Inhibition of Malaria Parasite Development by a Cyclic Peptide That Targets the Vital Parasite Protein SERA5

Abstract

The serine repeat antigen (SERA) proteins of the malaria parasites Plasmodium spp. contain a putative enzyme domain similar to that of papain family cysteine proteases. In Plasmodium falciparum parasites, more than half of the SERA family proteins, including the most abundantly expressed form, SERA5, have a cysteine-to-serine substitution within the putative catalytic triad of the active site. Although SERA5 is required for blood-stage parasite survival, the occurrence of a noncanonical catalytic triad casts doubt on the importance of the enzyme domain in this function. We used phage display to identify a small (14-residue) disulfidebonded cyclic peptide (SBP1) that targets the enzyme domain of SERA5. Biochemical characterization of the interaction shows that it is dependent on the conformation of both the peptide and protein. Addition of this peptide to parasite cultures compromised development of late-stage parasites compared to that of control parasites or those incubated with equivalent amounts of the carboxymethylated peptide. This effect was similar in two different strains of P. falciparum as well as in a transgenic strain where the gene encoding the related serine-type parasitophorous vacuole protein SERA4 was deleted. In compromised parasites, the SBP1 peptide crosses both the erythrocyte and parasitophorous vacuole membranes and accumulates within the parasitophorous vacuole. In addition, both SBP1 and SERA5 were identified in the parasite cytosol, indicating that the plasma membrane of the parasite was compromised as a result of SBP1 treatment. These data implicate an important role for SERA5 in the regulation of the intraerythrocytic development of late-stage parasites and as a target for drug development.

Structural Insights into the Protease-like Antigen Plasmodium falciparum SERA5 and Its Noncanonical Active-Site Serine

“…Our attempts to soak SERA5E crystals with the generic serine-protease inhibitors antipain and 4-(2-aminoethyl) benzenesulfonyl fluoride or with a cyclic peptide inhibitor 10 have failed to reveal anything bound to the protein. This may be attributable to some role of D594; in crystals grown in the absence of phosphate (structures 2 and 3), the carboxylate side chain of (one rotamer of) D594 occupies the phosphate-ion-binding site and limits access to the S 1 site.…”

“…10 Hence, compounds targeting the SERA5 active site may have potential as chemotherapeutic agents. Structural details of the unusual SERA active site presented here may offer opportunities for rational design of novel, parasitespecific inhibitory compounds.…”

“…High-resolution (1.8 Å) data were collected at the Advanced Photon Source on beamline 14-ID-B from a crystal grown in the presence of a putative peptide inhibitor (a 14-residue disulfide-bonded cyclic peptide with the sequence LVCHPAVPALLCAR). 10 However, there was no evidence of the peptide in the diffraction data, and this data set has been treated as "native." Data were collected from a SeMet-substituted SERA5E crystal at the Swiss Light Source, at 100 K. The crystals belong to the space group R3, with one monomer per asymmetric unit (Table 1).…”

“…[1][2][3] Moreover, the presence of protease domains and the prominence of one member, SERA5, as a strong antigen have led to interest in these proteins as targets for drug and vaccine development. [4][5][6][7][8][9][10] Blood-stage SERA proteins are synthesised as 100-to 130-kDa precursors, which are exported to the parasitophorous vacuole within an infected RBC. 1,8,11,12 In the case of SERA5, the most abundant and best-studied member of the family, the precursor is processed into three fragments.…”

Proteomic analysis of microparticles isolated from malaria positive blood samples

“…Precursors of serine-repeat antigen (SERA) proteins are synthesized in the late trophozoites [54]. Among organisms in the apicomplexan phylum and with the exception of Theileria found in cattle, species of Plasmodium are the only organisms that the gene family translating into Serine-repeat antigen (SERA) protein has been found [55].…”