From Branched Networks of Actin Filaments to Bundles

Abstract

Cross-linking proteins can mediate the emergence of rigid bundles from a dense branched network of actin filaments. To enable their binding, the filaments must first bend towards each other. We derive an explicit criterion for the onset of bundling, in terms of the initial length of filaments L, their spacing b, and cross-linker concentration f, reflecting the balance between bending and binding energies. Our model system contains actin, the branching complex Arp2/3 and the bundling protein fascin. In the first distinct stage, during which only actin and Arp2/3 are active, an entangled aster-like mesh of actin filaments is formed. Tens of seconds later, when filaments at the aster periphery are long and barely branched, a sharp transition takes place into a star-like structure, marking the onset of bundling. Now fascin and actin govern bundle growth; Arp2/3 plays no role. Using kinetic Monte Carlo simulations we calculate the temporal evolution of b and L, and predict the onset of bundling as a function of f. Our predictions are in good qualitative agreement with several new experiments that are reported herein and demonstrate how f controls the aster-star transition and bundle length. We also present two models for aster growth corresponding to different experimental realizations. The first treats filament and bundle association as an irreversible sequence of elongation-association steps. The second, applicable for low f, treats bundling as a reversible self-assembly process, where the optimal bundle size is dictated by the balance between surface and bending energies. Finally, we discuss the relevance of our conclusions for the lamellipodium to filopodia transition in living cells, noting that bundles are more likely nucleated by "tip complex" cross-linkers (e.g. mDia2 or Ena/VASP), whereas fascin is mainly involved in bundle maintenance.

The cytoskeletal mechanisms of cell–cell junction formation in endothelial cells

“…Accordingly, accumulation of fascin in bridge bundles progresses not in the tip-to-base direction, as in free-edge filopodia (Svitkina et al , 2003), but in a shaft-to-tip manner. Because fascin is believed to be recruited to preformed parallel bundles (Brill-Karniely et al , 2009; Courson and Rock, 2010), the mode of fascin accumulation in bridges supports an idea of bundle assembly in a proximal-to-distal direction. Of interest, fascin arrival to bridges occurs in discontinuous bursts, suggesting a similar mode of bundle formation.…”

Assembly kinetics determine the architecture of α-actinin crosslinked F-actin networks

“…Because previous experiments only assessed the architecture of reconstituted actin networks at steady state, these kinetics effects have not been deeply considered despite a long history of studying the morphology of cross-linked F-actin networks 3–6 . The majority of efforts to understand the morphology of cross-linked actin networks have focused on the role of cross-linker mediated aggregation of filaments with constant length 7,17,23 and the energetic or kinetic constraints of bundling filaments within meshwork 24 .…”

Filopodia initiation

“…Moreover, mathematical modeling of bundle formation in this system supported the convergent elongation mechanism, rather than the tip nucleation model. 52 A strong support for the convergent elongation model of filopodia initiation has been recently provided by in vitro reconstitution of filopodia-like structures self-assembling on supported PI(4,5)P2-containing lipid bilayers in the presence of cytoplasmic extracts. 53 Timeresolved recruitment of individual molecules to filopodia-like structures during their initiation in this system revealed that an F-BAR domain protein Toca-1 is first to be recruited to lipid bilayers through interaction with PI(4,5)P2.…”