Spectrin repeats

Law R, Carl P, Harper S, Dalhaimer P, Speicher DW, Discher DE. Cooperativity in forced unfolding of tandem spectrin repeats. Biophys J 2003 84 533-544. 

Fig. 1. Structure of spectrin superfamily proteins. Modular domains within each protein are clearly defined. Shaded spectrin repeats represent coiled coils involved in dimerization events incomplete repeats represent proportionally the number of coiled-coil helices contributed by a- and /3-spectrin when generating a complete spectrin repeat during formation of the spectrin tetramer. The dashed lines indicate how two spectrin heterodimers interact to form a functional spectrin tetramer. Asterisks in the dystrophin spectrin repeats represent the position of the two greater repeats in dystrophin with respect to utrophin, which in all other respects has a similar overall structure. Numbers in the EF hand regions represent the number of EF hand motifs.

The sequencing of a- and /3-spectrin, a-actinin, and dystrophin has revealed similarities not only within the spectrin repeat, but also the other domains and motifs present within these proteins. Subsequent analyses have revealed an evolutionary pathway for the divergence of spectrin and dystrophin from a common a-actinin ancestor via a series of rearrangements, duplications, and evolution of repeats and other domains, as well as the acquisition of unique domains such as PH, WW, and SHS (Fig. 2). 

Fig. 2. Evolution of the spectrin superfamily. Rounded rectangles represent spectrin repeats. Shaded rectangles denote a-actinin-like repeats involved in dimerization, whereas unshaded rectangles represent repeats that were involved in duplication and/ or elongation events. The incomplete spectrin repeats involved in tetramer formation are proportionally represented depending on the number of repeat helices each protein contributes to the formation of a complete spectrin repeat. (Adapted from Dubreuil, 1991 Pascual et al., 1997.) A dystrophin/utrophin ancestor probably diverged from a-actinin at a relatively early stage and then underwent its own series of duplications and acquisitions of new motifs.

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. 

Spectrin is a much more elongated protein compared to a-actinin due to the additional number of repeats. The additional repeats are more closely related to one another than repeats common to both a-actinin and spectrin. The spectrin repeat sequences are the most divergent in dystrophin and its homologue utrophin (Winder et al, 1995a), most likely reflecting an earlier divergent event when compared to spectrin (Pascual et al, 1997). 

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). 

Djinovic-Carugo, K., Gautel, M., Ylanne, J., and Young, P. (2002). The spectrin repeat A structural platform for cytoskeletal protein assemblies. FEBS Lett. 513, 119-123. 

Rief, M., Pascual, J., Saraste, M., and Gaub, H. E. (1999). Single molecule force spectroscopy of spectrin repeats Low unfolding forces in helix bundles. J. Mol. Biol. 286, 553-561. 

Sutherland-Smith, A. J., Moores, C. A., Norwood, F. L., Hatch, V., Craig, R., Kendrick-Jones, J., and Lehman, W. (2003). An atomic model for actin binding by the CH domains and spectrin-repeat modules of utrophin and dystrophin./. Mol. Biol. 329, 15-33. 

Ylanne, J., Scheffzek, K., Young, P., and Saraste, M. (2001b). Crystal structure of the alpha-actinin rod Four spectrin repeats forming a tight dimer. Cell Mol. Biol. Lett. 6, 234. 

Fig. 11. The structure of a-actinin and the two vertebrate Z-band lattices. (A) The ubiquitous protein a-actinin is an anti-parallel homodimer. Each 100 KDa monomer comprises four central spectrin repeats (SI to S4) an EF-hand domain and two calponin homology domains (CH) at the N-terminus. The EF-hand domains bind calcium in non-muscle cells. One a-actinin molecule binds two actin filaments via the calponin homology domains. a-Actinin binds titin via EF-hand domains. (B, C) The Z-band is the site where actin filaments from adjacent sarcomeres overlap in a tetragonal lattice and are crosslinked by a-actinin molecules. The polarity and origin of the actin filaments is indicated by U (up) and D (down). The appearance of the Z-band in cross-section is typically basketweave-like (B) or small square-like (G). The appearance is reported to transform between the two appearances depending on the state of the muscle.

Zhang Q, Skepper JN, Yang F, Davies JD, Hegyi L, et al. 2001. Nesprins A novel family of spectrin-repeat-containing proteins that localize to the nuclear membrane in multiple tissues. J Cell Sci 114 4485-4498. 

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