CSM News Electronic Edition Volume 7, number 10 October 12, 1996 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to CSM-News@worms.cmb.nwu.edu. Back issues of CSM-News, the CSM Reference database and other useful information is available by anonymous ftp from worms.cmb.nwu.edu [165.124.233.50], via Gopher at the same address, or by World Wide Web at the URL "http://worms.cmb.nwu.edu/dicty.html" =========== Abstracts =========== Developmental Signal Transduction Pathways Uncovered by Genetic Suppressors Gad Shaulsky, Ricardo Escalante and William F. Loomis* Center for Molecular Genetics, Department of Biology University of California San Diego, La Jolla, CA 92093 Proc. Natl. Acad. Sci. in press ABSTRACT We have found conditions for saturation mutagenesis by restriction enzyme mediated integration (REMI) that result in plasmid tagging of disrupted genes. Using this method we selected for mutations in genes that act at check points downstream of the intercellular signaling system that controls encapsulation in Dictyostelium discoideum. One of these genes, mkcA, is a member of the MAP-kinase cascade family while the other, regA, is a novel bi-partite gene homologous to response regulators in one part and to cyclic nucleotide phosphodiesterases in the other part. Disruption of either of these genes results in partial suppression of the block to spore formation resulting from the loss of the prestalk genes, tagB and tagC. The products of the tag genes have conserved domains of serine proteases attached to ATP driven transporters suggesting that they process and export peptide signals. Together, these genes outline an intercellular communication system that coordinates organismal shape with cellular differentiation during development. -------------------------------------------------------------------- Interaction of a Dictyostelium member of the plastin/fimbrin family with actin filaments and actin-myosin complexes Josef Prassler, Susanne Stocker, Gerard Marriott, Manfred Heidecker, Josef Kellermann, and Guenther Gerisch Max-Planck-Institut f=FCr Biochemie, D-82152 Martinsried, Germany Molecular Biology of the Cell, in press. ABSTRACT A protein purified from cytoskeletal fractions of Dictyostelium discoideum proved to be a member of the fimbrin/plastin family of actin-bundling proteins. Like other family members, this Ca2+-inhibited 67 kDa protein contains two EF-hands followed by two actin-binding sites of the a-actinin/=DF-spectrin type. Dd plastin interacted selectively with actin isoforms: it bound to D. discoideum actin and to b/g-actin from bovine spleen, but not to a-actin from rabbit skeletal muscle. Immunofluorescence labelling of growth-phase cells showed accumulation of Dd plastin in cortical structures associated with cell surface extensions. In the elongated, streaming cells of the early aggregation stage, Dd plastin was enriched in the front regions. In order to examine how the bundled actin filaments behave in myosin II-driven motility, complexes of F-actin and Dd plastin were bound to immobilized heavy meromyosin, and motility was started by photoactivating caged ATP. Actin filaments were immediately propelled out of bundles or even larger aggregates, and moved on the myosin as separate filaments. This result shows that myosin can disperse an actin network when it acts as a motor, and sheds light on the dynamics of protein-protein interactions in the cortex of a motile cell where myosin II and Dd plastin are simultaneously present. ---------------------------------------------------------------------- Cellulose microfibrils, cell motility, and plasma membrane protein organization change in parallel during culmination in Dictyostelium discoideum Mark J. Grimson, Candace H. Haigler, and Richard L. Blanton Department of Biological Sciences, Texas Tech University, Lubbock, TX 79409 J. Cell Science, in press Abstract Prestalk cells of Dictyostelium discoideum contribute cellulose to two distinct structures, the stalk tube and the stalk cell wall, during culmination. This paper demonstrates by freeze fracture electron microscopy that two distinct types of intramembrane particle aggregates, which can be characterized as cellulose microfibril terminal complexes, occur in the plasma membranes of cells synthesizing these different forms of cellulose. The same terminal complexes were observed in situ in developing culminants and in vitro in monolayer cells induced to synthesize the two types of cellulose. We propose that cessation of cell motility is associated with a change in packing and intramembrane mobility of the particle aggregates, which cause a change in the nature of the cellulose synthesized. The terminal complexes are compared to those described in other organisms and related to the previous hypothesis of two modes of cellulose synthesis in Dictyostelium. ---------------------------------------------------------------------- [End CSM News, volume 7, number 10]