Reversibly Reactive Affinity Selection-Mass Spectrometry Enables Identification of Covalent Peptide Binders.

Covalent peptide binders have found applications as activity-based probes and as irreversible therapeutic inhibitors. Currently, there is no rapid, label-free, and tunable affinity selection platform to enrich covalent reactive peptide binders from synthetic libraries. We address this challenge by developing a reversibly reactive affinity selection platform termed ReAct-ASMS enabled by tandem high-resolution mass spectrometry (MS/MS) to identify covalent peptide binders to native protein targets. It uses mixed disulfide-containing peptides to build reversible peptide-protein conjugates that can enrich for covalent variants, which can be sequenced by MS/MS after reduction. Using this platform, we identified covalent peptide binders against two oncoproteins, human papillomavirus 16 early protein 6 (HPV16 E6) and peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 protein (Pin1). The resulting peptide binders efficiently and selectively cross-link Cys58 of E6 at 37 °C and Cys113 of Pin1 at room temperature, respectively. ReAct-ASMS enables the identification of highly selective covalent peptide binders for diverse molecular targets, introducing an applicable platform to assist preclinical therapeutic development pipelines.

Structure- and function-based design of Plasmodium-selective proteasome inhibitors.

The proteasome is a multi-component protease complex responsible for regulating key processes such as the cell cycle and antigen presentation. Compounds that target the proteasome are potentially valuable tools for the treatment of pathogens that depend on proteasome function for survival and replication. In particular, proteasome inhibitors have been shown to be toxic for the malaria parasite Plasmodium falciparum at all stages of its life cycle. Most compounds that have been tested against the parasite also inhibit the mammalian proteasome, resulting in toxicity that precludes their use as therapeutic agents. Therefore, better definition of the substrate specificity and structural properties of the Plasmodium proteasome could enable the development of compounds with sufficient selectivity to allow their use as anti-malarial agents. To accomplish this goal, here we use a substrate profiling method to uncover differences in the specificities of the human and P. falciparum proteasome. We design inhibitors based on amino-acid preferences specific to the parasite proteasome, and find that they preferentially inhibit the ß2-subunit. We determine the structure of the P. falciparum 20S proteasome bound to the inhibitor using cryo-electron microscopy and single-particle analysis, to a resolution of 3.6 Å. These data reveal the unusually open P. falciparum ß2 active site and provide valuable information about active-site architecture that can be used to further refine inhibitor design. Furthermore, consistent with the recent finding that the proteasome is important for stress pathways associated with resistance of artemisinin family anti-malarials, we observe growth inhibition synergism with low doses of this ß2-selective inhibitor in artemisinin-sensitive and -resistant parasites. Finally, we demonstrate that a parasite-selective inhibitor could be used to attenuate parasite growth in vivo without appreciable toxicity to the host. Thus, the Plasmodium proteasome is a chemically tractable target that could be exploited by next-generation anti-malarial agents.

Membrane skeletal association and post-translational allosteric regulation of Toxoplasma gondii GAPDH1.

When Toxoplasma gondii egresses from the host cell, glyceraldehyde-3-phosphate dehydrogenase 1 (GAPDH1), which is primary a glycolysis enzyme but actually a quintessential multifunctional protein, translocates to the unique cortical membrane skeleton. Here, we report the 2.25 Å resolution crystal structure of the GAPDH1 holoenzyme in a quaternary complex providing the basis for the molecular dissection of GAPDH1 structure-function relationships Knockdown of GAPDH1 expression and catalytic site disruption validate the essentiality of GAPDH1 in intracellular replication but we confirmed that glycolysis is not strictly essential. We identify, for the first time, S-loop phosphorylation as a novel, critical regulator of enzymatic activity that is consistent with the notion that the S-loop is critical for cofactor binding, allosteric activation and oligomerization. We show that neither enzymatic activity nor phosphorylation state correlate with the ability to translocate to the cortex. However, we demonstrate that association of GAPDH1 with the cortex is mediated by the N-terminus, likely palmitoylation. Overall, glycolysis and cortical translocation are functionally decoupled by post-translational modifications.

Structure of the p53 degradation complex from HPV16.

Human papillomavirus (HPV) is a significant contributor to the global cancer burden, and its carcinogenic activity is facilitated in part by the HPV early protein 6 (E6), which interacts with the E3-ligase E6AP, also known as UBE3A, to promote degradation of the tumor suppressor, p53. In this study, we present a single-particle cryoEM structure of the full-length E6AP protein in complex with HPV16 E6 (16E6) and p53, determined at a resolution of ~3.3 Å. Our structure reveals extensive protein-protein interactions between 16E6 and E6AP, explaining their picomolar binding affinity. These findings shed light on the molecular basis of the ternary complex, which has been pursued as a potential therapeutic target for HPV-driven cervical, anal, and oropharyngeal cancers over the last two decades. Understanding the structural and mechanistic underpinnings of this complex is crucial for developing effective therapies to combat HPV-induced cancers. Our findings may help to explain why previous attempts to disrupt this complex have failed to generate therapeutic modalities and suggest that current strategies should be reevaluated.

Blocking Palmitoylation of Toxoplasma gondii Myosin Light Chain 1 Disrupts Glideosome Composition but Has Little Impact on Parasite Motility.

Toxoplasma gondii is a widespread apicomplexan parasite that causes severe disease in immunocompromised individuals and the developing fetus. Like other apicomplexans, T. gondii uses an unusual form of substrate-dependent gliding motility to invade cells of its hosts and to disseminate throughout the body during infection. It is well established that a myosin motor consisting of a class XIVa heavy chain (TgMyoA) and two light chains (TgMLC1 and TgELC1/2) plays an important role in parasite motility. The ability of the motor to generate force at the parasite periphery is thought to be reliant upon its anchoring and immobilization within a peripheral membrane-bound compartment, the inner membrane complex (IMC). The motor does not insert into the IMC directly; rather, this interaction is believed to be mediated by the binding of TgMLC1 to the IMC-anchored protein, TgGAP45. Therefore, the binding of TgMLC1 to TgGAP45 is considered a key element in the force transduction machinery of the parasite. TgMLC1 is palmitoylated, and we show here that palmitoylation occurs on two N-terminal cysteine residues, C8 and C11. Mutations that block TgMLC1 palmitoylation completely abrogate the binding of TgMLC1 to TgGAP45. Surprisingly, the loss of TgMLC1 binding to TgGAP45 in these mutant parasites has little effect on their ability to initiate or sustain movement. These results question a key tenet of the current model of apicomplexan motility and suggest that our understanding of gliding motility in this important group of human and animal pathogens is not yet complete.IMPORTANCE Gliding motility plays a central role in the life cycle of T. gondii and other apicomplexan parasites. The myosin motor thought to power motility is essential for virulence but distinctly different from the myosins found in humans. Consequently, an understanding of the mechanism(s) underlying parasite motility and the role played by this unusual myosin may reveal points of vulnerability that can be targeted for disease prevention or treatment. We show here that mutations that uncouple the motor from what is thought to be a key structural component of the motility machinery have little impact on parasite motility. This finding runs counter to predictions of the current, widely held "linear motor" model of motility, highlighting the need for further studies to fully understand how apicomplexan parasites generate the forces necessary to move into, out of, and between cells of the hosts they infect.

Toxoplasma gondii serine hydrolases regulate parasite lipid mobilization during growth and replication within the host.

The intracellular protozoan parasite Toxoplasma gondii must scavenge cholesterol and other lipids from the host to facilitate intracellular growth and replication. Enzymes responsible for neutral lipid synthesis have been identified but there is no evidence for enzymes that catalyze lipolysis of cholesterol esters and esterified lipids. Here, we characterize several T. gondii serine hydrolases with esterase and thioesterase activities that were previously thought to be depalmitoylating enzymes. We find they do not cleave palmitoyl thiol esters but rather hydrolyze short-chain lipid esters. Deletion of one of the hydrolases results in alterations in levels of multiple lipids species. We also identify small-molecule inhibitors of these hydrolases and show that treatment of parasites results in phenotypic defects reminiscent of parasites exposed to excess cholesterol or oleic acid. Together, these data characterize enzymes necessary for processing lipids critical for infection and highlight the potential for targeting parasite hydrolases for therapeutic applications.

The Antimalarial Natural Product Salinipostin A Identifies Essential α/ß Serine Hydrolases Involved in Lipid Metabolism in P. falciparum Parasites.

Salinipostin A (Sal A) is a potent antiplasmodial marine natural product with an undefined mechanism of action. Using a Sal A-derived activity-based probe, we identify its targets in the Plasmodium falciparum parasite. All of the identified proteins contain α/ß serine hydrolase domains and several are essential for parasite growth. One of the essential targets displays a high degree of homology to human monoacylglycerol lipase (MAGL) and is able to process lipid esters including a MAGL acylglyceride substrate. This Sal A target is inhibited by the anti-obesity drug Orlistat, which disrupts lipid metabolism. Resistance selections yielded parasites that showed only minor reductions in sensitivity and that acquired mutations in a PRELI domain-containing protein linked to drug resistance in Toxoplasma gondii. This inability to evolve efficient resistance mechanisms combined with the non-essentiality of human homologs makes the serine hydrolases identified here promising antimalarial targets.

Synthetic Fluorogenic Peptides Reveal Dynamic Substrate Specificity of Depalmitoylases.

Palmitoylation is a post-translational modification involving the thioesterification of cysteine residues with a 16-carbon-saturated fatty acid. Little is known about rates of depalmitoylation or the parameters that dictate these rates. Here we report a modular strategy to synthesize quenched fluorogenic substrates for the specific detection of depalmitoylase activity and for mapping the substrate specificity of individual depalmitoylases. We demonstrate that human depalmitoylases APT1 and APT2, and TgPPT1 from the parasite Toxoplasma gondii, have distinct specificities that depend on amino acid residues distal to the palmitoyl cysteine. This information informs the design of optimal and non-optimal substrates as well as isoform-selective substrates to detect the activity of a specific depalmitoylase in complex proteomes. In addition to providing tools for studying depalmitoylases, our findings identify a previously unrecognized mechanism for regulating steady-state levels of distinct palmitoylation sites by sequence-dependent control of depalmitoylation rates.

Erratum for Foe et al., "The Toxoplasma gondii Active Serine Hydrolase 4 Regulates Parasite Division and Intravacuolar Parasite Architecture".

[This corrects the article DOI: 10.1128/mSphere.00393-18.].

The Toxoplasma gondii Active Serine Hydrolase 4 Regulates Parasite Division and Intravacuolar Parasite Architecture.

Hydrolase are enzymes that regulate diverse biological processes, including posttranslational protein modifications. Recent work identified four active serine hydrolases (ASHs) in Toxoplasma gondii as candidate depalmitoylases. However, only TgPPT1 (ASH1) has been confirmed to remove palmitate from proteins. ASH4 (TgME49_264290) was reported to be refractory to genetic disruption. We demonstrate that recombinant ASH4 is an esterase that processes short acyl esters but not palmitoyl thioesters. Genetic disruption of ASH4 causes defects in cell division and premature scission of parasites from residual bodies. These defects lead to the presence of vacuoles with a disordered intravacuolar architecture, with parasites arranged in pairs around multiple residual bodies. Importantly, we found that the deletion of ASH4 correlates with a defect in radial dispersion from host cells after egress. This defect in dispersion of parasites is a general phenomenon that is observed for disordered vacuoles that occur at low frequency in wild-type parasites, suggesting a possible general link between intravacuolar organization and dispersion after egress.IMPORTANCE This work defines the function of an enzyme in the obligate intracellular parasite Toxoplasma gondii We show that this previously uncharacterized enzyme is critical for aspects of cellular division by the parasite and that loss of this enzyme leads to parasites with cell division defects and which also are disorganized inside their vacuoles. This leads to defects in the ability of the parasite to disseminate from the site of an infection and may have a significant impact on the parasite's overall infectivity of a host organism.

Development of an activity-based probe for acyl-protein thioesterases.

Protein palmitoylation is a dynamic post-translational modification (PTM) important for cellular functions such as protein stability, trafficking, localization, and protein-protein interactions. S-palmitoylation occurs via the addition of palmitate to cysteine residues via a thioester linkage, catalyzed by palmitoyl acyl transferases (PATs), with removal of the palmitate catalyzed by acyl protein thioesterases (APTs) and palmitoyl-protein thioesterases (PPTs). Tools that target the regulators of palmitoylation-PATs, APTs and PPTs-will improve understanding of this essential PTM. Here, we describe the synthesis and application of a cell-permeable activity-based probe (ABP) that targets APTs in intact mammalian cells and the parasite Toxoplasma gondii. Using a focused library of substituted chloroisocoumarins, we identified a probe scaffold with nanomolar affinity for human APTs (HsAPT1 and HsAPT2) and synthesized a fluorescent ABP, JCP174-BODIPY TMR (JCP174-BT). We use JCP174-BT to profile HsAPT activity in situ in mammalian cells, to detect an APT in T. gondii (TgPPT1). We show discordance between HsAPT activity levels and total protein concentration in some cell lines, indicating that total protein levels may not be representative of APT activity in complex systems, highlighting the utility of this probe.

Global Analysis of Palmitoylated Proteins in Toxoplasma gondii.

Post-translational modifications (PTMs) such as palmitoylation are critical for the lytic cycle of the protozoan parasite Toxoplasma gondii. While palmitoylation is involved in invasion, motility, and cell morphology, the proteins that utilize this PTM remain largely unknown. Using a chemical proteomic approach, we report a comprehensive analysis of palmitoylated proteins in T. gondii, identifying a total of 282 proteins, including cytosolic, membrane-associated, and transmembrane proteins. From this large set of palmitoylated targets, we validate palmitoylation of proteins involved in motility (myosin light chain 1, myosin A), cell morphology (PhIL1), and host cell invasion (apical membrane antigen 1, AMA1). Further studies reveal that blocking AMA1 palmitoylation enhances the release of AMA1 and other invasion-related proteins from apical secretory organelles, suggesting a previously unrecognized role for AMA1. These findings suggest that palmitoylation is ubiquitous throughout the T. gondii proteome and reveal insights into the biology of this important human pathogen.

Ubiquitination of Cdc20 by the APC occurs through an intramolecular mechanism.

BACKGROUND: Cells control progression through late mitosis by regulating Cdc20 and Cdh1, the two mitotic activators of the anaphase-promoting complex (APC). The control of Cdc20 protein levels during the cell cycle is not well understood. RESULTS: Here, we demonstrate that Cdc20 is degraded in budding yeast by multiple APC-dependent mechanisms. We find that the majority of Cdc20 turnover does not involve a second activator molecule but instead depends on in cis Cdc20 autoubiquitination while it is bound to its activator-binding site on the APC core. Unlike in trans ubiquitination of Cdc20 substrates, the APC ubiquitinates Cdc20 independent of APC activation by Cdc20's C box. Cdc20 turnover by this intramolecular mechanism is cell cycle regulated, contributing to the decline in Cdc20 levels that occurs after anaphase. Interestingly, high substrate levels in vitro significantly reduce Cdc20 autoubiquitination. CONCLUSION: We show here that Cdc20 fluctuates through the cell cycle via a distinct form of APC-mediated ubiquitination. This in cis autoubiquitination may preferentially occur in early anaphase, following depletion of Cdc20 substrates. This suggests that distinct mechanisms are able to target Cdc20 for ubiquitination at different points during the cell cycle.

Chocolate coated cats: TYRP1 mutations for brown color in domestic cats.

Brown coat color phenotypes caused by mutations in tyrosinase-related protein-1 (TYRP1) are recognized in many mammals. Brown variations are also recognized in the domestic cat, but the causative mutations are unknown. In cats, Brown, B, has a suggested allelic series, B > b > b1. The B allele is normal wild-type black coloration. Cats with the brown variation genotypes, bb or bb1, are supposedly phenotypically chocolate (aka chestnut) and the light brown genotype, b1b1, are supposedly phenotypically cinnamon (aka red). The complete coding sequence of feline TYRP1 and a portion of the 5' UTR was analyzed by direct sequencing of genomic DNA of wild-type and brown color variant cats. Sixteen single nucleotide polymorphisms (SNPs) were identified. Eight SNPs were in the coding regions, six are silent mutations. Two exon 2 on mutations cause amino acid changes. The C to T nonsense mutation at position 298 causes an arginine at amino acid 100 to be replaced by the opal (UGA) stop codon. This mutation is consistent with the cinnamon phenotype and is the putative light brown, b1, mutation. An intron 6 mutation that potentially disrupts the exon 6 downstream splice-donor recognition site is associated with the chocolate phenotype and is the putative brown, b, mutation. The allelic series was confirmed by segregation and sequence analyses. Three microsatellite makers had significant linkage to the brown phenotype and two for the TYRP1 mutations in a 60-member pedigree. These mutations could be used to identify carriers of brown phenotypes in the domestic cat.