Calcium-binding protein 1 of Entamoeba histolytica transiently associates with phagocytic cups in a calcium-independent manner

EhCaBP1, a calcium-binding protein of the parasite Entamoeba histolytica, is known to participate in cellular processes involving actin filaments. This may be due to its direct interaction with actin. In order to understand the kinetics of EhCaBP1 in such processes, its movement was studied in living cells expressing GFP-EhCaBP1. The results showed that EhCaBP1 accumulated at phagocytic cups and pseudopods transiently. The time taken for appearance and disappearance of EhCaBP1 was found to be around 12 s. Site-directed mutagenesis was used to generate an EhCaBP1 mutant with reduced Ca(2+)- and G-actin binding ability without any defect in its ability to bind F-actin. The overexpression of this mutant EhCaBP1 in the E. histolytica trophozoites resulted in the impairment of erythrophagocytosis, uptake of bacterial cells, killing of target cells but not fluid-phase pinocytosis. However, the mutant protein was still found to transiently localize with F-actin at the phagocytic cups and pseudopods. The mutant protein displayed reduced ability to activate endogenous kinase(s) suggesting that phagosome formation may require Ca(2+)-EhCaBP1 transducing downstream signalling but initiation of phagocytosis may be independent of its intrinsic ability to bind Ca(2+). The results suggest a dynamic association of EhCaBP1 with F-actin-mediated processes.