dictyNews Electronic Edition Volume 27, number 4 August 11, 2006 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to dicty@northwestern.edu or by using the form at http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit. Back issues of dictyNews, the Dicty Reference database and other useful information is available at dictyBase - http://dictybase.org. ============= Abstracts ============= Thirteen is enough: the myosins of Dictyostelium discoideum and their light chains Martin Kollmar BMC Genomics 2006, 7:183 http://www.biomedcentral.com/1471-2164/7/183 Background Dictyostelium discoideum is one of the most famous model organisms for studying motile processes like cell movement, organelle transport, cytokinesis, and endocytosis. Members of the myosin superfamily, that move on actin filaments and power many of these tasks, are tripartite proteins consisting of a conserved catalytic domain followed by the neck region consisting of a different number of so-called IQ motifs for binding of light chains. The tails contain functional motifs that are responsible for the accomplishment of the different tasks in the cell. Unicellular organisms like yeasts contain three to five myosins while vertebrates express over 40 different myosin genes. Recently, the question has been raised how many myosins a simple multicellular organism like Dictyostelium would need to accomplish all the different motility-related tasks. Results The analysis of the Dictyostelium genome revealed thirteen myosins of which three have not been described before. The phylogenetic analysis of the motor domains of the new myosins placed Myo1F to the class-I myosins and Myo5A to the class-V myosins. The third new myosin, an orphan myosin, has been named MyoG. It contains an N-terminal extension of over 400 residues, and a tail consisting of four IQ motifs and two MyTH4/FERM (myosin tail homology 4/band 4.1, ezrin, radixin, and moesin) tandem domains that are separated by a long region containing an SH3 (src homology 3) domain. In contrast to previous analyses, an extensive comparison with 126 class-VII, class-X, class-XV, and class-XXII myosins now showed that MyoI does not group into any of these classes and should not be used as a model for class-VII myosins. The search for calmodulin related proteins revealed two further potential myosin light chains. One is a close homolog of the two EF-hand motifs containing MlcB, and the other, CBP14, phylogenetically groups to the ELC/RLC/calmodulin (essential light chain/regulatory light chain) branch of the tree. Conclusions Dictyostelium contains thirteen myosins together with 6-8 MLCs (myosin light chain) to assist in a variety of actin-based processes in the cell. Although they are homologous to myosins of higher eukaryotes, the myosins of Dictyostelium should be considered with care as models for specific functions of vertebrate myosins. Submitted by: Martin Kollmar [mako@nmr.mpibpc.mpg.de] ----------------------------------------------------------------------------- Exocytosis of late endosomes does not directly contribute membrane to the formation of phagocytic cups or pseudopods in Dictyostelium Steve J. Charette and Pierre Cosson Universite de Geneve, Centre Medical Universitaire, Departement de physiologie cellulaire et metabolisme, 1 rue Michel Servet, CH-1211 Geneva 4, Switzerland. FEBS letters, in press Exocytosis of late endocytic compartments in Dictyostelium has mostly been studied by live microscopy. Here we show that this exocytosis is accompanied by a complete fusion of late endosomes with the plasma membrane resulting in the transient formation of membrane microdomains that can be visualized by immunofluorescence in fixed cells. This permitted to demonstrate that fusion of late endocytic compartments with the cell surface does not occur in regions of the plasma membrane engaged in the formation of pseudopods, macropinosomes or phagosomes. Our results propose that exocytosis of late endosomes and actin-driven membrane remodeling are mutually exclusive processes. Submitted by: Steve J. Charette [steve.charette@medecine.unige.ch] ----------------------------------------------------------------------------- A role for Adaptor Protein-3 complex in the organization of the endocytic pathway in Dictyostelium Steve J. Charette#1, Valentina Mercanti#1, Franois Letourneur2, Nelly Bennett3, and Pierre Cosson1 1. Universite de Geneve, Centre Medical Universitaire, Departement de Physiologie Cellulaire et Mtabolisme, 1 rue Michel Servet, CH-1211 Geneve 4, Switzerland. 2. Institut de Biologie et Chimie des Proteines (IBCP UMR 5086); CNRS; Universite Lyon1; IFR128 BioSciences Lyon-Gerland; 7, passage du Vercors, 69367 Lyon Cedex 07, France 3. Laboratoire de Biochimie et Biophysique des Systemes Integres, Departement de Reponse et Dynamique Cellulaires, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France. # These authors contributed equally to this work. Traffic, in press Dictyostelium discoideum cells continuously internalize extracellular material, which accumulates in well-characterized endocytic vacuoles. In the present study, we describe a new endocytic compartment identified by the presence of a specific marker, the p25 protein. This compartment presents features reminiscent of mammalian recycling endosomes: it is localized in the peri-centrosomal region but distinct from the Golgi apparatus. It contains specifically surface proteins that are continuously endocytosed but rapidly recycled to the cell surface and thus absent from maturing endocytic compartments. We evaluated the importance of each clathrin-associated adaptor complex in establishing a compartmentalized endocytic system by studying the phenotype of the corresponding mutants. In knockout cells for 3, a subunit of the AP-3 clathrin-associated complex, membrane proteins normally restricted to p25-positive endosomes were mislocalized to late endocytic compartments. Our results suggest that AP-3 plays an essential role in the compartmentalization of the endocytic pathway in Dictyostelium. Submitted by: Steve J. Charette [steve.charette@medecine.unige.ch] ============================================================================== [End dictyNews, volume 27, number 4]