Using correlated live-cell imaging and electron tomography we found that actin branch junctions in protruding and treadmilling lamellipodia are not concentrated at the front as previously supposed, but link actin filament subsets in which there is a continuum of distances from a junction to the filament plus ends, for up to at least 1 mm. When branch sites were observed closely spaced on the same filament their separation was commonly a multiple of the actin helical repeat of 36 nm. Image averaging of branch junctions in the tomograms yielded a model for the in vivo branch at 2.9 nm resolution, which was comparable with that derived for the in vitro actin- Arp2/3 complex. Lamellipodium initiation was monitored in an intracellular wound-healing model and was found to involve branching from the sides of actin filaments oriented parallel to the plasmalemma. Many filament plus ends, presumably capped, terminated behind the lamellipodium tip and localized on the dorsal and ventral surfaces of the actin network. These findings reveal how branching events initiate and maintain a network of actin filaments of variable length, and provide the first structural model of the branch junction in vivo. A possible role of filament capping in generating the lamellipodium leaflet is discussed and a mathematical model of protrusion is also presented.
Journal of Cell Science
This work was supported by the Austrian Science Fund [projects FWF I516-B09 and FWF P21292-B09 to J.V.S.]; the Vienna Science and Technology Fund [WWTF-grant numbers MA 09-004 to J.V.S. and C.S], ZIT - The Technology Agency of the City of Vienna [VSOE, CMCN to J.V.S. and G.P.R.]; the Deutsche Forschungsgemeinschaft [grant number RO 2414/1-2 to K.R.]; the Daiko research foundation [grant number 9134 to A.N.]; and a Grant-in-Aid for Scientific Research [S, grant number 20227008 to Y.M.] and a Grant-in-Aid for Young Scientists [B, grant number 22770145 to A.N.] (B) from The Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. Deposited in PMC for immediate release. We thank Tibor Kulcsar for assistance with graphics.
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Vinzenz M, Nemethova M, Schur FK, et al. Actin branching in the initiation and maintenance of lamellipodia. Journal of Cell Science. 2012;125(11):2775-2785. doi:10.1242/jcs.107623
Vinzenz, M., Nemethova, M., Schur, F. K., Mueller, J., Narita, A., Urban, E., … Small, J. (2012). Actin branching in the initiation and maintenance of lamellipodia. Journal of Cell Science, 125(11), 2775–2785. https://doi.org/10.1242/jcs.107623
Vinzenz, Marlene, Maria Nemethova, Florian KM Schur, Jan Mueller, Akihiro Narita, Edit Urban, Christoph Winkler, et al. “Actin Branching in the Initiation and Maintenance of Lamellipodia.” Journal of Cell Science 125, no. 11 (2012): 2775–85. https://doi.org/10.1242/jcs.107623.
M. Vinzenz et al., “Actin branching in the initiation and maintenance of lamellipodia,” Journal of Cell Science, vol. 125, no. 11, pp. 2775–2785, 2012.
Vinzenz M, Nemethova M, Schur FK, Mueller J, Narita A, Urban E, Winkler C, Schmeiser C, Koestler S, Rottner K, Resch G, Maéda Y, Small J. 2012. Actin branching in the initiation and maintenance of lamellipodia. Journal of Cell Science. 125(11), 2775–2785.
Vinzenz, Marlene, et al. “Actin Branching in the Initiation and Maintenance of Lamellipodia.” Journal of Cell Science, vol. 125, no. 11, Company of Biologists, 2012, pp. 2775–85, doi:10.1242/jcs.107623.
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