We have isolated 26 monoclonal antibodies which specifically recognize
the extreme apex of Toxoplasma gondii, a protozoan parasite which att
aches to and invades host cells via its specialized apical end. The un
ique apical organelles which define the phylum Apicomplexa are thought
to be involved in mechanical and enzymatic aspects of invasion. Immun
oblots, immunofluorescent morphology, and immunogold labeling define s
ix classes of apically localized antigens recognized by these antibodi
es. Three of the classes are detergent insoluble and localize to the c
onoid and the cytoplasmic face of the apical membrane, suggesting that
they may be part of the parasite's membrane cytoskeleton. The remaini
ng three classes extract with detergent and are associated with intern
al membrane bounded vesicles (micronemes and the upper necks of rhoptr
ies). One class of micronemal antigens appears to be cell cycle regula
ted. This antigen localizes to the cytoplasm, especially the perinucle
ar region, in thin (recently replicated) parasites, but is apical in l
arger parasites. (C) 1994 Academic Press, Inc.