Mise au point

L'actine cytosquelettique et ses protéines associées. Quelques exemples chez les protistes

Actin cytoskeleton and associated proteins: a few examples in protists

¹ Unité de Pathogénie Microbienne Moléculaire, INSERM U389. Institut Pasteur, 28, rue du Docteur Roux, F-75724 Paris Cedex 15..
² Dynamique du cytosquelette, Laboratoire d’Enzymologie et Biochimie Structurales, CNRS, 91198 Gif-sur-Yvette.
³ Biologie des Protistes, Université Biaise Pascal de Clermont-Ferrand, 63177 Aubière Cedex.
⁴ Unité de Biochimie et Biologie Moléculaire des Insectes, Institut Pasteur, 25, rue du Docteur Roux, F-75724 Paris Cedex 15.
⁵ Observatoire Océanologique, Université Pierre et Marie Curie, Paris 6, URA-2156 CNRS BP-44, F-66650 Banyuls-sur-mer Cedex.

^* Correspondance : Nancy Guillén. E-mail : e-mail : nguillen@pasteur.fr

Reçu : 11 Décembre 1997
Accepté : 2 Avril 1998

Résumé

La mise en œuvre de multiples processus biologiques nécessite des changements du cytosquelette riche en actine. Tel est le cas de la motilité cellulaire qui résulte de l'assemblage et désassemblage des filaments d'actine, une activité régulée par l'action de nombreuses protéines liant l'actine. Les organismes unicellulaires sont des modèles de choix pour l'étude des régulations engendrant les remaniements du cytosquelette car leur manipulation aisée s'adapte à diverses technologies. Lors de la XXXV^e réunion annuelle du Groupement des Protistologues de Langue Française (GPLF), un symposium concernant « L'actine cytosquelettique et ses protéines associées chez les protistes » a été organisé. Ce colloque reflète l'intérêt du GPLF pour le développement en France des thèmes de recherche concernant des protistes pathogènes pour l'homme et ceux, pathogènes ou non, servant de modèles d'analyse cellulaire. Un compte rendu de ce colloque est présenté dans cet article.

Abstract

Many processes, cell motility being an example, require cells to remodel the actin cytoskeleton in response to both intracellular and extracellular signals. Reorganization of the actin cytoskeleton involves the rapid disassembly and reassembly of actin filaments, a phenomenon regulated by the action of particular actin-binding proteins. In recent years, an interest in studying actin regulation in unicellular organisms has arisen. Parasitic protozoan are among these organisms and studies of the cytoskeleton functions of these protozoan are relevant related to either cell biology or pathogenicity. To discuss recent data in this field, a symposium concerning « Actin and actin-binding proteins in protists » was held on May 8-11 in Paris, France, during the XXXV meeting of the French Society of Protistology.

As a brief summary of the symposium we report here findings concerning the in vitro actin dynamic assembly, as well as the characterization of several actin-binding proteins from the parasitic protozoan Entamoeba histolytica Thrichomonas vaginalis, and Plasmodium knowlesi. In addition, localization of actin in non-pathogen protists such as Prorocentrum micans, and Crypthecodinium cohnii, is also presented.

The data show that some actin-binding proteins facilitate organization of filaments into higher order structures 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, was 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 participating in and changing the rate of actin assembly. Biochemical approaches allowed the identification of a protein complex formed by HSP/C70-cap32-34 which might also be involved in depolymerization of F-actin in P. knowlesi. Molecular and cellular approaches were used to identify 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 participates in the pseudopod formation. Similar approaches using T. vaginalis resulted in the discovery of an actin-binding protein that participate in the F-actin reorganization during adhesion of parasites to target cells. This protein is homologous to &-actinin from other eukaryotic 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 provide new insights to understand the roles of actin-binding proteins during cytoskeleton activities.