title: Actin branching in the initiation and maintenance of lamellipodia.
abstract: 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 μm. 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.
authors.affiliation: Institute of Molecular Biotechnology, Vienna, Austria.
authors.forename: Marlene
authors.initials: M
authors.lastname: Vinzenz
authors.forename: Maria
authors.initials: M
authors.lastname: Nemethova
authors.forename: Florian
authors.initials: F
authors.lastname: Schur
authors.forename: Jan
authors.initials: J
authors.lastname: Mueller
authors.forename: Akihiro
authors.initials: A
authors.lastname: Narita
authors.forename: Edit
authors.initials: E
authors.lastname: Urban
authors.forename: Christoph
authors.initials: C
authors.lastname: Winkler
authors.forename: Christian
authors.initials: C
authors.lastname: Schmeiser
authors.forename: Stefan A
authors.initials: SA
authors.lastname: Koestler
authors.forename: Klemens
authors.initials: K
authors.lastname: Rottner
authors.forename: Guenter P
authors.initials: GP
authors.lastname: Resch
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Maeda
authors.forename: J Victor
authors.initials: JV
authors.lastname: Small
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000199:Actins / Q000378:metabolism*; D000818:Animals; D042541:Intracellular Space / Q000378:metabolism; D008546:Melanoma, Experimental; D051379:Mice; D008954:Models, Biological; D041681:NIH 3T3 Cells; D011554:Pseudopodia / Q000378:metabolism* / Q000648:ultrastructure; D020744:rac GTP-Binding Proteins / Q000378:metabolism
chemical_list: D000199:Actins; D020744:rac GTP-Binding Proteins