ACTIN CYTOSKELETON AND ASSOCIATED PROTEIN S - A FEW EXAMPLES IN PROTISTS

Many processes, cell motility being on example, require cells to remod el the actin cytoskeleton in response to both intracellular and extrac ellular signals. Reorganization of the actin cytoskeleton involves the rapid disassembly and reassembly of actin filaments, a phenomenon reg ulated by the action of particular actin-binding proteins. In recent y ears, an interest in studying actin regulation in unicellular organism s has arisen. Parasitic protozoan are among these organisms and studie s of the cytoskeleton functions of these protozoon ore relevant relate d to either cell biology or pathogenicity To discuss recent data in th is field, a symposium concerning <<Actin and actin-binding proteins in protists>> was held on May 8-11 in Paris, France, during the XXXV mee ting of the French Society of Protistology. As a brief summary of the symposium we report here findings concerning the in vitro actin dynami c assembly, as well as the characterization of several actin-binding p roteins from the parasitic protozoan Entamoeba histolytica, Thrichomon as vaginalis and Plasmodium knowlesi. In addition, localization of act in in nonpathogen protists such as Prorocentrum micans and Crypthecodi nium cohnii is also presented. The data show that some actin-binding p roteins facilitate organization of filaments into higher order structu res as pseudopods, while others have regulatory functions, indicating very particular roles for actin-binding proteins. One of the proteins discussed during the symposium, the actin depolymerizing factor ADF, w as shown to enhance the treadmilling rate of actin filaments. in vitro , ADF binds to the ADP-bound forms of G-actin and F-actin, thereby par ticipating in and changing the rate of actin assembly. Biochemical app roaches allowed the identification of a protein complex formed by HSP/ C70-cap32-34 which might also be involved in depolymerization of F-act in in P. knowlesi. Molecular and cellular approaches were used to iden tify proteins such as ABP-120 and myosin IB at the leading edge of E. histolytica. ABP-120 organizes F-actin in a network and myosin IB part icipates in the pseudopod formation. Similar approaches using T. vagin alis resulted in the discovery of on actin-binding protein that partic ipate in the F-actin reorganization during adhesion of parasites to ta rget cells. This protein is homologous to alpha-actinin from other euk aryotic cells. Finally, by using cell biology approaches, F-actin was observed in the cytoplasm as well as in the nucleus of Dinoflagelates. The recent developments in the molecular genetics of protozoa will pr ovide new insights to understand the roles of actin-binding proteins d uring cytoskeleton activities.