Ultrastructure of protrusive actin filament arrays

Abstract

The actin cytoskeleton is the major force-generating machinery in the cell, which can produce pushing, pulling, and resistance forces. To accomplish these diverse functions, actin filaments, with help of numerous accessory proteins, form higher order ensembles, networks and bundles, adapted to specific tasks. Moreover, dynamic properties of the actin cytoskeleton allow a cell to constantly build, renew, and redesign actin structures according to its changing needs. High resolution architecture of actin filament arrays provides key information for understanding mechanisms of force generation. To generate pushing force, cells use coordinated polymerization of multiple actin filaments organized into branched (dendritic) networks or parallel bundles. This review summarizes our current knowledge of the structural organization of these two actin filament arrays.

Figure 1. Molecular architecture of protrusive actin filament arrays

(A) Dendritic networks in lamellipodia. (1) The Arp2/3 complex is coactivated at the plasma membrane by binding both a nucleation promoting factor (NPF) and a mother filament; (2,3) elongation of the newly nucleated branch, as well as of the mother filament, is transiently assisted by Ena/VASP (2) and/or formin (3) family proteins, which protect barbed ends from capping, recruit actin-profilin complexes, and mediate processive attachment of barbed ends to the plasma membrane. (4) During plasma membrane advance or retrograde flow, the Arp2/3 complex-containing actin filament branch becomes a part of the dendritic network and can be stabilized by cortactin. (5) After a period of elongation, some barbed ends lose elongation factors and become capped by capping protein; this process balances continuous nucleation of new filaments and is a part of filament length control in the dendritic network. Disassembly of the network is mediated by severing activity of ADF/cofilin and dissociation of branches (not depicted). (B) Parallel bundles in filopodia. Long actin filaments in the bundle are oriented with their barbed ends to the plasma membrane at the tip of the filopodium. Their elongation is assisted by formins and Ena/VASP proteins, as well as myosin X. Filaments are bundled by fascin and laterally attached to the plasma membrane by ERM proteins.

Figure 2. Functions of protrusive actin filament arrays

(A) Many cell types, such as fibroblasts, epithelial, endothelial, and immune cells employ dendritic networks and parallel bundle for different purposes. (1) Dendritic networks and parallel bundles at the leading edge drive membrane protrusion. (2) Dendritic networks induced by bacterial pathogens propel these bacteria throughout the bulk cytoplasm of a host cell; this motility is thought to mimic a normal membrane trafficking process. (3) During clathrin-mediated endocytosis, dendritic networks associate with clathrin-coated structures to promote their invagination, constriction and departure. (4) When bacterial comet tails become enclosed within a filopodium-like protrusion, the dendritic actin network appears to reorganize into a parallel bundle, although the continuing presence of a branched network at the actin-bacterium interface is a speculation at present. (5, lower part) Lamellipodial dendritic networks in contacting cells press against each other to bring their membranes into close proximity and facilitate cell-cell adhesion during adherens junction formation. (6, upper part) Retraction of an attached lamellipodium transforms its dendritic network into a parallel bundle in a base-to-tip direction, so that a mini-lamellipodium transiently remains at the tip of the bridge; the bundle subsequently recruits myosin II to exert tension on the forming adhesion and thus enhance its strength. (B) Neurons use protrusive actin arrays for similar purposes, but within neuron-specific strucutres. (7) Dendritic networks are present asymmetrically on both sides of an excitatory synapse, in the dendritic spine (left) and axonal presynaptic terminal (right), where they likely facilitate membrane juxtaposition; despite its narrow elongated shape, the spine neck also contains branched network. (8) Parallel bundles in filopodia are dominant protrusive structures in neuronal growth cones mediating elongation of axons and dendrites; however, dendritic networks are also present between growth cone filopodia and participate in filopodia initiation.