Proteolysis of myofibrillar proteins at neutral pH

Myofibrils that had been exposed to rat pancreatic trypsin-like serine proteinase and to beef heart Ca2+-activated thiol proteinase were examined in the electron microscope and by SDS-gel electrophoresis. The former enzyme caused more extensive disruption of the ultrastructure and degraded more of the myofibril proteins than the CA2+-activated proteinase. The susceptibilities of individual purified proteins to the two enzymes were also compared. Myosin was virtually resistant to the Ca2+-activated enzyme but with smooth muscle myosin/rat serine proteinase at a ratio of 20000/1, heavy chain degradation took place very rapidly and the ability of the degraded myosin to have its ATPase activity activated by actin in the presence of Ca2+ was lost at a similar rate. G-actin, troponins T and I and alpha-actinin were also degraded readily by the trypsin-like proteinase whereas tropomyosin, a negatively charged rodlike protein, was more resistant. The cellular location of both proteinases remains to be established but from these results obtained in vitro, consideration is given to whether these types of proteinase might work cooperatively in vivo to bring about the disassembly and turnover of myofibrillar proteins that is known to take place outside the lysosomes in muscle.