CHARACTERIZATION OF THE DYSTROPHIN-RELATED PROTEIN UTROPHIN IN HIGHLYPURIFIED SKELETAL-MUSCLE SARCOLEMMA VESICLES

Due to its restricted localisation to the neuromuscular junction and b ased on sequence homology to cytoskeletal proteins, the dystrophin-rel ated protein utrophin is thought to be an important constituent of the membrane cytoskeleton of the postsynaptic muscle membrane and may be involved in the clustering of acetylcholine receptors at the neuromusc ular junction. However, due to the low density of utrophin in microsom al muscle membranes, it is difficult to analyse the biochemical proper ties of the skeletal muscle isoform of utrophin. To overcome these tec hnical difficulties, we used here immunoblot analysis of highly purifi ed muscle surface membranes enriched even in sarcolemma markers of ver y low density such as ecto-5'nuclectidase and the calmodulin-sensitive Ca2+-ATPase. This enabled us to analyse the membrane biochemical prop erties of this dystrophin isoform of extremely low abundance. Since al kaline treatment released utrophin from the bilayer while it stayed as sociated with the :insoluble pellet following detergent extraction, ut rophin exhibits biochemical properties typical of a membrane cytoskele tal protein. Therefore, utrophin appears to be a specialised isoform w hich performs the membrane cytoskeletal function(s) of dystrophin at t he postsynaptic membrane of the neuromuscular junction.