Spectrin-actin interaction

Protein 4.1, a globular protein, binds tightly to the tail end of spectrin, near the actin-binding site of the latter, and thus is part of a protein 4.1-spectrin-actin ternary complex. Protein 4.1 also binds to the integral proteins, glycophorins A and C, thereby attaching the ternary complex to the membrane. In addition, protein 4.1 may interact with certain membrane phospholipids, thus connecting the lipid bilayer to the cytoskeleton. 

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). 

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). 

A primary function of the SH3 domains is to form fimctional oligomeric complexes at defined subcellular sites, frequently in cooperation with other modular domains. SH3 domains are foimd in many proteins associated with the cytoskeleton or with the plasma membrane. Examples are the actin binding protein a-spectrin and myosin lb. Furthermore, SH3 interactions are involved in signal transduction in the Ras pathway (see Chapter 9). 

More recent work suggested that one of four 4.1 proteins—4.1R—may associate with neurofilament proteins in forebrain postsynaptic densities, thus regulating the associated spectrin-rich cortex (Scott et al, 2001). Blot overlay analyses demonstrated that, in addition to spectrin and actin, postsynaptic density polypeptides included NF-L and o-internexin as interacting partners for 4.1R. Collectively, these studies emphasize that common themes are used in different cell types to both strengthen plasma membrane domains enriched in actin and IF polypeptides and to coordinate these sites with cytoplasmic architecture. 

N-terminal actin-binding domains and in the spectrin repeats that form the rod domains (Davison and Critchley, 1988). The spectrin repeats are found in distinct multiples in each protein, resulting in a characteristic actin crosslinking distance. a-Actinin contains four repeats, /3-spectrin contains 17, a-spectrin contains 20, and dystrophin contains 24. The sequences of some spectrin repeats of a- and /3-spectrin are similar in many ways to the four repeats present in a-actinin (Dubreuil, 1991). Within the cell, a-actinin and spectrin dimerize, although the spectrins interact further to generate a functional tetramer (Fig. 1). Most notable is that the ends of the native spectrin tetramer involved in the dimerization event show remarkable similarity to the rod domain repeats of a-actinin that also mediate dimer formation. 

The spectrin family of proteins, depending on the particular function, has numerous smaller motifs and binding sites for interaction with other proteins. These regions are important, as they are major protein-protein or protein-membrane interaction modules that bind to F-actin, proline-containing ligands, and/or phospholipids. Spectrin and dystrophin/utro-phin have all acquired copies of such domains since their evolution from a-actinin, presumably as a consequence of their more diverse roles in the cell. 

Proteomic studies have revealed additional FRAPs that are not PDZ proteins (17,24,29). Examples of these FRAPs include components of the actin-spectrin cytoskeleton (e.g., beta-actin, spectrin alpha II chain) and the intracellular signaling apparatus (e.g., calmodulin, and protein kinase C [PKC]0-interacting protein). 

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