THE 20S 26S PROTEASOMAL PATHWAY OF PROTEIN-DEGRADATION IN MUSCLE-TISSUE/

Similar to all other eukaryotic cells and tissues muscle tissue contai ns the proteolytic system of 20S/26S proteasomes with the 20S proteaso me existing predominantly in a latent state. Unlike with the mammalian enzyme in vitro transition from the latent to the activated state of the 20S proteasomes isolated from muscle of several fish species and f rom lobster can be achieved by heat shock. It is very likely that the activated state of the 20S proteasome corresponds to the physiological ly active form of the enzyme since only that one is able to attack sar coplasmic and myofibrillar proteins to any significant extent. As perf usion of rat hindquarters with presumptive low molecular mass activato rs like free fatty acids does not result in an activation of the muscl e proteasome other - possibly protein activators - may serve this purp ose in vivo. The 26S proteasome complex may be regarded as such a prot easome/activator complex. The 26S proteasome complex has the ability t o degrade protein (-ubiquitin-conjugates) by an ATP-consuming reaction . Since increased amounts of ubiquitinated proteins as well as an enha nced activity of the ATP (-ubiquitin)-dependent proteolytic system hav e been measured in rat muscle tissue during various catabolic conditio ns, it is not unlikely that this pathway is responsible for catalysis of muscle protein breakdown.