Résumé :
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Communication n° 490. Proteasomes are the major actors of non-lysosomal cytoplasmic protein degradation. In particular, these large protein complexes (about 2500 kDa) are considered to be responsible for muscular degradation during skeletal muscle atrophy. Despite their unusual and important size, they are widely described as soluble and mobile in the cytoplasm. In mature skeletal muscle, we have previously observed a sarcomeric distribution of proteasomes, as revealed by the distribution of a1/p27K, a subunit of the 20S core-particle (prosome) of proteasome. Here, we extend these observations at the electron microscopic level in vivo. We also show that this sarcomeric pattern is dependent of the extension of the sarcomere. Using isolated myofibrils, we demonstrate that a1/p27K is still attached to the myofibrils after the isolation procedure, and reproduce the observations made in vivo. The high-salt extraction of myosin from these isolated myofibrils do not alter the distribution or staining intensity of a1/p27K. By contrast, the extraction of actin by gelsolin largely removes a1/p27K from isolated myofibrils, but some of them are held in place after this extraction, showing a sarcomeric disposition in the absence of any detectable actin, and suggesting the existence of another molecular partner for these interactions. From these results, we conclude that most of detectable a1/p27K in skeletal muscle cells is tightly attached to the myofibrils, leading to the puzzling problem of the ability for proteasome to be present into the myofibrillar structure in regard to its size.
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