The microneme protein MIC3 of Toxoplasma gondii is a secretory adhesin that binds to both the surface of the host cells and the surface of the parasite

Assay of the adhesion of cultured cells on :Toxoplasma gondii tachyzoite pr otein Western blots identified a major adhesive protein, that migrated at 9 0 kDa in non-reducing gels. This band comigrated with the previously descri bed microneme protein MIC3. Cellular binding on Western blots was abolished by MIC3-specific monoclonal and polyclonal antibodies. The MIC3 protein af finity purified from tachyzoite lysates bound to the surface of putative ho st cells. In addition, T. gondii tachyzoites also bound to immobilized MIC3 . Immunofluorescence analysis of T. gondii tachyzoite invasion showed that MIC3 was exocytosed and relocalized to the surface of the parasite during i nvasion. The cDNA encoding MIC3 and the corresponding gene have been cloned , allowing the determination of the complete coding sequence. The MIC3 sequ ence has been confirmed by affinity purification of the native protein and N-terminal sequencing. The deduced protein sequence contains five partially overlapping EGF-like domains and a chitin binding-like domain, which can b e involved in protein-protein or protein-carbohydrate interactions. Taken t ogether, these results suggest that MIC3 is a new microneme adhesin of T. g ondii.