Actin binding protein, cross-linking

More than 50 proteins have been discovered in the cytosol of nonmuscle cells that bind to actin and affect the assembly and disassembly of actin filaments or the cross-linking of actin filaments with each other, with other filamentous components of the cytoskeleton, or with the plasma membrane. Collectively, these are known as actin-binding proteins (ABPs). Their mechanisms of actions are complex and are subject to regulation by specific binding affinities to actin and other molecules, cooperation or competition with other ABPs, local changes in the concentrations of ions in the cytosol, and physical forces (Way and Weeds, 1990). Classifications of ABPs have been proposed that are based on their site of binding to actin and on their molecular structure and function (Pollard and Cooper, 1986 Herrmann, 1989 Pollard et al., 1994). These include the following ... 

MAPs can be classified in a similar manner to actin-binding proteins. Like their actin-binding counterparts, MAPs can nucleate, cap, stabilize and cross-link microtubules. Several motor proteins have been identified in the last few years which are involved in force generation in a number of microtubule-based networks. As with actin-binding proteins, MAPs can be targeted, on a constitutive or transient basis, to certain regions of a cell. This allows specialized microtubule-based networks to be constructed and perform their tasks in specific locations. The properties and functions of some of these MAPs will be explored in the examples given below. 

Actin polymers form the thin filaments (also called microfilaments) in the cell that are organized into compact ordered bundles or loose network arrays by cross-linking proteins. Short actin filaments bind to the cross-linking protein spectrin to form the cortical actin skeleton network (see Fig. 10.6). In muscle cells, long actin fdaments combine with thick filaments, composed of the protein myosin, to produce muscle contraction. The assembly of G-actin subunits into polymers, bundling of fibers, and attachments of actin to spectrin and to the plasma membrane proteins and organelles, are mediated by a number of actin-binding proteins and G-proteins from the Rho family. 

The assembly of F-actin filaments into stable networks and dynamic bundles (essential for myosin-based motility) is mediated by cross-linking agents such as a-actinin or actin-binding proteins such as fascin, fimbrin, and filamin [71, 77]. Even in the absence of any cross-linking proteins, nanofibrils of F-actin can associate with each other to form gels [78, 79]. 

A large number of actin-binding proteins have been discovered from different cells. Their functions have been identified based on the scheme of helical polymerization. Some proteins bind to G-actin, inhibit the polymerization, and shift the G-F balance. Some bind to the end of F-actin and inhibit its growth. Some of them make nuclei for polymerization and break F-actin into fragments. Some proteins bind to the side of F-actin and change its interaction with other proteins. There are also proteins that make bundles or cross-links of F-actin and control the three-dimensional structure of the F-actin network. 

In cooperation with many kinds of actin-binding proteins, the G-F transformation of actin generates translational movement of a bacterial cell in a host cell, as in Figure 10 [46], Anchoring and nucleation proteins, depolymerizing proteins, and cross-linking proteins work to make possible a fast cycle of actin molecules from one end of F-actin to the other end, that is, tread-milling. This system has been artificially reconstructed in vitro [47]. 

The leucine zipper DNA-binding proteins, described in Chapter 10, are examples of globular proteins that use coiled coils to form both homo- and heterodimers. A variety of fibrous proteins also have heptad repeats in their sequences and use coiled coils to form oligomers, mainly dimers and trimers. Among these are myosin, fibrinogen, actin cross-linking proteins such as spectrin and dystrophin as well as the intermediate filament proteins keratin, vimentin, desmin, and neurofilament proteins. 

Proteins that cross-link actin filaments bind to their sides to produce bundles or three-dimensional networks (Otto, 1994). In microvilli, approximately 20 actin filaments of the core are cross-linked by villin (95 kD) and fimbrin (68 kD) in helical array to form a compact bundle (Figure 5). Filamin (2 x 250 kD) induces the formation of an actin network with gel formation. By immunofluorescence microscopy, this ABP is found in the ruffled, motile edge of cultured cells, where only actin filaments are abundant. 

Any of the cytoskeletal proteins that cross-link cytoskel-etal filaments into colinear arrays, such as the action of a-actinin in promoting the formation of actin stress fibers. Bundling proteins typically contain pairs of binding sites for attachment to cytoskeletal filaments. 

Dimeric S100P was found to bind and activate ezrin, a membrane/F-actin cross-linking protein (Koltzscher et al., 2003) possibly influencing cell morphology. 

Villin is an example of a bundling protein. Villin is found in the microvilli of, for example, intestinal brush border cells (Fig. 5-30). The microvilli greatly increase the surface area of the cells, which is essential for effective absorption to take place. Each microvillus extends about 2 p.m into the lumen of the gut and is supported by 20 or so actin filaments tightly bundled by villin (and other proteins) at regular intervals. In a feature common to many actin-based networks, all the filaments in the bundle are oriented with their barbed ends in the same direction, in this case toward the tip of the microvillus where they terminate. Cross-linking of the actin filaments to the plasma membrane occurs via a second protein from the myosin-1 family (a relative of the well-known contractile protein myosin-II). This protein binds its head domain to the sides of the filaments and embeds its tail domain into the membrane. 

Cross-linking proteins are placed into three gn>ups. Group I proteins have unique actin-binding domains Croup II have a 7.0(X. M V act in-binding domain and Group III have pairs of a 26,()(MI-MW actin-binding domain. 

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