Abstract
Filopodia are finger-like structures containing parallel bundles of actin filaments that are central to eukaryotic cell motility in a variety of contexts. K. Lee et al. (p. 1341) reconstituted filopodia-like structures that grow from supported lipid bilayers to explore filopodia assembly. A structural transition from actin networks to parallel bundles was observed that mediated self-assembly of filopodia-tip complexes on the membranes. Actin bundle structures formed on lipid bilayers give insight into formation of the finger-like structures involved in cell migration. Filopodia are finger-like protrusive structures, containing actin bundles. By incubating frog egg extracts with supported lipid bilayers containing phosphatidylinositol 4,5 bisphosphate, we have reconstituted the assembly of filopodia-like structures (FLSs). The actin assembles into parallel bundles, and known filopodial components localize to the tip and shaft. The filopodia tip complexes self-organize—they are not templated by preexisting membrane microdomains. The F-BAR domain protein toca-1 recruits N-WASP, followed by the Arp2/3 complex and actin. Elongation proteins, Diaphanous-related formin, VASP, and fascin are recruited subsequently. Although the Arp2/3 complex is required for FLS initiation, it is not essential for elongation, which involves formins. We propose that filopodia form via clustering of Arp2/3 complex activators, self-assembly of filopodial tip complexes on the membrane, and outgrowth of actin bundles.