Proteolytic Processes Underlying Molt-Induced Claw Muscle Atrophy in Decapod Crustaceans

The claw muscles of large-clawed decapod crustaceans undergoa programmed atrophy in preparation for molting, or ecdysis.This is mediated by five cytosolic proteinases organized intotwo proteolytic pathways: calcium-dependent and ubiquitin/proteasome-dependent.The calcium-dependent system consists of four calcium-dependentcysteine proteinases (CDPs I, IIa, IIb, and III; native masses310, 125, 195, and 59 kDa, respectively) that completely degrademyofibrillar proteins and are activated in atrophic muscles.Immunological analysis shows that the active-site sequence inCDP IIa (60-kDa subunit mass) is similar to that in mammalianCDPs (calpains), and that CDP IIb is homologous to a calpain-likegene isolated from Drosophila cDNA libraries. Increased intracellullarCa²⁺ stimulates proteolysis in situ, indicating CDPs play animportant role in muscle protein catabolism. The ubiquitin/proteasome-dependentsystem involves the ATP-dependent conjugation of multi-ubiquitinchains to protein by ubiquitin-conjugating enzymes. This actsas a signal for substrate degradation by the 26S proteasome,a multi-subunit complex consisting of a 20S proteasome catalytic"core" and two PA700 (19S) regulatory complexes. PolyubiquitinmRNA, ubiquitin-protein conjugates, and 20S proteasome are elevatedabout 5-, 8-, and 2-fold, respectively, during atrophy. A heat-inducedform of the 20S proteasome hydrolyzes myosin, troponin, andtropomyosin to large fragments in vitro. Biochemical studiesidentified the branched-chain amino acid-preferring (BrAAP)activity, one of six distinct catalytic components in the complex,as the activity that carries out these initial cleavages. Theseresults indicate that the ubiquitin/proteasome pathway is involved,but its precise role remains to be resolved.