Spectrin cytoskeletal

Bundles of parallel actin filaments with uniform polarity. The microvilli of intestinal epithelial cells (enterocytes) are packed with actin filaments that are attached to the overlying plasma membrane through a complex composed of a 110-kD protein and calmodulin. The actin filaments are attached to each other through fimbrin (68 kD) and villin (95 kD). The actin bundles that emerge out of the roots of microvilli disperse horizontally to form a filamentous complex, the terminal web, in which several cytoskeletal proteins, spectrin (fodrin), myosin, actinin, and tropomyosin are present. Actin in the terminal web also forms a peripheral ring, which is associated with the plasma membrane on the lateral surfaces of the enterocyte (see Figure 5, p. 24). 

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).

Mechanical functions of cells require interactions between integral membrane proteins and the cytoskeleton 29 The spectrin-ankyrin network comprises a general form of membrane-organizing cytoskeleton within which a variety of membrane-cytoskeletal specializations are interspersed 29 Interaction of rafts with cytoskeleton is suggested by the results of video microscopy 29... 

Pratt BM, Harris AS, Morrow JS et al (1984) Mechanisms of cytoskeletal regulation. Modulation of aortic endothelial cell spectrin by the extracellular matrix. Am J Pathol 117 ... 

Mangeat, P. H., and Burridge, K. (1984). Immunoprecipitation of nonerythrocyte spectrin within live cells following microinjection of specific antibodies Relation to cytoskeletal structures./ Cell. Biol. 98, 1363-1377. 

Roper, K., and Brown, N. H. (2003). Maintaining epithelial integrity A function for gigantic spectraplakin isoforms in adherens junctions. / Cell Biol. 162, 1305-1315. Roper, K., Gregory, S. L., and Brown, N. H. (2002). The spectraplakins Cytoskeletal giants with characteristics of both spectrin and plakin families. /. Cell Sci. 115, 4215-4225. 

Significant contributors to cell structure are those proteins that crosslink actin filaments or connect actin filaments to the cell membrane. Examples of such proteins can be found within the spectrin superfamily of cytoskeletal proteins. This discrete group is principally composed of the actin crosslinking protein o-actinin, and the membrane-associated actin-binding proteins spectrin and dystrophin. 

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). 

The /3-spectrin PH domain binds weakly to all phosphoinositides and is likely to associate with the negatively charged membrane surface via the positively charged face of the domain. Spectrin networks contain many spectrin molecules, and it is likely that the individual weak association with phosphoinositides is overcome by the overall collective interaction of many molecules. Such a mechanism of multivariant association allows only the assembled cytoskeletal components to interact strongly with cellular membranes, such as in the RBC. 

Spectrin forms an integral part of the erythrocyte cytoskeletal architecture any defects that disrupt the association of the spectrin heterotetra-mer or the interaction with any of the other submembranous proteins can result in RBC defects (reviewed in Hassoun and Palek, 1996). Indeed, abnormalities of the /3-spectrin N-terminus and the a-spectrin G-terminus affect the self-association site and result in hereditary elliptocytosis and hereditary pyropoikilocytosis (Delaunay, 1995 Delaunay and Dhermy, 1993 Palek and Jarolim, 1993). 

The spectrin superfamily is a group of cytoskeletal proteins that have been found to perform a variety of cellular functions. The role of each protein and their interactions within the cellular environment stem from the specific domains found within each protein and the manner in which they are organized. Each of the family members is formed from discrete modular domains that have the ability to interact or modulate specific interactions or impart physical abilities on the protein relevant to its function. The particular members of this protein family have been shown to be evolutionary related. a-Actinin is believed to be the ancestor of the whole group and, indeed, sequence and phylogenetic analysis has found this to be the case. It is astounding that from a simple precursor containing few domains such a family of functionally diverse proteins can be... 

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. 

Spectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane of many cell types, in a pentagonal or hexagonal arrangement, forming a scaffold and playing an important role in maintenance of plasma-membrane integrity and cytoskeletal structure. 

The pleckstrin-homology domain is named after pleckstrin, where it was found first (pleckstrin is a major platelet protein which is a substrate of protein kinase Pleckstrin and brain spectrin contain two domains, 2 about 120 residues each, with similar sequences in their amino- and carboxy-terminal parts. To date, at least 90 such PH-domain sequences have been described in proteins with very diverse functions, such as spectrin,43 cytosolic protein kinases, phospholipase C isoforms (PLC-P, -y, and the GTPase dynamin,44.45 guanine nucleotide exchange factors for monomeric GTP-bind-ing proteins, the GTPase-acdvating protein for Ras, the Ras-GAP, and cytoskeletal and several other proteins. 

Ankyrin is a 200 kDa protein that links cytoskeletal protdns to membrane proteins. It is bound tightly to the cytoplasmic surface of the erythrocyte plasma membrane to which it attaches the cytoskeletal protein spectrin. 

Figure 1 shows heat capacity profiles of human erythrocyte ghosts Incubated with Increasing concentrations of pyran copolymer In 310 Imosm phosphate buffer. The five endothermic transitions of the control sample (bottom curve) are labeled A, B2j C and D, In accord with earlier work by Brandts and coworkers (13-16) and from this laboratory (17-20). The A transition Is assigned quite securely as a partial denaturation of spectrin, the major cytoskeletal protein on the erythrocyte membrane (13). Bj. and B2 appear to Involve proteins and lipids (protein Bands IV.1 and... 

C. Heltianu, I. Bogdan, E. Constantinescu, and M. Simionescu. Endothelial cells express a spectrin-like cytoskeletal protein. Cite. Res. 58 605-700 (1986). 

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