title: The toxoplasma micronemal protein MIC4 is an adhesin composed of six conserved apple domains.
abstract: The initial stage of invasion by apicomplexan parasites involves the exocytosis of the micronemes-containing molecules that contribute to host cell attachment and penetration. MIC4 was previously described as a protein secreted by Toxoplasma gondii tachyzoites upon stimulation of micronemes exocytosis. We have microsequenced the mature protein, purified after discharge from micronemes and cloned the corresponding gene. The deduced amino acid sequence of MIC4 predicts a 61-kDa protein that contains 6 conserved apple domains. Apple domains are composed of six spacely conserved cysteine residues which form disulfide bridges and are also present in micronemal proteins from two closely related apicomplexan parasites, Sarcocystis muris and Eimeria species, and several mammalian serum proteins, including kallikrein. Here we show that MIC4 localizes in the micronemes of all the invasive forms of T. gondii, tachyzoites, bradyzoites, sporozoites, and merozoites. The protein is proteolytically processed both at the N and the C terminus only upon release from the organelle. MIC4 binds efficiently to host cells, and the adhesive motif maps in the most C-terminal apple domain.
authors.affiliation: Zentrum für Molekulare Biologie Heidelberg, Im Neuenheimer Feld 282, 69120 Heidelberg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Brecht
authors.forename: V B
authors.initials: VB
authors.lastname: Carruthers
authors.forename: D J
authors.initials: DJ
authors.lastname: Ferguson
authors.forename: O K
authors.initials: OK
authors.lastname: Giddings
authors.forename: G
authors.initials: G
authors.lastname: Wang
authors.forename: U
authors.initials: U
authors.lastname: Jakle
authors.forename: J M
authors.initials: JM
authors.lastname: Harper
authors.forename: L D
authors.initials: LD
authors.lastname: Sibley
authors.forename: D
authors.initials: D
authors.lastname: Soldati
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D015815:Cell Adhesion Molecules / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D002478:Cells, Cultured; D003001:Cloning, Molecular; D017124:Conserved Sequence; D016054:DNA, Protozoan; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D011499:Protein Processing, Post-Translational; D015800:Protozoan Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D013347:Subcellular Fractions / Q000378:metabolism; D014122:Toxoplasma / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D015815:Cell Adhesion Molecules; D016054:DNA, Protozoan; C428323:MIC4 protein, Toxoplasma gondii; D015800:Protozoan Proteins