THE SMALL GTP-BINDING PROTEIN RACG REGULATES UROID FORMATION IN THE PROTOZOAN PARASITE ENTAMOEBA-HISTOLYTICA

Entamoeba histolytica is a protozoan parasite that invades human intes tine leading to ulceration and destruction of tissue. Amoebic movement and phagocytosis of human cells is accompanied by characteristic chan ges in cell morphology. Amoebae become polarized, developing a frontal pseudopod and a well-defined rear zone of membrane accumulation desig nated the uroid, In motile eukaryotic cells, a phenomenon that contrib utes to movement is the capping of receptors at the cell surface. Duri ng the capping process, E. histolytica concentrates ligand-receptor co mplexes in the uroid, Interestingly, some of these surface receptors a re involved in the survival of the parasite. While looking for regulat ors of capping and uroid formation, we identified RacG, an E. histolyt ica protein that is homologous to human Rac1. This protein belongs to the Rac subfamily of small GTPases implicated in interactions between the actin cytoskeleton and the membrane of mammalian cells. Cloning of the EhracG gene and analysis of the protein activity either in murine fibroblasts or in E. histolytica revealed that EhRacG induces a chara cteristic Rac phenotype. When expressed in amoebae, an EhRacG-V12 muta nt protein not only deregulated cell polarity, but also caused a defec t in cytokinesis. Analysis of the cytoskeleton in amoebae bearing this mutant revealed that F-actin concentrated at the periphery of the cel l. In addition, the number and localization of uroids were modified. T hese results suggest a role for EhRacG in amoebic morphogenesis and cy tokinesis.