title: RALP1 is a rhoptry neck erythrocyte-binding protein of Plasmodium falciparum merozoites and a potential blood-stage vaccine candidate antigen.
abstract: Erythrocyte invasion by merozoites is an obligatory stage of Plasmodium infection and is essential to disease progression. Proteins in the apical organelles of merozoites mediate the invasion of erythrocytes and are potential malaria vaccine candidates. Rhoptry-associated, leucine zipper-like protein 1 (RALP1) of Plasmodium falciparum was previously found to be specifically expressed in schizont stages and localized to the rhoptries of merozoites by immunofluorescence assay (IFA). Also, RALP1 has been refractory to gene knockout attempts, suggesting that it is essential for blood-stage parasite survival. These characteristics suggest that RALP1 can be a potential blood-stage vaccine candidate antigen, and here we assessed its potential in this regard. Antibodies were raised against recombinant RALP1 proteins synthesized by using the wheat germ cell-free system. Immunoelectron microscopy demonstrated for the first time that RALP1 is a rhoptry neck protein of merozoites. Moreover, our IFA data showed that RALP1 translocates from the rhoptry neck to the moving junction during merozoite invasion. Growth and invasion inhibition assays revealed that anti-RALP1 antibodies inhibit the invasion of erythrocytes by merozoites. The findings that RALP1 possesses an erythrocyte-binding epitope in the C-terminal region and that anti-RALP1 antibodies disrupt tight-junction formation, are evidence that RALP1 plays an important role during merozoite invasion of erythrocytes. In addition, human sera collected from areas in Thailand and Mali where malaria is endemic recognized this protein. Overall, our findings indicate that RALP1 is a rhoptry neck erythrocyte-binding protein and that it qualifies as a potential blood-stage vaccine candidate.
authors.affiliation: Division of Malaria Research, Proteo-Science Center.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Ito
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Hasegawa
authors.forename: Kazutoyo
authors.initials: K
authors.lastname: Miura
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Yamasaki
authors.forename: Thangavelu U
authors.initials: TU
authors.lastname: Arumugam
authors.forename: Amporn
authors.initials: A
authors.lastname: Thongkukiatkul
authors.forename: Satoru
authors.initials: S
authors.lastname: Takeo
authors.forename: Eizo
authors.initials: E
authors.lastname: Takashima
authors.forename: Jetsumon
authors.initials: J
authors.lastname: Sattabongkot
authors.forename: Eun-Taek
authors.initials: ET
authors.lastname: Han
authors.forename: Carole A
authors.initials: CA
authors.lastname: Long
authors.forename: Motomi
authors.initials: M
authors.lastname: Torii
authors.forename: Takafumi
authors.initials: T
authors.lastname: Tsuboi
mesh_terms: D000328:Adult; D000818:Animals; D000913:Antibodies, Protozoan / Q000097:blood; D000953:Antigens, Protozoan / Q000276:immunology / Q000378:metabolism*; D004912:Erythrocytes / Q000469:parasitology*; D005455:Fluorescent Antibody Technique; D006801:Humans; D017780:Malaria Vaccines / Q000235:genetics / Q000276:immunology*; D008302:Mali; D016253:Microscopy, Immunoelectron; D010963:Plasmodium falciparum / Q000502:physiology*; D015800:Protozoan Proteins / Q000276:immunology / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000276:immunology / Q000378:metabolism*; D044967:Serum / Q000276:immunology; D013785:Thailand
chemical_list: D000913:Antibodies, Protozoan; D000953:Antigens, Protozoan; C066001:Duffy antigen binding protein, Plasmodium; D017780:Malaria Vaccines; D015800:Protozoan Proteins; D011956:Receptors, Cell Surface