actin related

 

Figure 16-38. Actin arrays in a cell.

Figure 16-30. Effects of thymosin and profilin on actin polymerization. An actin monomer bound to thymosin is sterically prevented from binding to and elongating the plus end of an actin filament. An actin monomer bound to profilin, on the other hand, is capable of elongating a filament. Thymosin and profilin cannot both bind to a single actin monomer at the same time. In a cell in which most of the actin monomer is bound to thymosin, the activation of a small amount of profilin can produce rapid filament assembly. As indicated, profilin binds to actin monomers that are transiently released from the thymosin-bound monomer pool, shuttles them onto the plus ends of actin filaments, and is then released and recycled for further rounds of filament elongation.

 

Figure 16-29. Profilin bound to an actin monomer. The profilin protein molecule is shown in blue, and the actin in red. ATP is shown in green. Profilin binds to the face of actin opposite the ATP-binding cleft. This profilin-actin heterodimer can therefore bind to and elongate the plus end of an actin filament, but it is sterically prevented from binding to the minus end. (Courtesy of Michael Rozycki and Clarence E. Schutt.)

 

Figure 16-35. Filament capping and its effects on filament dynamics. A population of uncapped filaments adds and loses subunits at both the plus and minus ends, resulting in rapid growth or shrinkage, depending on the concentration of available free monomers (green line). In the presence of a protein that caps the plus end (red line), only the minus end is able to add or lose subunits; consequently, filament growth will be slower at all monomer concentrations above the critical concentration, and filament shrinkage will be slower at all monomer concentrations below the critical concentration. Plus-end capping of this type is widely used for actin filaments but not for microtubules.

 

Figure 16-28. Nucleation and actin web formation by the ARP complex. (A)The structures of Arp2 and Arp3, compared to the structure of actin. Although the face of the molecule equivalent to the plus end (top) in both Arp2 and Arp3 is very similar to the plus end of actin itself, differences on the sides and minus end (bottom) prevent these actin-related proteins from forming filaments on their own or coassembling into filaments with actin. (B) A model for actin filament nucleation by the ARP complex. Arp2 and Arp3 may be held by their accessory proteins in an orientation that resembles the plus end of an actin filament. Actin subunits can then assemble onto this structure, bypassing the rate-limiting step of filament nucleation (see Figure 16-5). (C) The ARP complex nucleates filaments more efficiently when it is bound to the side of a preexisting actin filament. The result is a filament branch that grows at a 70° angle relative to the original filament. Repeated rounds of branching nucleation result in a treelike web of actin filaments. (D) Electron micrographs of branched actin filaments formed by mixing purified actin subunits with purified ARP complexes. (D, courtesy of R.D. Mullins et al., Proc. Natl. Acad. Sci. USA 95:6181–6186, 1998. © National Academy of Sciences.)