dictyNews Electronic Edition Volume 34, number 11 April 2, 2010 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. Follow dictyBase on twitter: http://twitter.com/dictybase ========= Abstracts ========= External and internal constraints to eukaryotic chemotaxis Danny Fuller1, Wen Chen2, Micha Adler2, Alex Groisman2, Herbert Levine2,3, Wouter-Jan Rappel2,3, William F. Loomis1 1 Division of Biological Sciences, 2 Department of Physics 3 Center for Theoretical Biological Physics, University of California San Diego, La Jolla, CA 92093 Proc. Natl. Acad. Sci., in press Chemotaxis, the chemically guided movement of cells, plays an important role in a number of biological processes including cancer, wound healing and embryogenesis. Chemotacting cells are able to sense shallow chemical gradients where the concentration of chemoattractant differs by only a few percent from one side of the cell to the other, over a wide range of local concentrations. Exactly what limits the chemotactic ability of these cells is presently unclear. Here we determine the chemotactic response of Dictyostelium cells to exponential gradients of varying steepness and local concentration of the chemoattractant cAMP. We find that the cells are sensitive to the steepness of the gradient as well as to the local concentration. Using information theory techniques, we derive a formula for the mutual information between the input gradient and the spatial distribution of bound receptors and also compute the mutual information between the input gradient and the motility direction in the experiments. A comparison between these two quantities reveals that for shallow gradients, in which the concentration difference between the back and the front of a 10 mm diameter cell is less than 5 %, and for small local concentrations (less than 10 nM) the intracellular information loss is insignificant. Thus, external fluctuations due to the finite number of receptors dominate and limit the chemotactic response. For steeper gradients and higher local concentrations, the intracellular information processing is sub-optimal and results in a much smaller mutual information between the input gradient and the motility direction than would have been predicted from the ligand-receptor binding process. Submitted by Bill Loomis [wloomis@ucsd.edu] -------------------------------------------------------------------------------- Self-organizing actin waves that simulate phagocytic cup structures Günther Gerisch PMC Biophysics 2010, 3:7 This report deals with actin waves that are spontaneously generated on the planar, substrate-attached surface of Dictyostelium cells. These waves have the following characteristics. (1) They are circular structures of varying shape, capable of changing the direction of propagation. (2) The waves propagate by treadmilling with a recovery of actin incorporation after photobleaching of less than 10 seconds. (3) The waves are associated with actin-binding proteins in an ordered 3-dimensional organization: with myosin-IB at the front and close to the membrane, the Arp2/3 complex throughout the wave, and coronin at the cytoplasmic face and back of the wave. Coronin is a marker of disassembling actin structures. (4) The waves separate two areas of the cell cortex that differ in actin structure and phosphoinositide composition of the membrane. The waves arise at the border of membrane areas rich in phosphatidylinositol (3,4,5) trisphosphate (PIP3). The inhibition of PIP3 synthesis reversibly inhibits wave formation. (5) The actin wave and PIP3 patterns resemble 2-dimensional projections of phagocytic cups, suggesting that they are involved in the scanning of surfaces for particles to be taken up. PACS Codes: 87.16.Ln, 87.19.lp, 89.75.Fb Submitted by Günther Gerisch [gerisch@biochem.mpg.de] -------------------------------------------------------------------------------- A low-affinity ground state conformation for the dynein microtubule binding domain. L. McNaughton, I. Tikhonenko, N. K. Banavali, D.M. LeMaster, and M. P. Koonce Wadsworth Center, Albany, NY J. Biol. Chem, in press Dynein interacts with microtubules through a dedicated binding domain that is dynamically controlled to achieve high or low affinity, depending on the state of nucleotide bound in a distant catalytic pocket. The active sites for microtubule binding and ATP hydrolysis communicate via conformational changes transduced through a ~10 nm length antiparallel coiled-coil stalk, which connects the binding domain to the roughly 300-kDa motor core. Recently, an X-ray structure of the murine cytoplasmic dynein microtubule binding domain (MTBD) in a weak-affinity conformation was published, containing a covalently constrained beta+ registry for the coiled-coil stalk segment (1). We here present an NMR analysis of the isolated MTBD from Dictyostelium discoideum that demonstrates the coiled-coil beta+ registry corresponds to the low energy conformation for this functional region of dynein. Addition of sequence encoding roughly half of the coiled-coil stalk proximal to the binding tip, results in a decreased affinity of the MTBD for microtubules. In contrast, addition of the complete coiled-coil sequence drives the MTBD to the conformationally unstable, high affinity binding state. These results suggest a thermodynamic coupling between conformational free energy differences in the alpha and beta+ registries of the coiled-coil stalk that acts as a switch between high and low affinity conformations of the MTBD. A balancing of opposing conformations in the stalk and MTBD enables potentially modest long-range interactions arising from ATP binding in the motor core to induce a relaxation of the MTBD into the stable low affinity state. Submitted by Michael Koonce [koonce@wadsworth.org] -------------------------------------------------------------------------------- Dictyostelium discoideum: A model system for ultrastructural analyses of cell motility and development M.P. Koonce and R.Gräf Wadsworth Center, Albany, NY and Department of Cell Biology, Institute for Biochemistry and Biology, University of Potsdam, Germany In press: Methods in Cell Biology. Dictyostelium occupies an interesting niche in the grand scheme of model organisms. On one hand, it is a compact, highly motile single cell that presents numerous opportunities to investigate the fundamental mechanisms of signal transduction, cell movement, and pathogen infection. However, upon starvation, individual cells enter a developmental pathway that involves cell aggregation, cell-cell adhesion, pattern formation, and differentiation. Thus, Dictyostelium is also well known as a basic model to study developmental processes. Electron microscopy (EM) has played a large role in both the unicellular and multicellular life stages; for example, providing image detail for structure/function relationships of cytoskeletal proteins, the deposition of cellulose fibrils in maturing spores, and the identification of intercellular junctional complexes. Powerful combinations of robust molecular genetic tools, high-resolution light microscopy and EM methods make this organism an attractive model for imaging dynamic cell processes. This methods chapter serves to highlight past and current EM approaches that have advanced our understanding of how cells and proteins function. Submitted by Michael Koonce [koonce@wadsworth.org] -------------------------------------------------------------------------------- Genetic control of lithium sensitivity and regulation of inositol biosynthetic genes. Jason King, Melanie Keim, Regina Teo, Karin E. Weening, Mridu Kapur, Karina McQuillan, Jonathan Ryves, Ben Rogers, Emma Dalton, Robin SB Williams and Adrian J. Harwood PLOSone Lithium (Li+) is a common treatment for bipolar mood disorder, a major psychiatric illness with a lifetime prevalence of more than 1%. Risk of bipolar disorder is heavily influenced by genetic predisposition, but is a complex genetic trait and to date, genetic studies have provided little insight into its molecular origins. An alternative approach is to investigate the genetics of Li+ sensitivity. Using the social amoeba Dictyostelium, we previously identified prolyl oligopeptidase (PO) as a modulator of Li+ sensitivity. In a link to the clinic, PO enzyme activity is altered in bipolar disorder patients. Further studies demonstrated that PO is a negative regulator of inositol(1,4,5)trisphosphate (IP3) synthesis, a Li+ sensitive intracellular signal. However, it was unclear how PO could influence either Li+ sensitivity or risk of bipolar disorder. Here we show that in both Dictyostelium and cultured human cells PO acts via Multiple Inositol Polyphosphate Phosphatase (Mipp1) to control gene expression. This reveals a novel, gene regulatory network that modulates inositol metabolism and Li+ sensitivity. Among its targets is the inositol monophosphatase gene IMPA2, which has also been associated with risk of bipolar disorder in some family studies, and our observations offer a cellular signalling pathway in which PO activity and IMPA2 gene expression converge. Submitted by Adrian Harwood [harwoodaj@cardiff.ac.uk] ============================================================== [End dictyNews, volume 34, number 11]