Self-compartmentalizing Proteases Research Articles

Structural insights into ClpP protease side exit pore-opening by a pH drop coupled with substrate hydrolysis.

The ClpP serine peptidase is a tetradecameric degradation molecular machine involved in many physiological processes. It becomes a competent ATP-dependent protease when coupled with Clp-ATPases. Small chemical compounds, acyldepsipeptides (ADEPs), are known to cause the dysregulation and activation of ClpP without ATPases and have potential as novel antibiotics. Previously, structural studies of ClpP from various species revealed its structural details, conformational changes, and activation mechanism. Although product release through side exit pores has been proposed, the detailed driving force for product release remains elusive. Herein, we report crystal structures of ClpP from Bacillus subtilis (BsClpP) in unforeseen ADEP-bound states. Cryo-electron microscopy structures of BsClpP revealed various conformational states under different pH conditions. To understand the conformational change required for product release, we investigated the relationship between substrate hydrolysis and the pH-lowering process. The production of hydrolyzed peptides from acidic and basic substrates by proteinase K and BsClpP lowered the pH values. Our data, together with those of previous findings, provide insight into the molecular mechanism of product release by the ClpP self-compartmentalizing protease.

Development of Antibiotics That Dysregulate the Neisserial ClpP Protease.

Evolving antimicrobial resistance has motivated the search for novel targets and alternative therapies. Caseinolytic protease (ClpP) has emerged as an enticing new target since its function is conserved and essential for bacterial fitness, and because its inhibition or dysregulation leads to bacterial cell death. ClpP protease function controls global protein homeostasis and is, therefore, crucial for the maintenance of the bacterial proteome during growth and infection. Previously, acyldepsipeptides (ADEPs) were discovered to dysregulate ClpP, leading to bactericidal activity against both actively growing and dormant Gram-positive pathogens. Unfortunately, these compounds had very low efficacy against Gram-negative bacteria. Hence, we sought to develop non-ADEP ClpP-targeting compounds with activity against Gram-negative species and called these activators of self-compartmentalizing proteases (ACPs). These ACPs bind and dysregulate ClpP in a manner similar to ADEPs, effectively digesting bacteria from the inside out. Here, we performed further ACP derivatization and testing to improve the efficacy and breadth of coverage of selected ACPs against Gram-negative bacteria. We observed that a diverse collection of Neisseria meningitidis and Neisseria gonorrhoeae clinical isolates were exquisitely sensitive to these ACP analogues. Furthermore, based on the ACP-ClpP cocrystal structure solved here, we demonstrate that ACPs could be designed to be species specific. This validates the feasibility of drug-based targeting of ClpP in Gram-negative bacteria.

Aggresomal sequestration and STUB1-mediated ubiquitylation during mammalian proteaphagy of inhibited proteasomes

The 26S proteasome, a self-compartmentalized protease complex, plays a crucial role in protein quality control. Multiple levels of regulatory systems modulate proteasomal activity for substrate hydrolysis. However, the destruction mechanism of mammalian proteasomes is poorly understood. We found that inhibited proteasomes are sequestered into the insoluble aggresome via HDAC6- and dynein-mediated transport. These proteasomes colocalized with the autophagic receptor SQSTM1 and cleared through selective macroautophagy, linking aggresomal segregation to autophagic degradation. This proteaphagic pathway was counterbalanced with the recovery of proteasomal activity and was critical for reducing cellular proteasomal stress. Changes in associated proteins and polyubiquitylation on inhibited 26S proteasomes participated in the targeting mechanism to the aggresome and autophagosome. The STUB1 E3 Ub ligase specifically ubiquitylated purified human proteasomes in vitro, mainly via Lys63-linked chains. Genetic and chemical inhibition of STUB1 activity significantly impaired proteasome processing and reduced resistance to proteasomal stress. These data demonstrate that aggresomal sequestration is the crucial upstream event for proteasome quality control and overall protein homeostasis in mammals.

Protective Roles of Cytosolic and Plastidal Proteasomes on Abiotic Stress and Pathogen Invasion.

Protein malfunction is typically caused by abiotic stressors. To ensure cell survival during conditions of stress, it is important for plant cells to maintain proteins in their respective functional conformation. Self-compartmentalizing proteases, such as ATP-dependent Clp proteases and proteasomes are designed to act in the crowded cellular environment, and they are responsible for degradation of misfolded or damaged proteins within the cell. During different types of stress conditions, the levels of misfolded or orphaned proteins that are degraded by the 26S proteasome in the cytosol and nucleus and by the Clp proteases in the mitochondria and chloroplasts increase. This allows cells to uphold feedback regulations to cellular-level signals and adjust to altered environmental conditions. In this review, we summarize recent findings on plant proteolytic complexes with respect to their protective functions against abiotic and biotic stressors.

Trisubstituted Aryl Cyclohexanecarboxylates (TACC): A Simple, New Molecular Scaffold for Antibiotics Design

A new class of potential antibacterial agents has been synthesized on a new molecular scaffold of cyclohexane carboxylate. We have tagged this new class of compounds TACCs (Trisubstituted Aryl Cyclohexanecarboxylate). These new molecules are structural analogues of an Activators of Self-Compartmentalizing Proteases 4 and 5 (ACP 4 and 5), and were synthesized to circumvent the drug-like property (drug-ability) challenges and liability noted in ACP 4 and 5. A pseudo-Robinson annulation protocol was used to furnish this new class of potential antibiotics. Structure-activity relationship (SAR) study was done to identify the pharmacophore(s) in this molecular scaffold. A selection of these compounds was used in our preliminary antibacterial inhibitory activities’ studies on Bacillus mycoides and Bacillus subtilis. These preliminary studies show that the TACCs exhibited equal, and in some cases better, antibacterial activity than ACP 4 and 5.

Structural characterization of the bacterial proteasome homolog BPH reveals a tetradecameric double-ring complex with unique inner cavity properties

Eukaryotic and archaeal proteasomes are paradigms for self-compartmentalizing proteases. To a large extent, their function requires interplay with hexameric ATPases associated with diverse cellular activities (AAA+) that act as substrate unfoldases. Bacteria have various types of self-compartmentalizing proteases; in addition to the proteasome itself, these include the proteasome homolog HslV, which functions together with the AAA+ HslU; the ClpP protease with its partner AAA+ ClpX; and Anbu, a recently characterized ancestral proteasome variant. Previous bioinformatic analysis has revealed a novel bacterial member of the proteasome family Betaproteobacteria proteasome homolog (BPH). Using cluster analysis, we here affirmed that BPH evolutionarily descends from HslV. Crystal structures of the Thiobacillus denitrificans and Cupriavidus metallidurans BPHs disclosed a homo-oligomeric double-ring architecture in which the active sites face the interior of the cylinder. Using small-angle X-ray scattering (SAXS) and electron microscopy averaging, we found that BPH forms tetradecamers in solution, unlike the dodecamers seen in HslV. Although the highly acidic inner surface of BPH was in striking contrast to the cavity characteristics of the proteasome and HslV, a classical proteasomal reaction mechanism could be inferred from the covalent binding of the proteasome-specific inhibitor epoxomicin to BPH. A ligand-bound structure implied that the elongated BPH inner pore loop may be involved in substrate recognition. The apparent lack of a partner unfoldase and other unique features, such as Ser replacing Thr as the catalytic residue in certain BPH subfamilies, suggest a proteolytic function for BPH distinct from those of known bacterial self-compartmentalizing proteases.

The Architecture of the Anbu Complex Reflects an Evolutionary Intermediate at the Origin of the Proteasome System

SummaryProteasomes are self-compartmentalizing proteases that function at the core of the cellular protein degradation machinery in eukaryotes, archaea, and some bacteria. Although their evolutionary history is under debate, it is thought to be linked to that of the bacterial protease HslV and the hypothetical bacterial protease Anbu (ancestral beta subunit). Here, together with an extensive bioinformatic analysis, we present the first biophysical characterization of Anbu. Anbu forms a dodecameric complex with a unique architecture that was only accessible through the combination of X-ray crystallography and small-angle X-ray scattering. While forming continuous helices in crystals and electron microscopy preparations, refinement of sections from the crystal structure against the scattering data revealed a helical open-ring structure in solution, contrasting the ring-shaped structures of proteasome and HslV. Based on this primordial architecture and exhaustive sequence comparisons, we propose that Anbu represents an ancestral precursor at the origin of self-compartmentalization.

Barrel-shaped ClpP Proteases Display Attenuated Cleavage Specificities.

ClpP is a self-compartmentalizing protease with crucial roles in bacterial and mitochondrial protein quality control. Although the ClpP homocomplex is composed of 14 equivalent active sites, it degrades a multitude of substrates to small peptides, demonstrating its capability to carry out diverse cleavage reactions. Here, we show that ClpP proteases from E. coli, S. aureus, and human mitochondria exhibit preferences for certain amino acids in the P1, P2, and P3 positions using a tailored fluorogenic substrate library. However, this high specificity is not retained during proteolysis of endogenous substrates as shown by mass spectrometric analysis of peptides produced in ClpXP-mediated degradation reactions. Our data suggest a mechanism that implicates the barrel-shaped architecture of ClpP not only in shielding the active sites to prevent uncontrolled proteolysis but also in providing high local substrate concentrations to enable efficient proteolytic processing. Furthermore, we introduce customized fluorogenic substrates with unnatural amino acids that greatly surpass the sensitivity of previously used tools. We used these to profile the activity of cancer-patient- and Perrault-syndrome-derived ClpP mutant proteins.

Functions of the proteasome on chromatin.

The proteasome is a large self-compartmentalized protease complex that recognizes, unfolds, and destroys ubiquitylated substrates. Proteasome activities are required for a host of cellular functions, and it has become clear in recent years that one set of critical actions of the proteasome occur on chromatin. In this review, we discuss some of the ways in which proteasomes directly regulate the structure and function of chromatin and chromatin regulatory proteins, and how this influences gene transcription. We discuss lingering controversies in the field, the relative importance of proteolytic versus non-proteolytic proteasome activities in this process, and highlight areas that require further investigation. Our intention is to show that proteasomes are involved in major steps controlling the expression of the genetic information, that proteasomes use both proteolytic mechanisms and ATP-dependent protein remodeling to accomplish this task, and that much is yet to be learned about the full spectrum of ways that proteasomes influence the genome.

Structure of Signal Peptide Peptidase A with C-Termini Bound in the Active Sites: Insights into Specificity, Self-Processing, and Regulation

Bacterial signal peptide peptidase A (SppA) is a membrane-bound enzyme that utilizes a serine/lysine catalytic dyad mechanism to cleave remnant signal peptides within the cellular membrane. Bacillus subtilis SppA (SppABS) oligomerizes into a homo-octameric dome-shaped complex with eight active sites, located at the interface between each protomer. In this study, we show that SppABS self-processes its own C-termini. We have determined the crystal structure of a proteolytically stable fragment of SppABSK199A that has its C-terminal peptide bound in each of the eight active sites, creating a perfect circle of peptides. Substrate specificity pockets S1, S3, and S2' are identified and accommodate C-terminal residues Tyr331, Met329, and Tyr333, respectively. Tyr331 at the P1 position is conserved among most Bacillus species. The structure reveals that the C-terminus binds within the substrate-binding grooves in an antiparallel β-sheet fashion. We show, by C-terminal truncations, that the C-terminus is not essential for oligomeric assembly. Kinetic analysis shows that a synthetic peptide corresponding to the C-terminus of SppABS competes with a fluorometric peptide substrate for the SppABS active site. A model is proposed for how the C-termini of SppA may function in the regulation of this membrane-bound self-compartmentalized protease.

CtpB Assembles a Gated Protease Tunnel Regulating Cell-Cell Signaling during Spore Formation in Bacillus subtilis

SummarySpore formation in Bacillus subtilis relies on a regulated intramembrane proteolysis (RIP) pathway that synchronizes mother-cell and forespore development. To address the molecular basis of this SpoIV transmembrane signaling, we carried out a structure-function analysis of the activating protease CtpB. Crystal structures reflecting distinct functional states show that CtpB constitutes a ring-like protein scaffold penetrated by two narrow tunnels. Access to the proteolytic sites sequestered within these tunnels is controlled by PDZ domains that rearrange upon substrate binding. Accordingly, CtpB resembles a minimal version of a self-compartmentalizing protease regulated by a unique allosteric mechanism. Moreover, biochemical analysis of the PDZ-gated channel combined with sporulation assays reveal that activation of the SpoIV RIP pathway is induced by the concerted activity of CtpB and a second signaling protease, SpoIVB. This proteolytic mechanism is of broad relevance for cell-cell communication, illustrating how distinct signaling pathways can be integrated into a single RIP module.

Control of Substrate Gating and Translocation into ClpP by Channel Residues and ClpX Binding

ClpP is a self-compartmentalized protease, which has very limited degradation activity unless it associates with ClpX to form ClpXP or with ClpA to form ClpAP. Here, we show that ClpX binding stimulates ClpP cleavage of peptides larger than a few amino acids and enhances ClpP active-site modification. Stimulation requires ATP binding but not hydrolysis by ClpX. The magnitude of this enhancement correlates with increasing molecular weight of the molecule entering ClpP. Amino-acid substitutions in the channel loop or helix A of ClpP enhance entry of larger substrates into the free enzyme, eliminate ClpX binding in some cases, and are not further stimulated by ClpX binding in other instances. These results support a model in which the channel residues of free ClpP exclude efficient entry of all but the smallest peptides into the degradation chamber, with ClpX binding serving to relieve these inhibitory interactions. Specific ClpP channel variants also prevent ClpXP translocation of certain amino-acid sequences, suggesting that the wild-type channel plays an important role in facilitating broad translocation specificity. In combination with previous studies, our results indicate that collaboration between ClpP and its partner ATPases opens a gate that functions to exclude larger substrates from isolated ClpP.

Structure of the Archaeal Pab87 Peptidase Reveals a Novel Self-Compartmentalizing Protease Family

Self-compartmentalizing proteases orchestrate protein turnover through an original architecture characterized by a central catalytic chamber. Here we report the first structure of an archaeal member of a new self-compartmentalizing protease family forming a cubic-shaped octamer with D 4 symmetry and referred to as CubicO. We solved the structure of the Pyrococcus abyssi Pab87 protein at 2.2 Å resolution using the anomalous signal of the high-phasing-power lanthanide derivative Lu-HPDO3A. A 20 Å wide channel runs through this supramolecular assembly of 0.4 MDa, giving access to a 60 Å wide central chamber holding the eight active sites. Surprisingly, activity assays revealed that Pab87 degrades specifically d-amino acid containing peptides, which have never been observed in archaea. Genomic context of the Pab87 gene showed that it is surrounded by genes involved in the amino acid/peptide transport or metabolism. We propose that CubicO proteases are involved in the processing of d-peptides from environmental origins.

Crystal Structure of the Caseinolytic Protease Gene Regulator, a Transcriptional Activator in Actinomycetes

Human pathogens of the genera Corynebacterium and Mycobacterium possess the transcriptional activator ClgR (clp gene regulator) which in Corynebacterium glutamicum has been shown to regulate the expression of the ClpCP protease genes. ClgR specifically binds to pseudo-palindromic operator regions upstream of clpC and clpP1P2. Here, we present the first crystal structure of a ClgR protein from C. glutamicum. The structure was determined from two different crystal forms to resolutions of 1.75 and 2.05 A, respectively. ClgR folds into a five-helix bundle with a helix-turn-helix motif typical for DNA-binding proteins. Upon dimerization the two DNA-recognition helices are arranged opposite to each other at the protein surface in a distance of approximately 30 A, which suggests that they bind into two adjacent major grooves of B-DNA in an anti-parallel manner. A binding pocket is situated at a strategic position in the dimer interface and could possess a regulatory role altering the positions of the DNA-binding helices.

Insights into the inter-ring plasticity of caseinolytic proteases from the X-ray structure ofMycobacterium tuberculosisClpP1

Mycobacterium tuberculosis caseinolytic protease ClpP1 (Mt ClpP1) is a self-compartmentalized protease consisting of two heptameric rings stacked on top of each other, thus enclosing a catalytic chamber. Within the chamber, which can be reached through two axial pores, each of the 14 identical monomers possesses a serine protease active site. The unfolding and translocation of substrates into the chamber are mediated by associated hexameric ATPases covering the axial pores. Three crystal structures of Mt ClpP1, determined by molecular replacement, are presented in this study. Two of the models were refined to a resolution of 2.6 A and the third to 3.0 A. It was found that disorder in the handle domain affects the formation and configuration of the tetradecamer and results in condensed structures with larger equatorial pores when compared with ClpPs from other species. Additionally, this disorder accompanies conformational changes of the residues in the catalytic triad. The models also reveal structural differences within the N-terminal hairpin-loop domain, which possibly reflect the significant differences in amino-acid sequence between Mt ClpP1 and other ClpP homologues in this region.

The Inner Cavity ofEscherichia coliDegP Protein Is Not Essentialfor Molecular Chaperone and Proteolytic Activity

The Escherichia coli DegP protein is an essential periplasmic protein for bacterial survival at high temperatures. DegP has the unusual property of working as a chaperone below 28 degrees C, but efficiently degrading unfolded proteins above 28 degrees C. Monomeric DegP contains a protease domain and two PDZ domains. It oligomerizes into a hexameric cage through the staggered association of trimers. The active sites are located in a central cavity that is only accessible laterally, and the 12 PDZ domains act as mobile sidewalls that mediate opening and closing of the gates. As access to the active sites is restricted, DegP is an example of a self-compartmentalized protease. To determine the essential elements of DegP that maintain the integrity of the hexameric cage, we constructed several deletion mutants of DegP that formed trimers rather than hexamers. We found that residues 39 to 78 within the LA loops, as well as the PDZ2 domains are essential for the integrity of the DegP hexamer. In addition, we asked whether an enclosed cavity or cage of specific dimensions is required for the protease and chaperone activities in DegP. Both activities were maintained in the trimeric DegP mutants without an enclosed cavity and in deletion DegP mutants with significantly reduced dimensions of the cage. We conclude that the functional unit for the protease and chaperone activities of DegP is a trimer and that neither a cavity of specific dimensions nor the presence of an enclosed cavity appears to be essential for the protease and chaperone activities of DegP.

An Archaeal Peptidase Assembles into Two Different Quaternary Structures

Cellular proteolysis involves large oligomeric peptidases that play key roles in the regulation of many cellular processes. The cobalt-activated peptidase TET1 from the hyperthermophilic Archaea Pyrococcus horikoshii (PhTET1) was found to assemble as a 12-subunit tetrahedron and as a 24-subunit octahedral particle. Both quaternary structures were solved by combining x-ray crystallography and cryoelectron microscopy data. The internal organization of the PhTET1 particles reveals highly self-compartmentalized systems made of networks of access channels extended by vast catalytic chambers. The two edifices display aminopeptidase activity, and their organizations indicate substrate navigation mechanisms different from those described in other large peptidase complexes. Compared with the tetrahedron, the octahedron forms a more expanded hollow structure, representing a new type of giant peptidase complex. PhTET1 assembles into two different quaternary structures because of quasi-equivalent contacts that previously have only been identified in viral capsids.

Inactivation of the 20S proteasome in Streptomyces lividans and its influence on the production of heterologous proteins

Proteasomes are self-compartmentalizing proteases first discovered in eukaryotes but also occurring in archaea and in bacteria belonging to the order Actinomycetales. In bacteria, proteasomes have so far no known function. In order to evaluate the influence of the 20S proteasome on the production of heterologous proteins by Streptomyces lividans TK24, the production of a number of heterologous proteins, including soluble human tumour necrosis factor receptor II (shuTNFRII) and salmon calcitonin (sCT), was compared with the wild-type TK24, a proteasome-deficient mutant designated PRO41 and a strain complemented for the disrupted proteasome genes (strain PRO41R). S. lividans cells lacking intact proteasome genes are phenotypically indistinguishable from the wild-type or the complemented strain containing functional proteasomes. Using the expression and secretion signals of the subtilisin inhibitor of Streptomyces venezuelae CBS762.70 (Vsi) for shuTNFRII and those of tyrosinase of Streptomyces antibioticus (MelC1) for the production of sCT, both proteins were secreted in significantly higher amounts in the strain PRO41 than in the wild-type S. lividans TK24 or the complemented strain PRO41R. However, the secretion of other heterologous proteins such as shuTNFRI was not enhanced in the proteasome-deficient strain. This suggests that S. lividans TK24 can degrade some heterologous proteins in a proteasome-dependent fashion. The proteasome-deficient strain may therefore be useful for the efficient production of these heterologous proteins.

Expression, purification, crystallization and preliminary crystallographic analysis of a deblocking aminopeptidase fromPyrococcus horikoshii

The deblocking aminopeptidase (DAP) of Pyrococcus horikoshii is a hyperthermophilic exoprotease that cleaves the N-terminal amino acid of peptide substrates with a putative deblocking activity for acylated peptides. DAP has been found to be homologous to a tetrahedral aminopeptidase from the halophilic Haloarcula marismortui. The latter enzyme is a dodecameric complex and has been revealed to be a self-compartmentalized protease whose central cavity harbouring the catalytic site is accessible through several channels of different size, unlike all other known proteolytic complexes. Three paralogues of DAP have been identified in P. horikoshii, with about 40% identity between them. Each of them has been overexpressed in Escherichia coli, purified and crystallized in the native and selenomethionine-substituted states. The results indicate that they form two kinds of assemblies, of 12 and of 24 subunits, with a molecular weight of approximately 400 and approximately 800 kDa, respectively. Crystals of the different variants of DAP and in their different oligomeric states diffract up to a resolution of 3 A.

Crystal Structure of a Dodecameric Tetrahedral-shaped Aminopeptidase

Protein turnover is an essential process in living cells. The degradation of cytosolic polypeptides is mainly carried out by the proteasome, resulting in 7-9-amino acid long peptides. Further degradation is usually carried out by energy-independent proteases like the tricorn protease from Thermoplasma acidophilum. Recently, a novel tetrahedral-shaped dodecameric 480-kDa aminopeptidase complex (TET) has been described in Haloarcula marismortui that differs from the known ring- or barrel-shaped self-compartmentalizing proteases. This complex is capable of degrading most peptides down to amino acids. We present here the crystal structure of the tetrahedral aminopeptidase homolog FrvX from Pyrococcus horikoshii. The monomer has a typical clan MH fold, as found for example in Aeromonas proteolytica aminopeptidase, containing a dinuclear zinc active center. The quaternary structure is built by dimers with a length of 100 A that form the edges of the tetrahedron. All 12 active sites are located on the inside of the tetrahedron. Substrate access is granted by pores with a maximal diameter of 10 A, allowing only small peptides and unfolded proteins access to the active site.