Two Novel Calcium-binding Proteins from Cytoplasmic Granules of the Protozoan Parasite Entamoeba Histolytica

We report on the molecular characterisation of two novel granule proteins of the protozoon and human pathogen Entamoeba histolytica. The proteins, which were named grainin 1 and 2, show a considerable structural similarity to calcium-binding proteins, particularly within EF-hand motifs. Each grainin possesses three of these putative calcium-binding sites. Based on careful inspection of known structures of protein families containing EF-hands, a domain of grainin 1 covering two EF-hand motifs was modeled by homology. Calcium-binding activity of grainins was demonstrated by two independent methods. These granule proteins may be implicated in functions vital for the primitive phagocyte and destructive parasite such as control of endocytotic pathways and granule discharge.