Characterisation of the dystrophin-related protein utrophin in highly purified skeletal muscle sarcolemma vesicles

Due to its restricted localisation to the neuromuscular junction and based on sequence homology to cytoskeletal proteins, the dystrophin-related 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 neuromuscular junction. However, due to the low density of utrophin in microsomal muscle membranes, it is difficult to analyse the biochemical properties of the skeletal muscle isoform of utrophin. To overcome these technical difficulties, we used here immunoblot analysis of highly purified muscle surface membranes enriched even in sarcolemma markers of very low density such as ecto-5'nuclectidase and the calmodulin-sensitive Ca2+-ATPase. This enabled us to analyse the membrane biochemical properties of this dystrophin isoform of extremely low abundance. Since alkaline treatment released utrophin from the bilayer while it stayed associated with the :insoluble pellet following detergent extraction, utrophin exhibits biochemical properties typical of a membrane cytoskeletal protein. Therefore, utrophin appears to be a specialised isoform which performs the membrane cytoskeletal function(s) of dystrophin at the postsynaptic membrane of the neuromuscular junction.