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Fig. 2. Interconnection of dynamin-mediated vesicle fission with Arp2/3-complex-dependent F-actin nucleation triggered by N-WASP and syndapins. (A) Early in vesicle formation, the membrane is deeply invaginated and dynamin starts to concentrate at the vesicle neck, which is still wide. Syndapin oligomers associated with dynamin may help recruit and activate the Arp2/3 complex activator N-WASP (1). In this way, actin nucleation by the Arp2/3 complex can be linked to dynamin-mediated fission control (2). Actin filaments can be generated de novo (2) and as new branches from already existing actin fibres that may be part of the cortical cytoskeleton (3). It remains to be investigated whether syndapin-dynamin complexes form first in the cytosol (4), after dynamin has been recruited to the plasma membrane (5) or both. (B) Late in vesicle formation, the vesicle neck is constricted and the vesicle is subsequently pinched off and detached from the plasma membrane. Dynamin oligomers surrounding the neck could be a spatial and temporal cue for Arp2/3-complex-mediated F-actin nucleation. Syndapins and N-WASP serve as connecting elements that ensure that actin polymerization is restricted to the neck region. Such a restriction of actin build-up and a polarization of actin fibres in a manner that orientates the fast-growing plus ends towards the forming/moving vesicle provides force and ensures the directionality of vesicle movement away from the donor membrane. Growing plus ends of actin filaments are marked by ATP-loaded actin monomers, which are depicted in darker blue. PIP2, phosphatidylinositol (4,5)-bisphosphate.
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