dictyNews Electronic Edition Volume 43, number 14 July 7, 2017 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 ========= Drinking problems: mechanisms of macropinosome formation and maturation. Catherine M Buckley and Jason S King Department of Biomedical Sciences, University of Sheffield, UK. FEBS Journal 2017 May 24 http://onlinelibrary.wiley.com/doi/10.1111/febs.14115/full Macropinocytosis is a mechanism for the nonspecific bulk uptake and internalisation of extracellular fluid. This plays specific and distinct roles in diverse cell types such as macrophages, dendritic cells and neurons, by allowing cells to sample their environment, extract extracellular nutrients and regulate plasma membrane turnover. Macropinocytosis has recently been implicated in several diseases including cancer, neurodegenerative diseases and atherosclerosis. Uptake by macropinocytosis is also exploited by several intracellular pathogens to gain entry into host cells. Both capturing and subsequently processing large volumes of extracellular fluid poses a number of unique challenges for the cell. Macropinosome formation requires coordinated three-dimensional manipulation of the cytoskeleton to form shaped protrusions able to entrap extracellular fluid. The following maturation of these large vesicles then involves a complex series of membrane rearrangements to shrink and concentrate their contents, while delivering components required for digestion and recycling. Recognition of the diverse importance of macropinocytosis in physiology and disease has prompted a number of recent studies. In this article, we summarise advances in our understanding of both macropinosome formation and maturation, and seek to highlight the important unanswered questions. submitted by: Jason King [jason.king@sheffield.ac.uk] ——————————————————————————————————————— Distinct VASP tetramers synergize in the processive elongation of individual actin filaments from clustered arrays Stefan Brühmann1, Dmitry S. Ushakov1, Moritz Winterhoff1, Richard B. Dickinson2, Ute Curth1, and Jan Faix1 1) Institute for Biophysical Chemistry, Hannover Medical School, 30625 Hannover, Germany; 2) Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 PNAS, in press Ena/VASP proteins act as actin polymerases that drive the processive elongation of filament barbed ends in membrane protrusions or at the surface of bacterial pathogens. Based on previous analyses of fast and slow elongating VASP proteins by in vitro total internal reflection fluorescence microscopy (TIRFM) and kinetic and thermodynamic measurements, we established a kinetic model of Ena/VASP-mediated actin filament elongation. At steady state, it entails that tetrameric VASP uses one of its arms to processively track growing filament barbed ends while three G-actin–binding sites (GABs) on other arms are available to recruit and deliver monomers to the filament tip, suggesting that VASP operates as a single tetramer in solution or when clustered on a surface, albeit processivity and resistance toward capping protein (CP) differ dramatically between both conditions. Here, we tested the model by variation of the oligomerization state and by increase of the number of GABs on individual polypeptide chains. In excellent agreement with model predictions, we show that in solution the rates of filament elongation directly correlate with the number of free GABs. Strikingly, however, irrespective of the oligomerization state or presence of additional GABs, filament elongation on a surface invariably proceeded with the same rate as with the VASP tetramer, demonstrating that adjacent VASP molecules synergize in the elongation of a single filament. Additionally, we reveal that actin ATP hydrolysis is not required for VASP-mediated filament assembly. Finally, we show evidence for the requirement of VASP to form tetramers and provide an amended model of processive VASPmediated actin assembly in clustered arrays. submitted by: Jan Faix [faix.jan@mh-hannover.de] ============================================================== [End dictyNews, volume 43, number 14]