Properties of a calcium-activated protease in squid axoplasm which selectively degrades neurofilament proteins

Axoplasm extruded from the giant axon of the squid contains Ca²⁺-activated proteases. The protease in the 100,000X g of supernatant of axoplasm is very specific and degrades only the 200,000 MW, neurofilament protein (NF₂₀₀), whereas the protease(s) in the pellet has a much wider range of substrate specificity. The activation of the supernatant protease is restricted to the Ca²⁺ ion, and no other divalent cation will substitute. The protease requires Ca²⁺ at a higher concentration than 0.5 mM for activation, and has a pH optimum of about 7.5. Degradation of the NF₂₀₀ appears to proceed through a 100,000 MW and possibly a 47,000–50,000-MW intermediate form before degradation to TCA-soluble peptides. Activity of the protease is inhibited by divalent cation chelators, Cu²⁺ and Fe²⁺, sulphydryl inhibitors, and leupeptin. This specific Ca²⁺-activated protease in squid axoplasm has identical properties to Ca²⁺-activated proteases found in various non-neural tissues. Despite its narrow protein substrate specificity, Ca²⁺-activated protease purified from human platelets effectively degrades squid NF₂₀₀, suggesting a possible structural relationship between platelet and muscle actin-binding proteins and neurofilament proteins.