Ultrastructure of the actin cytoskeleton

Abstract

The actin cytoskeleton is the primary force-generating machinery in the cell, which can produce pushing (protrusive) forces using energy of actin polymerization and pulling (contractile) forces via sliding of bipolar filaments of myosin II along actin filaments, as well as perform other key functions. These functions are essential for whole cell migration, cell interaction with the environment, mechanical properties of the cell surface and other key aspects of cell physiology. The actin cytoskeleton is a highly complex and dynamic system of actin filaments organized into various superstructures by multiple accessory proteins. High resolution architecture of functionally distinct actin arrays provides key clues for understanding actin cytoskeleton functions. This review summarizes recent advance in our understanding of the actin cytoskeleton ultrastructure.

Figure 1

Load-dependent changes in geometry of branched actin networks. (a) Two major branched network geometries characterized by either ±35° (“slingshot”) or −70° / 0° / +70° (“trident”) angle distribution. (b) Geometry of branched networks depends on resistive load. Conventional slingshot geometry exists under intermediate loads (middle), such as during steady state cell migration. Under increased resistance (left), branched networks switch to the trident geometry with higher filament density and more barbed ends. Under reduced load (right), branched networks switch to the trident geometry with lower density and predominant ~0° angle.

Figure 2

Roles of actin filament nucleators and elongators in lamellipodia. Both Arp2/3 complex and formins can nucleate lamellipodial filaments. Filament elongation can be promoted by the same or different formins, and by Ena/VASP proteins. NMII bipolar filaments reorganize branched networks into various bundles with potential preference for formin-polymerized filaments.

Figure 3

Formation of stress fibers from random networks by NMII activity. (a) NMII filaments form clusters within random actin networks in the cell lamella. Cluster formation occurs through spontaneous nucleation of NMII filaments followed by their local amplification. In clusters, NMII filaments interact with each other at the ends. (b) NMII filaments reorganize isotropic actin networks into stress fibers by moving along and pulling on actin filaments. Simultaneously, NMII filaments become aligned to form stacks, which can be separated by gaps of variable lengths. (c) Actin filaments in stress fibers have mixed polarity. Their barbed and pointed ends are preferentially enriched at NMII stacks and inter-NMII gaps, respectively.