C. Jacobs-Wagner, Cell 115:705–713, 2003. With permission from Elsevier.) accessory proteins cross-link and bundle the filaments together, making large-Figure 16–11 The structures of an actin monomer and actin filament. (A) The scale actin structures that are much more rigid than an individual actin filament.
ATP or ADP) bound in a deep cleft in the Nucleation Is the Rate-Limiting Step in the Formation of Actin center of the molecule. (B) Arrangement of two protofilaments, held together by lateral contacts, which wind around each
The regulation of actin filament formation is an important mechanism by which other as two parallel strands of a helix, cells control their shape and movement. Small oligomers of actin subunits can with a twist repeating every 37 nm. All the assemble spontaneously, but they are unstable and disassemble readily because subunits within the filament have the same each monomer is bound to only one or two other monomers. For a new actin fila-orientation. (C) Electron micrograph of ment to form, subunits must assemble into an initial aggregate, or nucleus, that is negatively stained actin filament.
(C, courtesy of Roger Craig.) stabilized by multiple subunit–subunit contacts and can then elongate rapidly by addition of more subunits. This process is called filament nucleation.
Many features of actin nucleation and polymerization have been studied with purified actin in a test tube (Figure 16–13). The instability of smaller actin aggregates creates a kinetic barrier to nucleation. When polymerization is initiated, this barrier results in a lag phase during which no filaments are observed. During this lag phase, however, a few of the small, unstable aggregates succeed in making the transition to a more stable form that resembles an actin filament. This leads to a 0.5 µm
Figure 16–12 Structural polarity of the actin filament. (A) This electron micrograph shows an actin filament polymerized from a short actin filament seed that was decorated with myosin motor domains, resulting in an arrowhead pattern. The filament has grown much faster at the barbed (plus) end than at the pointed (minus) end. (B) Enlarged image and model showing the arrowhead pattern. (A, courtesy of Tom Pollard; B, adapted  from M. Whittaker, B.O. Carragher and  K.A. Milligan, Ultramicro. 54:245–260, 1995.)
Figure 16–13 The time course of actin polymerization in a test tube. (A) Polymerization of pure actin subunits into filaments occurs after a lag phase. (B) Polymerization occurs more rapidly in the presence of preformed fragments of actin filaments, which act as nuclei for filament growth. As indicated, the % free subunits after polymerization reflects the critical concentration (Cc), at which there is no net change in polymer. Actin polymerization is often studied by observing the change in the light emission from a fluorescent probe, called pyrene, that has been covalently attached to the actin. Pyrene-actin fluoresces more brightly when it is incorporated into actin filaments.