Résumé :
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Dystrophin is a muscle scaffolding protein that establishes a structural link between the cytoskeleton and the extracellular matrix. Despite the large body of knowledge about the dystrophin gene and its interactions, the functional importance of the large central rod domain remains highly controversial. It is composed of 24 spectrin-like repeats interrupted by four hinges that delineate three sub-domains. We express repeat 1 to 3 (R1-3) and repeat 20 to 24 (R20-24) sub-domains, delineated by hinges 1-2 and 3-4 and the single repeats 2 (R2) and 23 R23). We determine their lipid-binding properties, thermal and urea stabilities and refolding velocities. By using intrinsic tryptophan fluorescence spectroscopy and size exclusion chromatography, we show that R2 and the R1-3 sub-domain strongly interact with anionic phospholipids. By contrast, R23 and R20-24 sub-domain do not interact with lipids. Therefore, the region R1-3 makes up a lipid-binding domain (LBD1). In addition, the R1-3 sub-domain and R2 are dramatically less stable with Tm of 51 and 54°C compared to R20-24 sub-domain and R23 with Tm of 62 and 66°C. In addition, refolding velocities are profoundly different, with rate constants for R2 and sub-domain R1-3 being twice as high as for their counterparts R23 and sub-domain R20-24, respectively. The contrasting properties of the two sub-domains clearly indicate that they make up two specific structural units within the rod domain that are not interchangeable. Due to their lipid binding properties, spectrin repeats have been shown to be involved in the modulation of the mechanical stability of the red cell membrane. The observation of sarcolemma ruptures in dystrophin-deficient muscles provides evidence for dystrophin involvement in plasma membrane resistance during muscle contraction. According to their homology with spectrin, we propose that the lipid-binding properties of dystrophin sub-domain R1-3 are involved in the maintenance of sarcolemma stability.
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