GETTING OUT OF A TIGHT SQUEEZE - ENZYMATIC REGULATION OF CLAW MUSCLE ATROPHY IN MOLTING

The claw closer muscle of the land crab, Gecarcinus lateralis, undergoes a cyclical atrophy and restoration during the interval between ecdyses. During proecdysis (stage D0), 30-60% of the muscle protein is degraded, which reduces tissue mass and facilitates withdrawal of the propodus at ecdysis. Protein is resynthesized as the muscle grows back to its previous size during metecdysis. This atrophy is specific to the claws and can be accentuated by multiple limb autotomy. Crustacean muscles contain five cytosolic proteinases that degrade myofibrillar proteins. Four of these constitute a family of enzymes requiring Ca2+ for activity. These calcium-dependent proteinases (CDPs) hydrolyze myofibrillar proteins in vitro and in situ and show increased activity in atrophic claw muscles, which suggests that CDPs play an important role in myofibrillar protein metabolism. The fifth enzyme is a multicatalytic proteinase (MCP), a multisubunit proteolytic complex that degrades a wide range of peptide and protein substrates. The catalytic properties of the complex are altered with low concentrations of sodium dodecyl sulfate or by brief heating at 60-degrees-C. Only the heat-activated form degrades myofibrillar proteins. Since the CDPs hydrolyze contractile proteins about 30-fold more rapidly than the heat-activated MCP, the MCP probably has a more limited or specialized function in molt-induced claw muscle atrophy.