Biochemical characterization of the apicoplast-targeted AAA+ ATPase ClpB from Plasmodium falciparum

Proteostasis is a key driver of the pathogenesis in Apicomplexa

Proteostasis, including protein folding mediated by molecular chaperones, protein degradation, and stress response pathways in organelles like ER (unfolded protein response: UPR), are responsible for cellular protein quality control. This is essential for cell survival as it regulates and reprograms cellular processes. Here, we underscore the role of the proteostasis pathway in Apicomplexan parasites with respect to their well-characterized roles as well as potential roles in many parasite functions, including survival, multiplication, persistence, and emerging drug resistance. In addition to the diverse physiological importance of proteostasis in Apicomplexa, we assess the potential of the pathway's components as chemotherapeutic targets.

Repurposing p97 inhibitors for chemical modulation of the bacterial ClpB-DnaK bichaperone system

The ClpB-DnaK bichaperone system reactivates aggregated cellular proteins and is essential for survival of bacteria, fungi, protozoa, and plants under stress. AAA+ ATPase ClpB is a promising target for the development of antimicrobials because a loss of its activity is detrimental for survival of many pathogens and no apparent ClpB orthologs are found in metazoans. We investigated ClpB activity in the presence of several compounds that were previously described as inhibitor leads for the human AAA+ ATPase p97, an antitumor target. We discovered that N2,N4-dibenzylquinazoline-2,4-diamine (DBeQ), the least potent among the tested p97 inhibitors, binds to ClpB with a Kd∼60 μM and inhibits the casein-activated, but not the basal, ATPase activity of ClpB with an IC50∼5 μM. The remaining p97 ligands, which displayed a higher affinity toward p97, did not affect the ClpB ATPase. DBeQ also interacted with DnaK with a Kd∼100 μM and did not affect the DnaK ATPase but inhibited the DnaK chaperone activity in vitro. DBeQ inhibited the reactivation of aggregated proteins by the ClpB-DnaK bichaperone system in vitro with an IC50∼5 μM and suppressed the growth of cultured Escherichia coli. The DBeQ-induced loss of E. coli proliferation was exacerbated by heat shock but was nearly eliminated in a ClpB-deficient E. coli strain, which demonstrates a significant selectivity of DBeQ toward ClpB in cells. Our results provide chemical validation of ClpB as a target for developing novel antimicrobials. We identified DBeQ as a promising lead compound for structural optimization aimed at selective targeting of ClpB and/or DnaK.

Role of ClpB From Corynebacterium crenatum in Thermal Stress and Arginine Fermentation

ClpB, an ATP-dependent molecular chaperone, is involved in metabolic pathways and plays important roles in microorganisms under stress conditions. Metabolic pathways and stress resistance are important characteristics of industrially -relevant bacteria during fermentation. Nevertheless, ClpB-related observations have been rarely reported in industrially -relevant microorganisms. Herein, we found a homolog of ClpB from Corynebacterium crenatum. The amino acid sequence of ClpB was analyzed, and the recombinant ClpB protein was purified and characterized. The full function of ClpB requires DnaK as chaperone protein. For this reason, dnaK/clpB deletion mutants and the complemented strains were constructed to investigate the role of ClpB. The results showed that DnaK/ClpB is not essential for the survival of C. crenatum MT under pH and alcohol stresses. The ClpB-deficient or DnaK-deficient C. crenatum mutants showed weakened growth during thermal stress. In addition, the results demonstrated that deletion of the clpB gene affected glucose consumption and L-arginine, L-glutamate, and lactate production during fermentation.

Plastid biogenesis in malaria parasites requires the interactions and catalytic activity of the Clp proteolytic system

The human malaria parasite, Plasmodium falciparum, contains an essential plastid called the apicoplast. Most apicoplast proteins are encoded by the nuclear genome and it is unclear how the plastid proteome is regulated. Here, we study an apicoplast-localized caseinolytic-protease (Clp) system and how it regulates organelle proteostasis. Using null and conditional mutants, we demonstrate that the P. falciparum Clp protease (PfClpP) has robust enzymatic activity that is essential for apicoplast biogenesis. We developed a CRISPR/Cas9-based system to express catalytically dead PfClpP, which showed that PfClpP oligomerizes as a zymogen and is matured via transautocatalysis. The expression of both wild-type and mutant Clp chaperone (PfClpC) variants revealed a functional chaperone-protease interaction. Conditional mutants of the substrate-adaptor (PfClpS) demonstrated its essential function in plastid biogenesis. A combination of multiple affinity purification screens identified the Clp complex composition as well as putative Clp substrates. This comprehensive study reveals the molecular composition and interactions influencing the proteolytic function of the apicoplast Clp system and demonstrates its central role in the biogenesis of the plastid in malaria parasites.

Directing traffic: Chaperone-mediated protein transport in malaria parasites

The ability of eukaryotic parasites from the phylum Apicomplexa to cause devastating diseases is predicated upon their ability to maintain faithful and precise protein trafficking mechanisms. Their parasitic life cycle depends on the trafficking of effector proteins to the infected host cell, transport of proteins to several critical organelles required for survival, as well as transport of parasite and host proteins to the digestive organelles to generate the building blocks for parasite growth. Several recent studies have shed light on the molecular mechanisms parasites utilise to transform the infected host cells, transport proteins to essential metabolic organelles and for biogenesis of organelles required for continuation of their life cycle. Here, we review key pathways of protein transport originating and branching from the endoplasmic reticulum, focusing on the essential roles of chaperones in these processes. Further, we highlight key gaps in our knowledge that prevents us from building a holistic view of protein trafficking in these deadly human pathogens.

Relict plastidic metabolic process as a potential therapeutic target

The alignment of the evolutionary history of parasites with that of plants provides a different panorama in the drug development process. The housing of different metabolic processes, essential for parasite survival, adds to the indispensability of the apicoplast. The different pathways responsible for fueling the apicoplast and parasite offer a myriad of proteins responsible for the apicoplast function. The studies emphasizing the target-based approaches might help in the discovery of antimalarials. The different putative drug targets and their roles are highlighted. In addition, the origin of the apicoplast and metabolic processes are reviewed and the different drugs acting upon the enzymes of the apicoplast are discussed.

Toxoplasma gondii Clp family protein: TgClpB1 plays a crucial role in thermotolerance

Caseinolytic peptidase B (ClpB) plays a pivotal role in suppressing and reversing protein aggregation. Toxoplasma gondii is an intracellular parasitic protozoan that infects a wide variety of mammals and birds and therefore is exposed to a broad range of living condition. We screened ToxoDB (http://ToxoDB.org) and identified 10 putative T. gondii genes encoding members of the Clp superfamily of caseinolytic proteases and chaperones. Of these, we focused on characterizing the Class I ATP-dependent molecular chaperones TgClpB1, TgClpB2, and TgClpB3. We found that TgClpB1, the most divergent of the five T. gondii Class I Clp ATPases, is cytoplasmic, TgClpB2 is found in the mitochondria of the parasites, and TgClpB3 is a ClpB with novel apicoplast localization. Knockout strains of TgClpB1 and TgClpB2 were established by CRISPR/Cas9 mutagenesis, and their complementing strains were constructed with FLAG-tag. Although knockout of TgClpB1 or TgClpB2 did not affect growth under normal circumstances, TgClpB1 was required for T. gondii thermotolerance. The growth, replication, and invasion capabilities of TgClpB1-deficient mutants were significantly inhibited after extracellular parasites were pretreated at 45°C. Moreover, TgClpB1 were observed at the poles of the ΔTgClpB1 FLAG-tagged strain treated at 42°C.

Characterization of the molecular chaperone ClpB from the pathogenic spirochaete Leptospira interrogans

Leptospira interrogans is a spirochaete responsible for leptospirosis in mammals. The molecular mechanisms of the Leptospira virulence remain mostly unknown. Recently, it has been demonstrated that an AAA+ chaperone ClpB (a member of the Hsp100 family) from L. interrogans (ClpBLi) is not only essential for survival of Leptospira under the thermal and oxidative stresses, but also during infection of a host. The aim of this study was to provide further insight into the role of ClpB in the pathogenic spirochaetes and explore its biochemical properties. We found that a non-hydrolysable ATP analogue, ATPγS, but not AMP-PNP induces the formation of ClpBLi hexamers and stabilizes the associated form of the chaperone. ADP also induces structural changes in ClpBLi and promotes its self-assembly, but does not produce full association into the hexamers. We also demonstrated that ClpBLi exhibits a weak ATPase activity that is stimulated by κ-casein and poly-lysine, and may mediate protein disaggregation independently from the DnaK chaperone system. Unexpectedly, the presence of E. coli DnaK/DnaJ/GrpE did not significantly affect the disaggregation activity of ClpBLi and ClpBLi did not substitute for the ClpBEc function in the clpB-null E. coli strain. This result underscores the species-specificity of the ClpB cooperation with the co-chaperones and is most likely due to a loss of interactions between the ClpBLi middle domain and the E. coli DnaK. We also found that ClpBLi interacts more efficiently with the aggregated G6PDH in the presence of ATPγS rather than ATP. Our results indicate that ClpB's importance during infection might be due to its role as a molecular chaperone involved in reactivation of protein aggregates.

Structure of a putative ClpS N-end rule adaptor protein from the malaria pathogen Plasmodium falciparum

The N-end rule pathway uses an evolutionarily conserved mechanism in bacteria and eukaryotes that marks proteins for degradation by ATP-dependent chaperones and proteases such as the Clp chaperones and proteases. Specific N-terminal amino acids (N-degrons) are sufficient to target substrates for degradation. In bacteria, the ClpS adaptor binds and delivers N-end rule substrates for their degradation upon association with the ClpA/P chaperone/protease. Here, we report the first crystal structure, solved at 2.7 Å resolution, of a eukaryotic homolog of bacterial ClpS from the malaria apicomplexan parasite Plasmodium falciparum (Pfal). Despite limited sequence identity, Plasmodium ClpS is very similar to bacterial ClpS. Akin to its bacterial orthologs, plasmodial ClpS harbors a preformed hydrophobic pocket whose geometry and chemical properties are compatible with the binding of N-degrons. However, while the N-degron binding pocket in bacterial ClpS structures is open and accessible, the corresponding pocket in Plasmodium ClpS is occluded by a conserved surface loop that acts as a latch. Despite the closed conformation observed in the crystal, we show that, in solution, Pfal-ClpS binds and discriminates peptides mimicking bona fide N-end rule substrates. The presence of an apicoplast targeting peptide suggests that Pfal-ClpS localizes to this plastid-like organelle characteristic of all Apicomplexa and hosting most of its Clp machinery. By analogy with the related ClpS1 from plant chloroplasts and cyanobacteria, Plasmodium ClpS likely functions in association with ClpC in the apicoplast. Our findings open new venues for the design of novel anti-malarial drugs aimed at disrupting parasite-specific protein quality control pathways.

Structural mapping of the ClpB ATPases of Plasmodium falciparum: Targeting protein folding and secretion for antimalarial drug design

Caseinolytic chaperones and proteases (Clp) belong to the AAA+ protein superfamily and are part of the protein quality control machinery in cells. The eukaryotic parasite Plasmodium falciparum, the causative agent of malaria, has evolved an elaborate network of Clp proteins including two distinct ClpB ATPases. ClpB1 and ClpB2 are involved in different aspects of parasitic proteostasis. ClpB1 is present in the apicoplast, a parasite-specific and plastid-like organelle hosting various metabolic pathways necessary for parasite growth. ClpB2 localizes to the parasitophorous vacuole membrane where it drives protein export as core subunit of a parasite-derived protein secretion complex, the Plasmodium Translocon of Exported proteins (PTEX); this process is central to parasite virulence and survival in the human host. The functional associations of these two chaperones with parasite-specific metabolism and protein secretion make them prime drug targets. ClpB proteins function as unfoldases and disaggregases and share a common architecture consisting of four domains-a variable N-terminal domain that binds different protein substrates, followed by two highly conserved catalytic ATPase domains, and a C-terminal domain. Here, we report and compare the first crystal structures of the N terminal domains of ClpB1 and ClpB2 from Plasmodium and analyze their molecular surfaces. Solution scattering analysis of the N domain of ClpB2 shows that the average solution conformation is similar to the crystalline structure. These structures represent the first step towards the characterization of these two malarial chaperones and the reconstitution of the entire PTEX to aid structure-based design of novel anti-malarial drugs.