A metal ion-dependent conformational switch modulates activity of the Plasmodium M17 aminopeptidase.

The metal-dependent M17 aminopeptidases are conserved throughout all kingdoms of life. This large enzyme family is characterized by a conserved binuclear metal center and a distinctive homohexameric arrangement. Recently, we showed that hexamer formation in Plasmodium M17 aminopeptidases was controlled by the metal ion environment, although the functional necessity for hexamer formation is still unclear. To further understand the mechanistic role of the hexameric assembly, here we undertook an investigation of the structure and dynamics of the M17 aminopeptidase from Plasmodium falciparum, PfA-M17. We describe a novel structure of PfA-M17, which shows that the active sites of each trimer are linked by a dynamic loop, and loop movement is coupled with a drastic rearrangement of the binuclear metal center and substrate-binding pocket, rendering the protein inactive. Molecular dynamics simulations and biochemical analyses of PfA-M17 variants demonstrated that this rearrangement is inherent to PfA-M17, and that the transition between the active and inactive states is metal dependent and part of a dynamic regulatory mechanism. Key to the mechanism is a remodeling of the binuclear metal center, which occurs in response to a signal from the neighboring active site and serves to moderate the rate of proteolysis under different environmental conditions. In conclusion, this work identifies a precise mechanism by which oligomerization contributes to PfA-M17 function. Furthermore, it describes a novel role for metal cofactors in the regulation of enzymes, with implications for the wide range of metalloenzymes that operate via a two-metal ion catalytic center, including DNA processing enzymes and metalloproteases.

Toxoplasma gondii Toxolysin 4 Contributes to Efficient Parasite Egress from Host Cells.

Egress from host cells is an essential step in the lytic cycle of T. gondii and other apicomplexan parasites; however, only a few parasite secretory proteins are known to affect this process. The putative metalloproteinase toxolysin 4 (TLN4) was previously shown to be an extensively processed microneme protein, but further characterization was impeded by the inability to genetically ablate TLN4. Here, we show that TLN4 has the structural properties of an M16 family metalloproteinase, that it possesses proteolytic activity on a model substrate, and that genetic disruption of TLN4 reduces the efficiency of egress from host cells. Complementation of the knockout strain with the TLN4 coding sequence significantly restored egress competency, affirming that the phenotype of the Δtln4 parasite was due to the absence of TLN4. This work identifies TLN4 as the first metalloproteinase and the second microneme protein to function in T. gondii egress. The study also lays a foundation for future mechanistic studies defining the precise role of TLN4 in parasite exit from host cells. IMPORTANCE After replicating within infected host cells, the single-celled parasite Toxoplasma gondii must rupture out of such cells in a process termed egress. Although it is known that T. gondii egress is an active event that involves disruption of host-derived membranes surrounding the parasite, very few proteins that are released by the parasite are known to facilitate egress. In this study, we identify a parasite secretory protease that is necessary for efficient and timely egress, laying the foundation for understanding precisely how this protease facilitates T. gondii exit from host cells.

The molecular assembly of the marsupial γµ T cell receptor defines a third T cell lineage.

αß and γδ T cell receptors (TCRs) are highly diverse antigen receptors that define two evolutionarily conserved T cell lineages. We describe a population of γµTCRs found exclusively in non-eutherian mammals that consist of a two-domain (Vγ-Cγ) γ-chain paired to a three-domain (Vµ-Vµj-Cµ) µ-chain. γµTCRs were characterized by restricted diversity in the Vγ and Vµj domains and a highly diverse unpaired Vµ domain. Crystal structures of two distinct γµTCRs revealed the structural basis of the association of the γµTCR heterodimer. The Vµ domain shared the characteristics of a single-domain antibody within which the hypervariable CDR3µ loop suggests a major antigen recognition determinant. We define here the molecular basis underpinning the assembly of a third TCR lineage, the γµTCR.

Structure and substrate fingerprint of aminopeptidase P from Plasmodium falciparum.

Malaria is one of the world's most prevalent parasitic diseases, with over 200 million cases annually. Alarmingly, the spread of drug-resistant parasites threatens the effectiveness of current antimalarials and has made the development of novel therapeutic strategies a global health priority. Malaria parasites have a complicated lifecycle, involving an asymptomatic 'liver stage' and a symptomatic 'blood stage'. During the blood stage, the parasites utilise a proteolytic cascade to digest host hemoglobin, which produces free amino acids absolutely necessary for parasite growth and reproduction. The enzymes required for hemoglobin digestion are therefore attractive therapeutic targets. The final step of the cascade is catalyzed by several metalloaminopeptidases, including aminopeptidase P (APP). We developed a novel platform to examine the substrate fingerprint of APP from Plasmodium falciparum (PfAPP) and to show that it can catalyze the removal of any residue immediately prior to a proline. Further, we have determined the crystal structure of PfAPP and present the first examination of the 3D structure of this essential malarial enzyme. Together, these analyses provide insights into potential mechanisms of inhibition that could be used to develop novel antimalarial therapeutics.

Identification and Validation of a Potent Dual Inhibitor of the P. falciparum M1 and M17 Aminopeptidases Using Virtual Screening.

The Plasmodium falciparum PfA-M1 and PfA-M17 metalloaminopeptidases are validated drug targets for the discovery of antimalarial agents. In order to identify dual inhibitors of both proteins, we developed a hierarchical virtual screening approach, followed by in vitro evaluation of the highest scoring hits. Starting from the ZINC database of purchasable compounds, sequential 3D-pharmacophore and molecular docking steps were applied to filter the virtual 'hits'. At the end of virtual screening, 12 compounds were chosen and tested against the in vitro aminopeptidase activity of both PfA-M1 and PfA-M17. Two molecules showed significant inhibitory activity (low micromolar/nanomolar range) against both proteins. Finally, the crystal structure of the most potent compound in complex with both PfA-M1 and PfA-M17 was solved, revealing the binding mode and validating our computational approach.

X-ray crystal structures of an orally available aminopeptidase inhibitor, Tosedostat, bound to anti-malarial drug targets PfA-M1 and PfA-M17.

New anti-malarial treatments are desperately required to face the spread of drug resistant parasites. Inhibition of metalloaminopeptidases, PfA-M1 and PfA-M17, is a validated therapeutic strategy for treatment of Plasmodium falciparum malaria. Here, we describe the crystal structures of PfA-M1 and PfA-M17 bound to chemotherapeutic agent Tosedostat. The inhibitor occupies the enzymes' putative product egress channels in addition to the substrate binding pockets; however, adopts different binding poses when bound to PfA-M1 and PfA-M17. These findings will be valuable for the continued development of selective inhibitors of PfA-M1 and PfA-M17.

Two-pronged attack: dual inhibition of Plasmodium falciparum M1 and M17 metalloaminopeptidases by a novel series of hydroxamic acid-based inhibitors.

Plasmodium parasites, the causative agents of malaria, have developed resistance to most of our current antimalarial therapies, including artemisinin combination therapies which are widely described as our last line of defense. Antimalarial agents with a novel mode of action are urgently required. Two Plasmodium falciparum aminopeptidases, PfA-M1 and PfA-M17, play crucial roles in the erythrocytic stage of infection and have been validated as potential antimalarial targets. Using compound-bound crystal structures of both enzymes, we have used a structure-guided approach to develop a novel series of inhibitors capable of potent inhibition of both PfA-M1 and PfA-M17 activity and parasite growth in culture. Herein we describe the design, synthesis, and evaluation of a series of hydroxamic acid-based inhibitors and demonstrate the compounds to be exciting new leads for the development of novel antimalarial therapeutics.

X-ray crystal structure and specificity of the Plasmodium falciparum malaria aminopeptidase PfM18AAP.

The malarial aminopeptidases have emerged as promising new drug targets for the development of novel antimalarial drugs. The M18AAP of Plasmodium falciparum malaria is a metallo-aminopeptidase that we show demonstrates a highly restricted specificity for peptides with an N-terminal Glu or Asp residue. Thus, the enzyme may function alongside other aminopeptidases in effecting the complete degradation or turnover of proteins, such as host hemoglobin, which provides a free amino acid pool for the growing parasite. Inhibition of PfM18AAP's function using antisense RNA is detrimental to the intra-erythrocytic malaria parasite and, hence, it has been proposed as a potential novel drug target. We report the X-ray crystal structure of the PfM18AAP aminopeptidase and reveal its complex dodecameric assembly arranged via dimer and trimer units that interact to form a large tetrahedron shape that completely encloses the 12 active sites within a central cavity. The four entry points to the catalytic lumen are each guarded by 12 large flexible loops that could control substrate entry into the catalytic sites. PfM18AAP thus resembles a proteasomal-like machine with multiple active sites able to degrade peptide substrates that enter the central lumen. The Plasmodium enzyme shows significant structural differences around the active site when compared to recently determined structures of its mammalian and human homologs, which provides a platform from which a rational approach to inhibitor design of new malaria-specific drugs can begin.