potential therapeutics in malignant astrocytomas

The authors have withdrawn their manuscript owing to massive revision and data validation. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author.

Differential sensitivity of the yeast Lon protease Pim1p to impaired mitochondrial respiration

Mitochondria are essential organelles whose proteome is well protected by regulated protein degradation and quality control. While the ubiquitin-proteasome system can monitor mitochondrial proteins that reside at the mitochondrial outer membrane or are not successfully imported, resident proteases generally act on proteins within mitochondria. Herein, we assess the degradative pathways for mutant forms of three mitochondrial matrix proteins (mas1-1HA, mas2-11HA, and tim44-8HA) in Saccharomyces cerevisiae. The degradation of these proteins is strongly impaired by loss of either the matrix AAA-ATPase (m-AAA) (Afg3p/Yta12p) or Lon (Pim1p) protease. We determine that these mutant proteins are all bona fide Pim1p substrates whose degradation is also blocked in respiratory-deficient "petite" yeast cells, such as in cells lacking m-AAA protease subunits. In contrast, matrix proteins that are substrates of the m-AAA protease are not affected by loss of respiration. The failure to efficiently remove Pim1p substrates in petite cells has no evident relationship to Pim1p maturation, localization, or assembly. However, Pim1p's autoproteolysis is intact, and its overexpression restores substrate degradation, indicating that Pim1p retains some functionality in petite cells. Interestingly, chemical perturbation of mitochondria with oligomycin similarly prevents degradation of Pim1p substrates. Our results demonstrate that Pim1p activity is highly sensitive to mitochondrial perturbations such as loss of respiration or drug treatment in a manner that we do not observe with other proteases.

Structure, substrate binding and activity of a unique AAA+ protein: the BrxL phage restriction factor

Bacteriophage exclusion ('BREX') systems are multi-protein complexes encoded by a variety of bacteria and archaea that restrict phage by an unknown mechanism. One BREX factor, termed BrxL, has been noted to display sequence similarity to various AAA+ protein factors including Lon protease. In this study we describe multiple CryoEM structures of BrxL that demonstrate it to be a chambered, ATP-dependent DNA binding protein. The largest BrxL assemblage corresponds to a dimer of heptamers in the absence of bound DNA, versus a dimer of hexamers when DNA is bound in its central pore. The protein displays DNA-dependent ATPase activity, and ATP binding promotes assembly of the complex on DNA. Point mutations within several regions of the protein-DNA complex alter one or more in vitro behaviors and activities, including ATPase activity and ATP-dependent association with DNA. However, only the disruption of the ATPase active site fully eliminates phage restriction, indicating that other mutations can still complement BrxL function within the context of an otherwise intact BREX system. BrxL displays significant structural homology to MCM subunits (the replicative helicase in archaea and eukaryotes), implying that it and other BREX factors may collaborate to disrupt initiation of phage DNA replication.

Mitochondrial matrix proteases: quality control and beyond

To ensure correct function, mitochondria have developed several mechanisms of protein quality control (QC). Protein homeostasis highly relies on chaperones and proteases to maintain proper folding and remove damaged proteins that might otherwise form cell-toxic aggregates. Besides quality control, mitochondrial proteases modulate and regulate many essential functions, such as trafficking, processing and activation of mitochondrial proteins, mitochondrial dynamics, mitophagy and apoptosis. Therefore, the impaired function of mitochondrial proteases is associated with various pathological conditions, including cancer, metabolic syndromes and neurodegenerative disorders. This review recapitulates and discusses the emerging roles of two major proteases of the mitochondrial matrix, LON and ClpXP. Although commonly acknowledge for their protein quality control role, recent advances have uncovered several highly regulated processes controlled by the LON and ClpXP connected to mitochondrial gene expression and respiratory chain function maintenance. Furthermore, both proteases have been lately recognized as potent targets for anticancer therapies, and we summarize those findings.

Comprehensive characterization of the Hsp70 interactome reveals novel client proteins and interactions mediated by posttranslational modifications

Hsp70 interactions are critical for cellular viability and the response to stress. Previous attempts to characterize Hsp70 interactions have been limited by their transient nature and the inability of current technologies to distinguish direct versus bridged interactions. We report the novel use of cross-linking mass spectrometry (XL-MS) to comprehensively characterize the Saccharomyces cerevisiae (budding yeast) Hsp70 protein interactome. Using this approach, we have gained fundamental new insights into Hsp70 function, including definitive evidence of Hsp70 self-association as well as multipoint interaction with its client proteins. In addition to identifying a novel set of direct Hsp70 interactors that can be used to probe chaperone function in cells, we have also identified a suite of posttranslational modification (PTM)-associated Hsp70 interactions. The majority of these PTMs have not been previously reported and appear to be critical in the regulation of client protein function. These data indicate that one of the mechanisms by which PTMs contribute to protein function is by facilitating interaction with chaperones. Taken together, we propose that XL-MS analysis of chaperone complexes may be used as a unique way to identify biologically important PTMs on client proteins.

Cryo-EM structure of hexameric yeast Lon protease (PIM1) highlights the importance of conserved structural elements

Lon protease is a conserved ATP-dependent serine protease composed of an AAA+ domain that mechanically unfolds substrates and a serine protease domain that degrades these unfolded substrates. In yeast, dysregulation of Lon protease (PIM1) attenuates lifespan and leads to gross mitochondrial morphological perturbations. Although structures of the bacterial and human Lon protease reveal a hexameric assembly, yeast PIM1 was speculated to form a heptameric assembly and is uniquely characterized by a ∼50-residue insertion between the ATPase and protease domains. To further understand the yeast-specific properties of PIM1, we determined a high-resolution cryo-electron microscopy structure of PIM1 in a substrate-translocating state. Here, we reveal that PIM1 forms a hexamer, conserved with that of bacterial and human Lon proteases, wherein the ATPase domains form a canonical closed spiral that enables pore loop residues to translocate substrates to the protease chamber. In the substrate-translocating state, PIM1 protease domains form a planar protease chamber in an active conformation and are uniquely characterized by a ∼15-residue C-terminal extension. These additional C-terminal residues form an α-helix located along the base of the protease domain. Finally, we did not observe density for the yeast-specific insertion between the ATPase and protease domains, likely due to high conformational flexibility. Biochemical studies to investigate the insertion using constructs that truncated or replaced the insertion with a glycine-serine linker suggest that the yeast-specific insertion is dispensable for PIM1's enzymatic function. Altogether, our structural and biochemical studies highlight unique components of PIM1 machinery and demonstrate evolutionary conservation of Lon protease function.

Oxidative Modification of Proteins: From Damage to Catalysis, Signaling, and Beyond

Significance: The systematic investigation of oxidative modification of proteins by reactive oxygen species started in 1980. Later, it was shown that reactive nitrogen species could also modify proteins. Some protein oxidative modifications promote loss of protein function, cleavage or aggregation, and some result in proteo-toxicity and cellular homeostasis disruption. Recent Advances: Previously, protein oxidation was associated exclusively to damage. However, not all oxidative modifications are necessarily associated with damage, as with Met and Cys protein residue oxidation. In these cases, redox state changes can alter protein structure, catalytic function, and signaling processes in response to metabolic and/or environmental alterations. This review aims to integrate the present knowledge on redox modifications of proteins with their fate and role in redox signaling and human pathological conditions. Critical Issues: It is hypothesized that protein oxidation participates in the development and progression of many pathological conditions. However, no quantitative data have been correlated with specific oxidized proteins or the progression or severity of pathological conditions. Hence, the comprehension of the mechanisms underlying these modifications, their importance in human pathologies, and the fate of the modified proteins is of clinical relevance. Future Directions: We discuss new tools to cope with protein oxidation and suggest new approaches for integrating knowledge about protein oxidation and redox processes with human pathophysiological conditions. Antioxid. Redox Signal. 35, 1016-1080.

Mitochondrial Protein Synthesis and mtDNA Levels Coordinated through an Aminoacyl-tRNA Synthetase Subunit

The aminoacylation of tRNAs by aminoacyl-tRNA synthetases (ARSs) is a central reaction in biology. Multiple regulatory pathways use the aminoacylation status of cytosolic tRNAs to monitor and regulate metabolism. The existence of equivalent regulatory networks within the mitochondria is unknown. Here, we describe a functional network that couples protein synthesis to DNA replication in animal mitochondria. We show that a duplication of the gene coding for mitochondrial seryl-tRNA synthetase (SerRS2) generated in arthropods a paralog protein (SLIMP) that forms a heterodimeric complex with a SerRS2 monomer. This seryl-tRNA synthetase variant is essential for protein synthesis and mitochondrial respiration. In addition, SLIMP interacts with the substrate binding domain of the mitochondrial protease LON, thus stimulating proteolysis of the DNA-binding protein TFAM and preventing mitochondrial DNA (mtDNA) accumulation. Thus, mitochondrial translation is directly coupled to mtDNA levels by a network based upon a profound structural modification of an animal ARS.

The biology of Lonp1: More than a mitochondrial protease

Initially discovered as a protease responsible for degradation of misfolded or damaged proteins, the mitochondrial Lon protease (Lonp1) turned out to be a multifaceted enzyme, that displays at least three different functions (proteolysis, chaperone activity, binding of mtDNA) and that finely regulates several cellular processes, within and without mitochondria. Indeed, LONP1 in humans is ubiquitously expressed, and is involved in regulation of response to oxidative stress and, heat shock, in the maintenance of mtDNA, in the regulation of mitophagy. Furthermore, its proteolytic activity can regulate several biochemical pathways occurring totally or partially within mitochondria, such as TCA cycle, oxidative phosphorylation, steroid and heme biosynthesis and glutamine production. Because of these multiple activities, Lon protease is highly conserved throughout evolution, and mutations occurring in its gene determines severe diseases in humans, including a rare syndrome characterized by Cerebral, Ocular, Dental, Auricular and Skeletal anomalies (CODAS). Finally, alterations of LONP1 regulation in humans can favor tumor progression and aggressiveness, further highlighting the crucial role of this enzyme in mitochondrial and cellular homeostasis.

The Mitochondrial Lon Protease: Novel Functions off the Beaten Track?

To maintain organellar function, mitochondria contain an elaborate endogenous protein quality control system. As one of the two soluble energy-dependent proteolytic enzymes in the matrix compartment, the protease Lon is a major component of this system, responsible for the degradation of misfolded proteins, in particular under oxidative stress conditions. Lon defects have been shown to negatively affect energy production by oxidative phosphorylation but also mitochondrial gene expression. In this review, recent studies on the role of Lon in mammalian cells, in particular on its protective action under diverse stress conditions and its relationship to important human diseases are summarized and commented.

Cryo-EM structure of substrate-free E. coli Lon protease provides insights into the dynamics of Lon machinery

Energy-dependent Lon proteases play a key role in cellular regulation by degrading short-lived regulatory proteins and misfolded proteins in the cell. The structure of the catalytically inactive S679A mutant of Escherichia coli LonA protease (EcLon) has been determined by cryo-EM at the resolution of 3.5 Å. EcLonA without a bound substrate adopts a hexameric open-spiral quaternary structure that might represent the resting state of the enzyme. Upon interaction with substrate the open-spiral hexamer undergoes a major conformational change resulting in a compact, closed-circle hexamer as in the recent structure of a complex of Yersinia pestis LonA with a protein substrate. This major change is accomplished by the rigid-body rearrangement of the individual domains within the protomers of the complex around the hinge points in the interdomain linkers. Comparison of substrate-free and substrate-bound Lon structures allows to mark the location of putative pivotal points involved in such conformational changes.

Comprehensive analysis of Lon proteases in plants highlights independent gene duplication events

The degradation of damaged proteins is essential for cell viability. Lon is a highly conserved ATP-dependent serine-lysine protease that maintains proteostasis. We performed a comparative genome-wide analysis to determine the evolutionary history of Lon proteases. Prokaryotes and unicellular eukaryotes retained a single Lon copy, whereas multicellular eukaryotes acquired a peroxisomal copy, in addition to the mitochondrial gene, to sustain the evolution of higher order organ structures. Land plants developed small Lon gene families. Despite the Lon2 peroxisomal paralog, Lon genes triplicated in the Arabidopsis lineage through sequential evolutionary events including whole-genome and tandem duplications. The retention of Lon1, Lon4, and Lon3 triplicates relied on their differential and even contrasting expression patterns, distinct subcellular targeting mechanisms, and functional divergence. Lon1 seems similar to the pre-duplication ancestral gene unit, whereas the duplication of Lon3 and Lon4 is evolutionarily recent. In the wider context of plant evolution, papaya is the only genome with a single ancestral Lon1-type gene. The evolutionary trend among plants is to acquire Lon copies with ambiguous pre-sequences for dual-targeting to mitochondria and chloroplasts, and a substrate recognition domain that deviates from the ancestral Lon1 type. Lon genes constitute a paradigm of dynamic evolution contributing to understanding the functional fate of gene duplicates.

Drosophila protease ClpXP specifically degrades DmLRPPRC1 controlling mitochondrial mRNA and translation

ClpXP is the major protease in the mitochondrial matrix in eukaryotes, and is well conserved among species. ClpXP is composed of a proteolytic subunit, ClpP, and a chaperone-like subunit, ClpX. Although it has been proposed that ClpXP is required for the mitochondrial unfolded protein response, additional roles for ClpXP in mitochondrial biogenesis are unclear. Here, we found that Drosophila leucine-rich pentatricopeptide repeat domain-containing protein 1 (DmLRPPRC1) is a specific substrate of ClpXP. Depletion or introduction of catalytically inactive mutation of ClpP increases DmLRPPRC1 and causes non-uniform increases of mitochondrial mRNAs, accumulation of some unprocessed mitochondrial transcripts, and modest repression of mitochondrial translation in Drosophila Schneider S2 cells. Moreover, DmLRPPRC1 over-expression induces the phenotypes similar to those observed when ClpP is depleted. Taken together, ClpXP regulates mitochondrial gene expression by changing the protein level of DmLRPPRC1 in Drosophila Schneider S2 cells.

Defining the crucial domain and amino acid residues in bacterial Lon protease for DNA binding and processing of DNA-interacting substrates

Lon protease previously has been shown to interact with DNA, but the role of this interaction for Lon proteolytic activity has not been characterized. In this study, we used truncated Escherichia coli Lon constructs, bioinformatics analysis, and site-directed mutagenesis to identify Lon domains and residues crucial for Lon binding with DNA and effects on Lon proteolytic activity. We found that deletion of Lon's ATPase domain abrogated interactions with DNA. Substitution of positively charged amino acids in this domain in full-length Lon with residues conferring a net negative charge disrupted binding of Lon to DNA. These changes also affected the degradation of nucleic acid-binding protein substrates of Lon, intracellular localization of Lon, and cell morphology. In vivo tests revealed that Lon-DNA interactions are essential for Lon activity in cell division control. In summary, we demonstrate that the ability of Lon to bind DNA is determined by its ATPase domain, that this binding is required for processing protein substrates in nucleoprotein complexes, and that Lon may help regulate DNA replication in response to growth conditions.

Assessing heterogeneity in oligomeric AAA+ machines

ATPases Associated with various cellular Activities (AAA+ ATPases) are molecular motors that use the energy of ATP binding and hydrolysis to remodel their target macromolecules. The majority of these ATPases form ring-shaped hexamers in which the active sites are located at the interfaces between neighboring subunits. Structural changes initiate in an active site and propagate to distant motor parts that interface and reshape the target macromolecules, thereby performing mechanical work. During the functioning cycle, the AAA+ motor transits through multiple distinct states. Ring architecture and placement of the catalytic sites at the intersubunit interfaces allow for a unique level of coordination among subunits of the motor. This in turn results in conformational differences among subunits and overall asymmetry of the motor ring as it functions. To date, a large amount of structural information has been gathered for different AAA+ motors, but even for the most characterized of them only a few structural states are known and the full mechanistic cycle cannot be yet reconstructed. Therefore, the first part of this work will provide a broad overview of what arrangements of AAA+ subunits have been structurally observed focusing on diversity of ATPase oligomeric ensembles and heterogeneity within the ensembles. The second part of this review will concentrate on methods that assess structural and functional heterogeneity among subunits of AAA+ motors, thus bringing us closer to understanding the mechanism of these fascinating molecular motors.

Mini review: ATP-dependent proteases in bacteria

AAA(+) proteases are universal barrel-like and ATP-fueled machines preventing the accumulation of aberrant proteins and regulating the proteome according to the cellular demand. They are characterized by two separate operating units, the ATPase and peptidase domains. ATP-dependent unfolding and translocation of a substrate into the proteolytic chamber is followed by ATP-independent degradation. This review addresses the structure and function of bacterial AAA(+) proteases with a focus on the ATP-driven mechanisms and the coordinated movements in the complex mainly based on the knowledge of ClpXP. We conclude by discussing strategies how novel protease substrates can be trapped by mutated AAA(+) protease variants. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 505-517, 2016.

The N-terminal domain plays a crucial role in the structure of a full-length human mitochondrial Lon protease

Lon is an essential, multitasking AAA⁺ protease regulating many cellular processes in species across all kingdoms of life. Altered expression levels of the human mitochondrial Lon protease (hLon) are linked to serious diseases including myopathies, paraplegia, and cancer. Here, we present the first 3D structure of full-length hLon using cryo-electron microscopy. hLon has a unique three-dimensional structure, in which the proteolytic and ATP-binding domains (AP-domain) form a hexameric chamber, while the N-terminal domain is arranged as a trimer of dimers. These two domains are linked by a narrow trimeric channel composed likely of coiled-coil helices. In the presence of AMP-PNP, the AP-domain has a closed-ring conformation and its N-terminal entry gate appears closed, but in ADP binding, it switches to a lock-washer conformation and its N-terminal gate opens, which is accompanied by a rearrangement of the N-terminal domain. We have also found that both the enzymatic activities and the 3D structure of a hLon mutant lacking the first 156 amino acids are severely disturbed, showing that hLon’s N-terminal domains are crucial for the overall structure of the hLon, maintaining a conformation allowing its proper functioning.

Structural Basis for the Magnesium-Dependent Activation and Hexamerization of the Lon AAA+ Protease

The Lon AAA+ protease (LonA) plays important roles in protein homeostasis and regulation of diverse biological processes. LonA behaves as a homomeric hexamer in the presence of magnesium (Mg(2+)) and performs ATP-dependent proteolysis. However, it is also found that LonA can carry out Mg(2+)-dependent degradation of unfolded protein substrate in an ATP-independent manner. Here we show that in the presence of Mg(2+) LonA forms a non-secluded hexameric barrel with prominent openings, which explains why Mg(2+)-activated LonA can operate as a diffusion-based chambered protease to degrade unstructured protein and peptide substrates efficiently in the absence of ATP. A 1.85 Å crystal structure of Mg(2+)-activated protease domain reveals Mg(2+)-dependent remodeling of a substrate-binding loop and a potential metal-binding site near the Ser-Lys catalytic dyad, supported by biophysical binding assays and molecular dynamics simulations. Together, these findings reveal the specific roles of Mg(2+) in the molecular assembly and activation of LonA.

Emerging role of Lon protease as a master regulator of mitochondrial functions

Lon protease is a nuclear-encoded, mitochondrial ATP-dependent protease highly conserved throughout the evolution, crucial for the maintenance of mitochondrial homeostasis. Lon acts as a chaperone of misfolded proteins, and is necessary for maintaining mitochondrial DNA. The impairment of these functions has a deep impact on mitochondrial functionality and morphology. An altered expression of Lon leads to a profound reprogramming of cell metabolism, with a switch from respiration to glycolysis, which is often observed in cancer cells. Mutations of Lon, which likely impair its chaperone properties, are at the basis of a genetic inherited disease named of the cerebral, ocular, dental, auricular, skeletal (CODAS) syndrome. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

The peroxisomal Lon protease LonP2 in aging and disease: functions and comparisons with mitochondrial Lon protease LonP1

Peroxisomes are ubiquitous eukaryotic organelles with the primary role of breaking down very long- and branched-chain fatty acids for subsequent β-oxidation in the mitochondrion. Like mitochondria, peroxisomes are major sites for oxygen utilization and potential contributors to cellular oxidative stress. The accumulation of oxidatively damaged proteins, which often develop into inclusion bodies (of oxidized, aggregated, and cross-linked proteins) within both mitochondria and peroxisomes, results in loss of organelle function that may contribute to the aging process. Both organelles possess an isoform of the Lon protease that is responsible for degrading proteins damaged by oxidation. While the importance of mitochondrial Lon (LonP1) in relation to oxidative stress and aging has been established, little is known regarding the role of LonP2 and aging-related changes in the peroxisome. Recently, peroxisome dysfunction has been associated with aging-related diseases indicating that peroxisome maintenance is a critical component of 'healthy aging'. Although mitochondria and peroxisomes are both needed for fatty acid metabolism, little work has focused on understanding the relationship between these two organelles including how age-dependent changes in one organelle may be detrimental for the other. Herein, we summarize findings that establish proteolytic degradation of damaged proteins by the Lon protease as a vital mechanism to maintain protein homeostasis within the peroxisome. Due to the metabolic coordination between peroxisomes and mitochondria, understanding the role of Lon in the aging peroxisome may help to elucidate cellular causes for both peroxisome and mitochondrial dysfunction.

Protection of scaffold protein Isu from degradation by the Lon protease Pim1 as a component of Fe-S cluster biogenesis regulation

Fe–S clusters are built on and transferred from the scaffold Isu. Isu is a substrate of Lon protease. Binding Nfs1, the sulfur donor for cluster assembly, or Jac1, the protein initiating cluster transfer, protects Isu from degradation. Such protection increases Isu levels, likely serving to rapidly up-regulate cellular Fe–S cluster biogenesis capacity.

Iron–sulfur (Fe–S) clusters, essential protein cofactors, are assembled on the mitochondrial scaffold protein Isu and then transferred to recipient proteins via a multistep process in which Isu interacts sequentially with multiple protein factors. This pathway is in part regulated posttranslationally by modulation of the degradation of Isu, whose abundance increases >10-fold upon perturbation of the biogenesis process. We tested a model in which direct interaction with protein partners protects Isu from degradation by the mitochondrial Lon-type protease. Using purified components, we demonstrated that Isu is indeed a substrate of the Lon-type protease and that it is protected from degradation by Nfs1, the sulfur donor for Fe–S cluster assembly, as well as by Jac1, the J-protein Hsp70 cochaperone that functions in cluster transfer from Isu. Nfs1 and Jac1 variants known to be defective in interaction with Isu were also defective in protecting Isu from degradation. Furthermore, overproduction of Jac1 protected Isu from degradation in vivo, as did Nfs1. Taken together, our results lead to a model of dynamic interplay between a protease and protein factors throughout the Fe–S cluster assembly and transfer process, leading to up-regulation of Isu levels under conditions when Fe–S cluster biogenesis does not meet cellular demands.

Mitochondrial Lon protease at the crossroads of oxidative stress, ageing and cancer

Lon protease is a nuclear DNA-encoded mitochondrial enzyme highly conserved throughout evolution, involved in the degradation of damaged and oxidized proteins of the mitochondrial matrix, in the correct folding of proteins imported in mitochondria, and in the maintenance of mitochondrial DNA. Lon expression is induced by various stimuli, including hypoxia and reactive oxygen species, and provides protection against cell stress. Lon down-regulation is associated with ageing and with cell senescence, while up-regulation is observed in tumour cells, and is correlated with a more aggressive phenotype of cancer. Lon up-regulation contributes to metabolic reprogramming observed in cancer, favours the switch from a respiratory to a glycolytic metabolism, helping cancer cell survival in the tumour microenvironment, and contributes to epithelial to mesenchymal transition. Silencing of Lon, or pharmacological inhibition of its activity, causes cell death in various cancer cells. Thus, Lon can be included in the growing class of proteins that are not responsible for oncogenic transformation, but that are essential for survival and proliferation of cancer cells, and that can be considered as a new target for development of anticancer drugs.

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.

Backbone resonance assignments of the α sub-domain of Brevibacillus thermoruber Lon protease

Lon is an ATPases associated with diverse cellular activities protease and belongs to a unique group that binds DNA. The α sub-domain of Lon protease is responsible for DNA-binding, but the structural information for its DNA-recognition mode is still limited. Here, we report (1)H, (15)N and (13)C backbone assignment for the α sub-domain from Brevibacillus thermoruber Lon protease as the basis for the elucidation of its structure and interactions with DNA, necessary for understanding the allosteric regulatory mechanism of the enzymatic function.

Mutations to a glycine loop in the catalytic site of human Lon changes its protease, peptidase and ATPase activities

Lon, also called protease La, is an ATP-dependent protease present in all kingdoms of life. It is involved in protein quality control and several regulatory processes. Eukaryotic Lon possesses three domains, an N-terminal domain, an ATPase domain and a proteolytic domain. It requires ATP hydrolysis to digest larger, intact proteins, but can cleave small, fluorogenic peptides such as Glu-Ala-Ala-Phe-MNA by only binding, but not hydrolyzing, ATP. Both ATPase and peptidase activities can be stimulated by the binding of a larger protein substrate, such as β-casein. To better understand its mechanism of action, we have prepared several point mutants of four conserved residues of human Lon (G893A, G893P, G894A, G894P, G894S, G893A-G894A, G893P-G894A, G893A-G894P, T880V, W770A, W770P) and studied their ATPase, protease and peptidase activities. Our results show that mutations to Gly894 enhance its basal ATPase activity but do not change its β-casein-stimulated activity. The loop containing Gly893 and Gly894, which flanks Lon's proteolytic active site, therefore appears to be involved in the conformational change that occurs upon substrate binding. Furthermore, mutations to Trp770 have the same general effects on the ATPase activity as mutations to Gly893, indicating that Trp770 is involved in ATPase stimulation. We have also established that this loop does not need to move in order to cleave small, fluorogenic peptides, but does move during the digestion of β-casein. Finally, we also noted that Lon's ability to digest small peptides can be inhibited by moderate ATP concentrations.

DATABASE

Lon (Endopeptidase La), EC 4.4.21.53 STRUCTURED DIGITAL ABSTRACT: • hLonP cleaves beta casein by protease assay (1, 2, 3, 4, 5, 6) • hLon and hLon bind by cross-linking study (View interaction).

Evolution and significance of the Lon gene family in Arabidopsis organelle biogenesis and energy metabolism

Lon is the first identified ATP-dependent protease highly conserved across all kingdoms. Model plant species Arabidopsis thaliana has a small Lon gene family of four members. Although these genes share common structural features, they have distinct properties in terms of gene expression profile, subcellular targeting and substrate recognition motifs. This supports the notion that their functions under different environmental conditions are not necessarily redundant. This article intends to unravel the biological role of Lon proteases in energy metabolism and plant growth through an evolutionary perspective. Given that plants are sessile organisms exposed to diverse environmental conditions and plant organelles are semi-autonomous, it is tempting to suggest that Lon genes in Arabidopsis are paralogs. Adaptive evolution through repetitive gene duplication events of a single archaic gene led to Lon genes with complementing sets of subfunctions providing to the organism rapid adaptability for canonical development under different environmental conditions. Lon1 function is adequately characterized being involved in mitochondrial biogenesis, modulating carbon metabolism, oxidative phosphorylation and energy supply, all prerequisites for seed germination and seedling establishment. Lon is not a stand-alone proteolytic machine in plant organelles. Lon in association with other nuclear-encoded ATP-dependent proteases builds up an elegant nevertheless, tight interconnected circuit. This circuitry channels properly and accurately, proteostasis and protein quality control among the distinct subcellular compartments namely mitochondria, chloroplasts, and peroxisomes.

Crystallization and preliminary X-ray diffraction analysis of the α subdomain of Lon protease from Brevibacillus thermoruber

DNA-binding ability has previously been reported as a novel function for the thermostable Lon protease from Brevibacillus thermoruber WR-249 (Bt-Lon), and the α subdomain (amino acids 491-605) of Bt-Lon has been identified as being responsible for DNA binding. However, the physiological role and DNA-recognition mode of Bt-Lon still remain unclear. In this study, the crystallization and preliminary crystallographic analysis of the Bt-Lon α subdomain are presented. Native diffraction data to 2.88 Å resolution were obtained from a vitrified crystal at 100 K on the BL13C1 beamline at the NSRRC (National Synchrotron Radiation Research Center), Taiwan. The crystals belonged to space group P23, with unit-cell parameters a = b = c = 94.28 Å. Solvent-content calculations and molecular-replacement results suggest that there are two molecules of Bt-Lon α subdomain per asymmetric unit.

Mitochondrial matrix proteases as novel therapeutic targets in malignancy

Although mitochondrial function is often altered in cancer, it remains essential for tumor viability. Tight control of protein homeostasis is required for the maintenance of mitochondrial function, and the mitochondrial matrix houses several coordinated protein quality control systems. These include three evolutionarily conserved proteases of the AAA+ superfamily-the Lon, ClpXP and m-AAA proteases. In humans, these proteases are proposed to degrade, process and chaperone the assembly of mitochondrial proteins in the matrix and inner membrane involved in oxidative phosphorylation, mitochondrial protein synthesis, mitochondrial network dynamics and nucleoid function. In addition, these proteases are upregulated by a variety of mitochondrial stressors, including oxidative stress, unfolded protein stress and imbalances in respiratory complex assembly. Given that tumor cells must survive and proliferate under dynamic cellular stress conditions, dysregulation of mitochondrial protein quality control systems may provide a selective advantage. The association of mitochondrial matrix AAA+ proteases with cancer and their potential for therapeutic modulation therefore warrant further consideration. Although our current knowledge of the endogenous human substrates of these proteases is limited, we highlight functional insights gained from cultured human cells, protease-deficient mouse models and other eukaryotic model organisms. We also review the consequences of disrupting mitochondrial matrix AAA+ proteases through genetic and pharmacological approaches, along with implications of these studies on the potential of these proteases as anticancer therapeutic targets.

Discovery of functional toxin/antitoxin systems in bacteria by shotgun cloning

Toxin-antitoxin (TA) modules, composed of a toxic protein and a counteracting antitoxin, play important roles in bacterial physiology. We examined the experimental insertion of 1.5 million genes from 388 microbial genomes into an Escherichia coli host using more than 8.5 million random clones. This revealed hundreds of genes (toxins) that could only be cloned when the neighboring gene (antitoxin) was present on the same clone. Clustering of these genes revealed TA families widespread in bacterial genomes, some of which deviate from the classical characteristics previously described for such modules. Introduction of these genes into E. coli validated that the toxin toxicity is mitigated by the antitoxin. Infection experiments with T7 phage showed that two of the new modules can provide resistance against phage. Moreover, our experiments revealed an "antidefense" protein in phage T7 that neutralizes phage resistance. Our results expose active fronts in the arms race between bacteria and phage.

The role of bacterial enhancer binding proteins as specialized activators of σ54-dependent transcription

Bacterial enhancer binding proteins (bEBPs) are transcriptional activators that assemble as hexameric rings in their active forms and utilize ATP hydrolysis to remodel the conformation of RNA polymerase containing the alternative sigma factor σ(54). We present a comprehensive and detailed summary of recent advances in our understanding of how these specialized molecular machines function. The review is structured by introducing each of the three domains in turn: the central catalytic domain, the N-terminal regulatory domain, and the C-terminal DNA binding domain. The role of the central catalytic domain is presented with particular reference to (i) oligomerization, (ii) ATP hydrolysis, and (iii) the key GAFTGA motif that contacts σ(54) for remodeling. Each of these functions forms a potential target of the signal-sensing N-terminal regulatory domain, which can act either positively or negatively to control the activation of σ(54)-dependent transcription. Finally, we focus on the DNA binding function of the C-terminal domain and the enhancer sites to which it binds. Particular attention is paid to the importance of σ(54) to the bacterial cell and its unique role in regulating transcription.

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.

The Lon AAA+ protease

As the first ATP-dependent protease to be identified, Lon holds a special place in the history of cellular biology. In fact, the concept of ATP-dependent protein degradation was established through the findings that led to the discovery of Lon. Therefore, this chapter begins with a historical perspective, describing the milestones that led to the discovery of Lon and ATP-dependent proteolysis, starting from the early findings in the 1960s until the demonstration of Lon's ATP-dependent proteolytic activity in vitro, in 1981. Most of our knowledge on Lon derives from studies of the Escherichia coli Lon ortholog, and, therefore, most of this chapter relates to this particular enzyme. Nonetheless, Lon is not only found in most bacterial species, it is also found in Archaea and in the mitochondrion and chloroplast of eukaryotic cells. Therefore many of the conclusions gained from studies on the E. coli enzyme are relevant to Lon proteases in other organisms. Lon, more than any other bacterial or organellar protease, is associated with the degradation of misfolded proteins and protein quality control. In addition, Lon also degrades many regulatory proteins that are natively folded, thus it also plays a prominent role in regulation of physiological processes. Throughout the years, many Lon substrates have been identified, confirming its role in the regulation of diverse cellular processes, including cell division, DNA replication, differentiation, and adaptation to stress conditions. Some examples of these functions are described and discussed here, as is the role of Lon in the degradation of misfolded proteins and in protein quality control. Finally, this chapter deals with the exquisite sensitivity of protein degradation inside a cell. How can a protease distinguish so many substrates from cellular proteins that should not be degraded? Can the specificity of a protease be regulated according to the physiological needs of a cell? This chapter thus broadly discusses the substrate specificity of Lon and its allosteric regulation.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.

Peroxisomal proteostasis involves a Lon family protein that functions as protease and chaperone

Proteins are subject to continuous quality control for optimal proteostasis. The knowledge of peroxisome quality control systems is still in its infancy. Here we show that peroxisomes contain a member of the Lon family of proteases (Pln). We show that Pln is a heptameric protein and acts as an ATP-fueled protease and chaperone. Hence, Pln is the first chaperone identified in fungal peroxisomes. In cells of a PLN deletion strain peroxisomes contain protein aggregates, a major component of which is catalase-peroxidase. We show that this enzyme is sensitive to oxidative damage. The oxidatively damaged, but not the native protein, is a substrate of the Pln protease. Cells of the pln strain contain enhanced levels of catalase-peroxidase protein but reduced catalase-peroxidase enzyme activities. Together with the observation that Pln has chaperone activity in vitro, our data suggest that catalase-peroxidase aggregates accumulate in peroxisomes of pln cells due to the combined absence of Pln protease and chaperone activities.

The influence of ATP-dependent proteases on a variety of nucleoid-associated processes

ATP-dependent proteases are crucial components of all living cells and are involved in a variety of responses to physiological and environmental changes. Nucleoids are dynamic nucleoprotein complexes present in bacteria and eukaryotic organelles (mitochondria and plastids) and are the place where the majority of cellular responses to stress begin. These structures are actively remodeled in reaction to changing environmental and physiological conditions. The levels of nucleoid protein components (e.g. DNA-stabilizing proteins, transcription factors, replication proteins) therefore have to be continually regulated. ATP-dependent proteases have all the characteristics needed to fulfill this requirement. Some of them bind nucleic acids, but above all, they control and maintain the level of many DNA-binding proteins. In this review we will discuss the roles of the Lon, ClpAP, ClpXP, HslUV and FtsH proteases in the maintenance, stability, transcription and repair of DNA in eubacterial and mitochondrial nucleoids.

Identification of a region in the N-terminus of Escherichia coli Lon that affects ATPase, substrate translocation and proteolytic activity

Lon, also known as protease La, is an AAA+ protease machine that contains the ATPase and proteolytic domain within each enzyme subunit. Three truncated Escherichia coli Lon (ELon) mutants were generated based on a previous limited tryptic digestion result and hydrogen-deuterium exchange mass spectrometry analyses performed in this study. Using methods developed for characterizing wild-type (WT) Lon, we compared the ATPase, ATP-dependent protein degradation and ATP-dependent peptidase activities. With the exception of not degrading a putative structured substrate known as CcrM (cell-cycle-regulated DNA methyltransferase), the mutant lacking the first 239 residues behaved like WT ELon. Comparing the activity data of WT and ELon mutants reveals that the first 239 residues are not needed for minimal enzyme catalysis. The mutants lacking the first 252 residues or residues 232-252 displayed compromised ATPase, protein degradation and ATP-dependent peptide translocation abilities but retained WT-like steady-state peptidase activity. The binding affinities of WT and Lon mutants were evaluated by determining the concentration of λ N (K(λN)) needed to achieve 50% maximal ATPase stimulation. Comparing the K(λN) values reveals that the region encompassing 232-252 of ELon could contribute to λ N binding, but the effect is modest. Taken together, results generated from this study reveal that the region constituting residues 240-252 of ELon is important for ATPase activity, substrate translocation and protein degradation.

Mitochondrial calpain 10 is degraded by Lon protease after oxidant injury

Calpain 10 is ubiquitously expressed and is one of four mitochondrial matrix proteases. We determined that over-expression or knock-down of mitochondrial calpain 10 results in cell death, demonstrating that mitochondrial calpain 10 is required for viability. Thus, we studied calpain 10 degradation in isolated mitochondrial matrix, mitochondria and in renal proximal tubular cells (RPTC) under control and toxic conditions. Using isolated renal cortical mitochondria and mitochondrial matrix, calpain 10 underwent rapid degradation at 37°C that was blocked with Lon inhibitors but not by calpain or proteasome inhibitors. While exogenous Ca(2+) addition, Ca(2+) chelation or exogenous ATP addition had no effect on calpain 10 degradation, the oxidants tert-butyl hydroperoxide (TBHP) or H(2)O(2) increased the rate of degradation. Using RPTC, mitochondrial and cytosolic calpain 10 increased in the presence of MG132 (Lon/proteasome inhibitor) but only cytosolic calpain 10 increased in the presence of epoxomicin (proteasome inhibitor). Furthermore, TBHP and H(2)O(2) oxidized mitochondrial calpain 10, decreased mitochondrial, but not cytosolic calpain 10, and pretreatment with MG132 blocked TBHP-induced degradation of calpain 10. In summary, mitochondrial calpain 10 is selectively degraded by Lon protease under basal conditions and is enhanced under and oxidizing conditions, while cytosolic calpain 10 is degraded by the proteasome.

Matrix proteases in mitochondrial DNA function

Lon, ClpXP and m-AAA are the three major ATP-dependent proteases in the mitochondrial matrix. All three are involved in general quality control by degrading damaged or abnormal proteins. In addition to this role, they are proposed to serve roles in mitochondrial DNA functions including packaging and stability, replication, transcription and translation. In particular, Lon has been implicated in mtDNA metabolism in yeast, fly and humans. Here, we review the role of Lon protease in mitochondrial DNA functions, and discuss a putative physiological role for mitochondrial transcription factor A (TFAM) degradation by Lon protease. We also discuss the possible roles of m-AAA and ClpXP in mitochondrial DNA functions, and the putative candidate substrates for the three matrix proteases. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Multitasking in the mitochondrion by the ATP-dependent Lon protease

The AAA(+) Lon protease is a soluble single-ringed homo-oligomer, which represents the most streamlined operational unit mediating ATP-dependent proteolysis. Despite its simplicity, the architecture of Lon proteases exhibits a species-specific diversity. Homology modeling provides insights into the structural features that distinguish bacterial and human Lon proteases as hexameric complexes from yeast Lon, which is uniquely heptameric. The best-understood functions of mitochondrial Lon are linked to maintaining proteostasis under normal metabolic conditions, and preventing proteotoxicity during environmental and cellular stress. An intriguing property of human Lon is its specific binding to G-quadruplex DNA, and its association with the mitochondrial genome in cultured cells. A fraction of Lon preferentially binds to the control region of mitochondrial DNA where transcription and replication are initiated. Here, we present an overview of the diverse functions of mitochondrial Lon, as well as speculative perspectives on its role in protein and mtDNA quality control.

Mitochondrial Lon protease regulates mitochondrial DNA copy number and transcription by selective degradation of mitochondrial transcription factor A (TFAM)

Lon is the major protease in the mitochondrial matrix in eukaryotes, and is well conserved among species. Although a role for Lon in mitochondrial biogenesis has been proposed, the mechanistic basis is unclear. Here, we demonstrate a role for Lon in mtDNA metabolism. An RNA interference (RNAi) construct was designed that reduces Lon to less than 10% of its normal level in Drosophila Schneider cells. RNAi knockdown of Lon results in increased abundance of mitochondrial transcription factor A (TFAM) and mtDNA copy number. In a corollary manner, overexpression of Lon reduces TFAM levels and mtDNA copy number. Notably, induction of mtDNA depletion in Lon knockdown cells does not result in degradation of TFAM, thereby causing a dramatic increase in the TFAMmtDNA ratio. The increased TFAMmtDNA ratio in turn causes inhibition of mitochondrial transcription. We conclude that Lon regulates mitochondrial transcription by stabilizing the mitochondrial TFAMmtDNA ratio via selective degradation of TFAM.

Crystal structure of Lon protease: molecular architecture of gated entry to a sequestered degradation chamber

Lon proteases are distributed in all kingdoms of life and are required for survival of cells under stress. Lon is a tandem fusion of an AAA+ molecular chaperone and a protease with a serine-lysine catalytic dyad. We report the 2.0-Å resolution crystal structure of Thermococcus onnurineus NA1 Lon (TonLon). The structure is a three-tiered hexagonal cylinder with a large sequestered chamber accessible through an axial channel. Conserved loops extending from the AAA+ domain combine with an insertion domain containing the membrane anchor to form an apical domain that serves as a gate governing substrate access to an internal unfolding and degradation chamber. Alternating AAA+ domains are in tight- and weak-binding nucleotide states with different domain orientations and intersubunit contacts, reflecting intramolecular dynamics during ATP-driven protein unfolding and translocation. The bowl-shaped proteolytic chamber is contiguous with the chaperone chamber allowing internalized proteins direct access to the proteolytic sites without further gating restrictions.

Crystal structures of Bacillus subtilis Lon protease

Lon ATP-dependent proteases are key components of the protein quality control systems of bacterial cells and eukaryotic organelles. Eubacterial Lon proteases contain an N-terminal domain, an ATPase domain, and a protease domain, all in one polypeptide chain. The N-terminal domain is thought to be involved in substrate recognition, the ATPase domain in substrate unfolding and translocation into the protease chamber, and the protease domain in the hydrolysis of polypeptides into small peptide fragments. Like other AAA+ ATPases and self-compartmentalising proteases, Lon functions as an oligomeric complex, although the subunit stoichiometry is currently unclear. Here, we present crystal structures of truncated versions of Lon protease from Bacillus subtilis (BsLon), which reveal previously unknown architectural features of Lon complexes. Our analytical ultracentrifugation and electron microscopy show different oligomerisation of Lon proteases from two different bacterial species, Aquifex aeolicus and B. subtilis. The structure of BsLon-AP shows a hexameric complex consisting of a small part of the N-terminal domain, the ATPase, and protease domains. The structure shows the approximate arrangement of the three functional domains of Lon. It also reveals a resemblance between the architecture of Lon proteases and the bacterial proteasome-like protease HslUV. Our second structure, BsLon-N, represents the first 209 amino acids of the N-terminal domain of BsLon and consists of a globular domain, similar in structure to the E. coli Lon N-terminal domain, and an additional four-helix bundle, which is part of a predicted coiled-coil region. An unexpected dimeric interaction between BsLon-N monomers reveals the possibility that Lon complexes may be stabilised by coiled-coil interactions between neighbouring N-terminal domains. Together, BsLon-N and BsLon-AP are 36 amino acids short of offering a complete picture of a full-length Lon protease.

Structure of the catalytic domain of the human mitochondrial Lon protease: proposed relation of oligomer formation and activity

ATP-dependent proteases are crucial for cellular homeostasis. By degrading short-lived regulatory proteins, they play an important role in the control of many cellular pathways and, through the degradation of abnormally misfolded proteins, protect the cell from a buildup of aggregates. Disruption or disregulation of mammalian mitochondrial Lon protease leads to severe changes in the cell, linked with carcinogenesis, apoptosis, and necrosis. Here we present the structure of the proteolytic domain of human mitochondrial Lon at 2 A resolution. The fold resembles those of the three previously determined Lon proteolytic domains from Escherichia coli, Methanococcus jannaschii, and Archaeoglobus fulgidus. There are six protomers in the asymmetric unit, four arranged as two dimers. The intersubunit interactions within the two dimers are similar to those between adjacent subunits of the hexameric ring of E. coli Lon, suggesting that the human Lon proteolytic domain also forms hexamers. The active site contains a 3(10) helix attached to the N-terminal end of alpha-helix 2, which leads to the insertion of Asp852 into the active site, as seen in M. jannaschii. Structural considerations make it likely that this conformation is proteolytically inactive. When comparing the intersubunit interactions of human with those of E. coli Lon taken with biochemical data leads us to propose a mechanism relating the formation of Lon oligomers with a conformational shift in the active site region coupled to a movement of a loop in the oligomer interface, converting the proteolytically inactive form seen here to the active one in the E. coli hexamer.

Oxidized mitochondrial protein degradation and repair in aging and oxidative stress

Proteins are main targets for oxidative damage that occurs during aging and in oxidative stress situations. Since the mitochondria is a major source of reactive oxygen species, mitochondrial proteins are especially exposed to oxidative modification, and elimination of oxidized proteins is crucial for maintaining the integrity of this organelle. Hence, enzymatic reversal of protein oxidation and protein degradation is critical for protein homeostasis while protein maintenance failure has been implicated in the age-related accumulation of oxidized proteins. Within the mitochondrial matrix, the ATP-stimulated mitochondrial Lon protease is believed to play an important role in the degradation of oxidized protein, and age-associated impairment of Lon-like protease activity has been suggested to contribute to oxidized protein buildup in the mitochondria. Oxidized protein repair is limited to certain oxidation products of the sulfur-containing amino acids cysteine and methionine. Oxidized protein repair systems, thioredoxin/thioredoxin reductase or glutaredoxin/glutathione/glutathione reductase that catalytically reduce disulfide bridges or sulfenic acids, and methionine sulfoxide reductase that reverses methionine sulfoxide back to methionine within proteins, are present in the mitochondrial matrix. Thus, the role of the mitochondrial Lon protease and the oxidized protein repair system methionine sulfoxide reductase is further addressed in the context of oxidative stress and aging.