Purification and characterization of a novel isoform of myosinase from spear squid liver

A novel isoform of myosinase was purified to homogeneity from liver of spear squid by sequential chromatographies using SP Sephadex, hydroxylapatite, Zn/Co chelating affinity, and TSK-gel G2000SW columns. Myosinase activity was detected as a single peak of 45-kDa protein by gel filtration. The novel isoform of myosinase specifically hydrolyzed a rabbit skeletal muscle myosin heavy chain into products of 120 and 100 kDa in the presence of Co(2+) ions, and the cleavage site in the myosin heavy chain was quite different from those of two known myosinase isoforms, I and II. Therefore, we named the novel isoform myosinase III. Myosinase III was also distinguishable from myosinase I by its amino-terminal sequence. The sequence showed similarity to an internal sequence of the astacin family.