dictyNews Electronic Edition Volume 40, number 10 April 4, 2014 Please submit abstracts of your papers as soon as they have been accepted for publication by by using the form at http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit or by sending them to dicty@northwestern.edu Back issues of dictyNews, the Dicty Reference database and other useful information is available at dictyBase - http://dictybase.org. Follow dictyBase on twitter: http://twitter.com/dictybase ========= Abstracts ========= The IQGAP-related protein DGAP1 mediates signaling to the actin cytoskeleton as an effector and a sequestrator of Rac1 GTPases Vedrana Filic, Maja Marinovic, Jan Faix, Igor Weber Cellular and Molecular Life Sciences, in press http://link.springer.com/article/10.1007%2Fs00018-014-1606-3 Proteins are typically categorized into protein families based on their domain organization. Yet, evolutionarily unrelated proteins can also be grouped together according to their common functional roles. Sequestering proteins constitute one such functional class, acting as macromolecular buffers and serving as an intracellular reservoir ready to release large quantities of bound proteins or other molecules upon appropriate stimulation. Another functional protein class comprises effector proteins, which constitute essential components of many intracellular signal transduction pathways. For instance, effectors of small GTP-hydrolases are activated upon binding a GTP-bound GTPase and thereupon participate in downstream interactions. Here we describe a member of the IQGAP family of scaffolding proteins, DGAP1 from Dictyostelium, which unifies the roles of an effector and a sequestrator in regard to the small GTPase Rac1. Unlike classical effectors, which bind their activators transiently leading to short-lived signaling complexes, interaction between DGAP1 and Rac1-GTP is stable and induces formation of a complex with actin-bundling proteins cortexillins at the back end of the cell. An oppositely localized Rac1 effector, the Scar/WAVE complex, promotes actin polymerization at the cell front. Competition between DGAP1 and Scar/WAVE for the common activator Rac1-GTP might provide the basis for the oscillatory re- polarization typically seen in randomly migrating Dictyostelium cells. We discuss the consequencesof the dual roles exerted by DGAP1 and Rac1 in the regulation of cell motility and polarity, and propose that similar signaling mechanisms may be of general importance in regulating spatiotemporal dynamics of the actin cytoskeleton by small GTPases. Submitted by Vedrana FilicÊ [vedrana.filic@irb.hr]] --------------------------------------------------------------------------- Pore-forming toxins from pathogenic amoebae. Leippe M. Appl Microbiol Biotechnol. 2014 Mar 28. [Epub ahead of print], DOI: 10.1007/s00253-014-5673-z Some amoeboid protozoans are facultative or obligate parasites in humans and bear an enormous cytotoxic potential that can result in severe destruction of host tissues and fatal diseases. Pathogenic amoebae produce soluble pore-forming polypeptides that bind to prokaryotic and eukaryotic target cell membranes and generate pores upon insertion and oligomerization. This review summerizes the current knowledge of such small protein toxins from amoebae, compares them with related proteins from other species, focuses on their three-dimensional structures, and gives insights into divergent activation mechanisms. The potential use of pore-forming toxins in biotechnology will be briefly outlined. Submitted by Matthias Leippe [mleippe@zoologie.uni-kiel.de] --------------------------------------------------------------------------- Requirements for Hirano Body formation. Paul Griffin, Ruth Furukawa, Cleveland Piggott, Andrew Maselli, and Marcus Fechheimer. Eukaryotic Cell, In press Hirano bodies are paracrystalline F-actin-rich structures associated with diverse conditions including neurodegeneration and aging. Generation of model Hirano bodies using altered forms of Dictyostelium 34 kDa actin- bundling protein allows studies of their physiological function and mechanism of formation. We describe a novel 34 kDa protein mutant, E60K, with a point mutation within the inhibitory domain of 34 kDa protein. Expression of E60K in Dictyostelium induces formation of model Hirano bodies. The E60K protein has activated actin binding and is calcium- regulated unlike other forms of the 34 kDa protein that induce Hirano bodies that have activated actin binding but lack calcium-regulation. Actin filaments in the presence of E60K in vitro show enhanced resistance to disassembly induced by latrunculin B. Actin filaments in model Hirano bodies are also protected from latrunculin induced depolymerization. We used nocodazole and blebbistatin to probe the role of the microtubules and myosin II, respectively, in formation of model Hirano bodies. In the presence of these inhibitors, model Hirano bodies can form, but are smaller than control at early times of formation. The ultrastructure of model Hirano bodies did not reveal any major difference in the structure and organization in the presence of inhibitors. In summary, these results support the conclusion that formation of model Hirano bodies is promoted by gain-of- function actin filament bundling which enhances actin filament stabilization. Microtubules and myosin II contribute to but are not required for formation of model Hirano bodies. Submitted by Ruth Furukawa [furukawa@uga.edu] --------------------------------------------------------------------------- The association of myosin IB with actin waves in Dictyostelium requires both the plasma membrane-binding site and actin-binding region in the myosin tail Hanna Brzeska1*, Kevin Pridham1, Godefroy Chery1, Margaret A. Titus2 and Edward D. Korn1 1 Laboratory of Cell Biology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland, United States of America, 2 Department of Genetics, Cell Biology and Development, University of Minnesota, Minneapolis, Minnesota, United States of America. *E-mail: brzeskah@mail.nih.gov PLOS ONE, accepted F-actin structures and their distribution are important determinants of the dynamic shapes and functions of eukaryotic cells. Actin waves are F-actin formations that move along the ventral cell membrane driven by actin polymerization. Dictyostelium myosin IB is associated with actin waves but its role in the wave is unknown. Myosin IB is a monomeric, non-filamentous myosin with a globular head that binds to F-actin and has motor activity, and a non-helical tail comprising a basic region, a glycine-proline-glutamine-rich region and an SH3-domain. The basic region binds to acidic phospholipids in the plasma membrane through a short basic-hydrophobic site and the Gly-Pro-Gln region binds F-actin. In the current work we found that both the basic-hydrophobic site in the basic region and the Gly-Pro-Gln region of the tail are required for the association of myosin IB with actin waves. This is the first evidence that the Gly-Pro-Gln region is required for localization of myosin IB to a specific actin structure in situ. The head is not required for myosin IB association with actin waves but binding of the head to F-actin strengthens the association of myosin IB with waves and stabilizes waves. Neither the SH3-domain nor motor activity is required for association of myosin IB with actin waves. We conclude that myosin IB contributes to anchoring actin waves to the plasma membranes by binding of the basic-hydrophobic site to acidic phospholipids in the plasma membrane and binding of the Gly-Pro-Gln region to F-actin in the wave. Submitted by Hanna Brzeska [brzeska@helix.nih.gov] --------------------------------------------------------------------------- Protection of spores from ultraviolet-C irradiation by auto-fluorescent substances in the spore mass of the cellular slime mold Dictyostelium discoideum Saburo Uchiyama and Ikuo Hatakeyama Graduate School of Education, Iwate University, Morioka 020-8550, Japan Pteridines, accepted In this study, native spores surrounded by fluorescent substances in the spore mass of Dictyostelium discoideum were found to be resistant to relatively strong ultraviolet-C (UV-C) irradiation (2880 J/m2). The remaining emergency activity of the native mass of spores was over 80% even after exposure to strong UV-C irradiation (2880 J/m2). On the other hand, the washed spores were very sensitive to weak UV-C irradiation (144 J/m2). The mass of spores in the fruiting body formed by amoebae with a low concentration of fluorescent substances was less resistant to UV-C compared to that in the fruiting body formed by normally grown amoebae. Based on the remaining emergency activity of washed spores with appropriate lumazine solution, the concentration of fluorescent substances in the native mass of spores was estimated to be equivalent to approximately 5 mmol/L lumazine. Submitted by Saburo Uchiyama [uchiyama@iwate-u.ac.jp] ============================================================== [End dictyNews, volume 40, number 10]