Actin-spectrin network

Actin is plentiful in the cytoplasm of many animal cells, comprising 5% or more of the total protein present. Four principal patterns of arrangement can be recognized (1) three-dimensional networks of filaments, (2) bundles of parallel filaments with the same polarity, (3) submembranous actin-spectrin (fodrin) networks, and (4) bundles of parallel filaments with alternating polarities. 

This spectrin network further binds to actin microfilaments and to numerous other ligands. These associations are probably dynamic. For example, phosphorylation of ankyrin can alter its affinity for spectrin. The functions of the multiple protein-interaction domains of both spectrin and ankyrin have been as yet only partially defined (see Ch. 8). 

The integral membrane proteins glycophorin and anion exchange protein are components in a network of linkages that connect the plasma membrane to structural elements of the cytoskeleton (e.g., actin, spectrin, protein 4.1, and ankyrin). 

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. 

Figured. Diagrammatic representation of the red blood cell cytoskeletal-plasma membrane complex. Spectrin is made up of many homologous triple-helical segments joined by nonhelical regions (Speicher and Marchesi, 1984). Spectrin and actin require accessory proteins to form a membrane-associated network. (This diagram is constructed from data previously published for example, see Stryer, 1988 Davies and Lux, 1989 Bennett and Gilligan, 1993).

The function of spectrin superfamily proteins is particularly evident when taken in context of their cellular localization. They often form flexible links or structures that allow interactions with the cellular cyto-skeletal architecture and the membrane. In both spectrin and dystrophin, such a function is performed, but the spectrin repeats of these molecules are also able to interact with actin and contribute to binding. A portion of the dystrophin rod domain that spans residues 11-17 contains a number of basic repeats that allow a lateral interaction with filamentous actin (Rybakova et al., 2002). The homologous utrophin can also interact laterally with actin. This interaction is distinct from that of dystrophin, as the utrophin rod domain lacks the basic repeat cluster and associates with actin via the first ten spectrin repeats (Rybakova et al., 2002). /3-Spectrin also exhibits an extended contact with actin via the first spectrin repeat. In this situation, it was found that the extended contact increased the association of the adjacent ABD with actin (Li and Bennett, 1996). In conjunction with this interaction, it has been found that the second repeat is also required for maximal interaction with adducin (Li and Bennett, 1996), a protein localized at the spectrin-actin junction that is believed to contribute to the assembly of this structure in the membrane skeletal network (Gardner and Bennett, 1987). In the erythrocyte cytoskeletal lattice, /3-spectrin interacts with ankyrin, which in turn binds to the cytoplasmic domain of the membrane-associated anion exchanger. This indirect link to the cellular membrane occurs via repeat 15 of /3-spectrin (Kennedy et al., 1991) and is largely responsible for the attachment of the spectrin-actin network to the erythrocyte membrane (reviewed in Bennett and Baines, 2001). A much larger number of direct links to transmembrane proteins have been determined for the spectrin repeats of o-actinin (reviewed in Djinovic-Carugo et al, 2002). 

In addition to organelles, the cell cytoplasm contains actin filaments that make up the cellular cytoskeleton that controls shape. Myosin and a-actinin are also found in the cytoplasm and are believed to be involved in cell contraction. Other filaments including intermediate filaments, tubulin, calmodulin, and spectrin form networks within the cytoplasm that modify cell and organelle mobility and shape. 

Cortical spectrin-actin networks are attached to the cell membrane by bivalent membrane-microfilament binding proteins such as ankyrin and band 4.1 (see Figure 5-31). 

The q toskeleton of eukaryotic cells is generally considered to be a meshwork of protein filaments that spans the space between the nucleus and the plasma membrane. In many cell types, the three-dimensional (3D) composite network of actin filaments, microtubules (MTs), and intermediate filaments (IPs) in the cytoplasm interfaces with two-dimensional networks composed largely of spectrins that line the plasma membrane and nuclear lamins that line the inner surface of the nuclear membrane. A few eukaryotic cell types contain an entirely different cytoskeleton that powers their locomotion and which is constmcted from the cationic major sperm protein instead of actin. The three cytoskeletal proteins, acdn, tubulin, and IF subunits, constitute a significant fraction of... 

The peripheral proteins have been identified as follows (bands are numbered from the top of the SDS-polyacrylamide gel pattern band 1 is therefore the slowest and has the highest apparent molecular mass). Bands 1 and 2 dimeric and monomeric form of spectrin, which forms a network and stabilizes and controls the shape of the erythrotyte by interacting with other proteins. Band 4.1 a protein that links spectrin to the membrane. Band S actin. Band 6 gly-ceraldehyde 3-phosphate dehydrogenase. 

The study of tethered polymer chains is an area which has received increasing attention in recent years. These are systems in which one or both ends of the chain are constrained in their motion because they are attached to a d dimensional surface. This surface could be a point or small central core (d = 0) as in the case of a many-arm star polymer, a line (d = 1) as in the case of a comb polymer, or a flat surface (d = 2) as in the case of a polymer brush. Polymers attached to themselves to form a polymer network or a tethered membrane are also examples of tethered chain systems. An interesting example of a tethered membrane is the spectrin/actin membrane skeleton of the red blood cell skeleton. A schematic illustration of these four examples of tethered chain is shown in Fig. 9.1. Additional interest in tethered chains is due to their technological applications in colloidal stabilization and lubrication. ... 

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