Journal
JOURNAL OF CELL SCIENCE
Volume 125, Issue 11, Pages 2775-2785Publisher
COMPANY BIOLOGISTS LTD
DOI: 10.1242/jcs.107623
Keywords
Actin; arp2/3 complex; Cytoskeleton; Electron tomography; Lamellipodia
Categories
Funding
- Austrian Science Fund [FWF I516-B09, FWF P21292-B09]
- Vienna Science and Technology Fund [WWTF] [MA 09-004]
- Technology Agency of the City of Vienna [VSOE, CMCN]
- Deutsche Forschungsgemeinschaft [RO 2414/1-2]
- Daiko research foundation [9134]
- Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government [20227008, 22770145]
- Grants-in-Aid for Scientific Research [22770145, 20227008] Funding Source: KAKEN
- Austrian Science Fund (FWF) [P 21292, I 516] Funding Source: researchfish
- Austrian Science Fund (FWF) [I516, P21292] Funding Source: Austrian Science Fund (FWF)
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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 mu m. 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.
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