Copy Link
Add to Bookmark
Report
dictyNews Volume 27 Number 04
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 µ3, 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] 
