Dystrophin and Utrophin Bind Actin through Distinct Modes of Contact

Abstract

This study was designed to define the molecular epitopes of dystrophin-actin interaction and to directly compare the actin binding properties of dystrophin and utrophin. According to our data, dystrophin and utrophin both bound alongside actin filaments with submicromolar affinities. However, the molecular epitopes involved in actin binding differed between the two proteins. In utrophin, the amino-terminal domain and an adjacent string of the first 10 spectrin-like repeats more fully recapitulated the activities measured for full-length protein. The homologous region of dystrophin bound actin with low affinity and near 1:1 stoichiometry as previously measured for the isolated amino-terminal, tandem (CH) domain. In contrast, a dystrophin construct including a cluster of basic spectrin-like repeats and spanning from the amino terminus through repeat 17, bound actin with properties most similar to full-length dystrophin. Dystrophin and utrophin both stabilized preformed actin filaments from forced depolymerization with similar efficacies but did not appear to compete for binding sites on actin. We also found that dystrophin binding to F-actin was markedly sensitive to increasing ionic strength, although utrophin binding was unaffected. Although dystrophin and utrophin are functionally homologous actin-binding proteins, these results indicate that their respective modes of contact with actin filaments are markedly different. Finally, we reassessed the abundance of dystrophin in striated muscle using full-length protein as the standard and measured greater than 10-fold higher values than previously reported.Dystrophin and utrophin are homologous proteins with similar interacting partners. By providing a link between the actin cytoskeleton and the extracellular matrix, dystrophin functions to maintain the integrity of the cell membrane during muscle contraction. Consequently, genetic ablation of dystrophin leads to increased fragility in the muscle membrane and results in the pathologies observed in Duchenne and Becker muscular dystrophies and some forms of cardiomyopathy. The functional role of utrophin is not completely understood. However, utrophin overexpression in the dystrophin-deficient mdx mouse has been shown to correct all known parameters of the dystrophic phenotype (1). Notably, utrophin overexpression rescued the mechanical linkage between costameric actin and the sarcolemma of the dystrophin-deficient mdx mouse muscle (2).Like other members of the spectrin superfamily of proteins, both dystrophin and utrophin interact with actin via the amino-terminal tandem calponin homology (CH) 2 actin-binding domain (3-6). Additionally, in both dystrophin and utrophin, the spectrin-like repeats of the rod domain have been shown to contribute to actin binding. According to our recent study, the first 10 spectrin-like repeats of utrophin increase the affinity and capacity of its amino-terminal domain for actin (7). The actin binding activity of the homologous region of dystrophin has not been investigated yet. However,...

β-Dystroglycan binds caveolin-1 in smooth muscle: a functional role in caveolae distribution and Ca2+ release

“…The DGC interacts with and stabilizes actin filaments through a link involving dystrophin (Rybakova et al, 2006). This led us to hypothesize that actin tethering is important for stabilization and organization of the DGC and its role in orchestrating caveolae distribution via caveolin-1 on the sarcolemma.…”

Basement Membranes: Cell Scaffoldings and Signaling Platforms

“…In other tissues, bdystroglycan is found bound to homologs of dystrophin that include utrophin (also found in muscle), Dp160 and Dp116. Dystrophin and utrophin bind in a noncompetitive manner to F-actin with similar affinities (Rybakova et al 2006). Dystroglycan plays a crucial role in preserving the sarcolemma in the face of muscle contraction/relaxation and is important for development of the Schwann cell nodes of Ranvier, brain cortex laminations, and formation and/or survival of the parietal endoderm (Ervasti and Campbell 1993;Williamson et al 1997;Cohn et al 2002;Moore et al 2002;Nodari et al 2008).…”

The genetic and molecular basis of muscular dystrophy: roles of cell–matrix linkage in the pathogenesis

“…Dystrophin contains actin-binding domains at the N-terminal part. The central rod domain of dystrophin consists of spectrin-like repeats and serves as the second site for actin binding (Rybakova et al 2006). Therefore, one of the functions of the DGC is to connect the extracellular matrix to the actin cytoskeleton, which is thought to provide mechanical stability to the muscle plasma membrane.…”