dictyNews Electronic Edition Volume 30, number 11 March 28, 2008 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 ========= Filopodia formation induced by active mDia2/Drf3 1Jennifer Block, 2Theresia E.B. Stradal , 2Jan Haenisch, 3Robert Geffers, 4Stefan A. Koestler, 4Edit Urban, 4J. Victor Small, 1,*Klemens Rottner, and 5,*Jan Faix 1Cytoskeleton Dynamics Group, 2Signalling and Motility Group, 3Mucosal Immunity Group, Helmholtz Centre for Infection Research (HZI), Inhoffen Strasse 7, D-38124 Braunschweig, Germany; 4Institute of Molecular Biotechnology, Austrian Academy of Sciences, Dr. Bohr Gasse 3, A-1030 Vienna, Austria; 5Institute for Biophysical Chemistry, Hannover Medical School, Carl-Neubergstr. 1, D-30623 Hannover, Germany. *Corresponding authors Filopodia are rod-shaped cell surface protrusions composed of a parallel bundle of actin filaments. Since filopodia frequently emanate from lamellipodia, it has been proposed that they form exclusively by the convergence and elongation of actin filaments generated in lamellipodia networks. However, filopodia form without Arp2/3-complex, which is essential for lamellipodia formation, indicating that actin filaments in filopodia may be generated by other nucleators. Here we analysed the effects of ectopic expression of GFP-tagged full length or a constitutively active variant of the human formin mDia2/Drf3. In contrast to the full-length molecule, which did not affect cell behaviour and was entirely cytosolic, active Drf3 lacking the C-terminal regulatory region (Drf3DelDAD) induced the formation of filopodia and accumulated at their tips. Low expression of Drf3DelDAD induced rod-shaped or tapered filopodia, whereas overexpression resulted in multiple, club-shaped filopodia. The clubs were filled with densely bundled actin filaments, whose number but not packing density decreased further away from the tip. Interestingly, clubs frequently increased in width after protrusion beyond the cell periphery, which correlated with increased amounts of Drf3DelDAD at their tips. These data suggest Drf3-induced filopodia form and extend by de novo nucleation of actin filaments instead of convergent elongation. Finally, Drf3DelDAD also induced the formation of unusual, lamellipodia-like structures, which contained both lamellipodial markers and the prominent filopodial protein fascin. Microarray analyses revealed highly variable Drf3 expression levels in different commonly used cell lines, reflecting the need for more detailed analyses of the functions of distinct formins in actin cytoskeleton turnover and different cell types. Submitted by: Jan Faix [faix@bpc.mh-hannover.de] -------------------------------------------------------------------------------- Interactions between myosin and actin crosslinkers control cytokinesis contractility dynamics and mechanics. Reichl E.M., Ren Y., Morphew M.K., Delannoy M., Effler J.C., Girard K.D., Divi S., Iglesias P.A., Kuo S.C. and Robinson D.N. Curr. Biol. 2008; 18(7), in press. Introduction: Contractile networks are fundamental to many cellular functions, particularly cytokinesis and cell motility. Contractile networks depend on myosin-II mechanochemistry to generate sliding force on the actin polymers. However, to be contractile, the networks must also be crosslinked by crosslinking proteins and to change the shape of the cell, the network must be linked to the plasma membrane. Discerning how this integrated network operates is essential for understanding cytokinesis contractility and shape control. Here, we analyzed the cytoskeletal network that drives furrow ingression in Dictyostelium. Results: We establish that the actin polymers are assembled into a meshwork and that myosin-II does not assemble into a discrete ring in the Dictyostelium cleavage furrow of adherent cells. We show that myosin-II generates regional mechanics by increasing cleavage furrow stiffness and slows furrow ingression during late cytokinesis as compared to myoII nulls. Actin crosslinkers dynacortin and fimbrin similarly slow furrow ingression and contribute to cell mechanics in a myosin-II-dependent manner. Using FRAP, we show that the actin crosslinkers have slower kinetics in the cleavage furrow cortex than in the pole, that their kinetics differ between wild type and myoII null cells, and that the protein dynamics of each crosslinker correlate with its impact on cortical mechanics. Conclusions: These observations suggest that myosin-II along with actin crosslinkers establish local cortical tension and elasticity, allowing for contractility independent of a circumferential cytoskeletal array. Furthermore, myosin-II and actin crosslinkers may influence each other as they modulate the dynamics and mechanics of cell shape change. Submitted by: Doug Robinson [dnr@jhmi.edu] ============================================================== [End dictyNews, volume 30, number 11]