Recent studies have suggested that activation of the ubiquitin-proteasome pathway is primarily responsible for the rapid loss of muscle proteins in various types of atrophy. The present studies were undertaken to test if different classes of muscle proteins are degraded by this pathway. In extracts of rabbit psoas muscle, the complete degradation of soluble proteins to amino acids was stimulated up to 6-fold by ATP. Peptide aldehyde inhibitors of the proteasome or the removal of proteasomes markedly inhibited only the ATP-dependent process. Addition of purified myosin, actin, troponin, or tropomyosin to these extracts showed that these proteins served as substrates for the ubiquitin-proteasome pathway. By contrast, degradation of myoglobin did not require ATP, proteasomes, or any known proteases in muscles. When myosin, actin, and troponin were added as actomyosin complexes or as intact myofibrils to these extracts, they were not hydrolyzed at a significant rate, probably because in these multicomponent complexes, these proteins are protected from degradation. Accordingly, actin (but not albumin or troponin) inhibited the degradation of 125I-myosin, and actin was found to selectively inhibit ubiquitin conjugation to 125I-myosin. Also, the presence of tropomyosin inhibited the degradation of 125I-troponin. However, neither actin nor tropomyosin inhibited the degradation of 125I-lysozyme or soluble muscle proteins. Thus, specific interactions between the myofibrillar proteins appear to protect them from ubiquitin-dependent degradation, and the rate-limiting step in their degradation is probably their dissociation from the myofibril.
Solomon et al. (1996) studied this question.