Chemical compounds that stabilize or destabilize actin filaments are important tools in studies of the filaments’ dynamic behavior and function in cells. The cytochalasins are fungal products that prevent actin polymerization by binding to the plus end of actin filaments. Latrunculin prevents actin polymerization by binding to actin subunits. The phalloidins are toxins isolated from the Amanita mushroom that bind tightly all along the side of actin filaments and stabilize them against depolymerization. All of these compounds cause dramatic changes in the actin cytoskeleton and are toxic to cells, indicating that the function of actin filaments depends on a dynamic equilibrium between filaments and actin monomers (Table 16–1).
In a test tube, polymerization of actin is controlled simply by its concentration, as described above, and by pH and the concentrations of salts and ATP. Within a cell, however, actin behavior is also regulated by numerous accessory proteins that bind actin monomers or filaments (summarized in Panel 16–3). At steady state the filament grows, elongation is faster than hydrolysis at the plus end in this example, and the terminal subunits at this end are therefore always in the T form. However, hydrolysis is faster than elongation at the minus end, and so terminal subunits at this end are in the D form. (B) Treadmilling occurs at intermediate concentrations of free subunits. The critical concentration for polymerization on a filament end in the T form is lower than for a filament end in the D form. If the actual subunit concentration is somewhere between these two values, the plus end grows while the minus end shrinks, resulting in treadmilling.
nucleates assembly associated with the growing to form a web and remains plus end associated with the minus end binds subunits, prevents assembly proflin binds subunits, – + binds ADP-actin flaments, binds to plus end flament bundling, cross-linking, and attachment to membranes
Some of the major accessory proteins of the actin cytoskeleton. Except for the myosin motor proteins, an example of each major type is shown. Each of these is discussed in the text. However, most cells contain more than a hundred different actin-binding proteins, and it is likely that there are important types of actin-associated proteins that are not yet recognized.
in vitro, when the monomer concentration is 0.2 μM, filament half-life, a measure of how long an individual actin monomer spends in a filament as it treadmills, is approximately 30 minutes. In a non-muscle vertebrate cell, actin half-life in filaments is only 30 seconds, demonstrating that cellular factors modify the dynamic behavior of actin filaments. Actin-binding proteins dramatically alter actin filament dynamics and organization through spatial and temporal control of monomer availability, filament nucleation, elongation, and depolymerization. In the following sections, we describe the ways in which these accessory proteins modify actin function in the cell.
In most non-muscle vertebrate cells, approximately 50% of the actin is in filaments and 50% is soluble—and yet the soluble monomer concentration is 50–200 μM, well above the critical concentration. Why does so little of the actin polymerize into filaments? The reason is that the cell contains proteins that bind to the actin monomers and make polymerization much less favorable (an action similar to that of the drug latrunculin). A small protein called thymosin is the most abundant of these proteins. Actin monomers bound to thymosin are in a locked state, where they cannot associate with either the plus or minus ends of actin filaments and can neither hydrolyze nor exchange their bound nucleotide.