Comparative studies on troponin, a Ca2+-dependent regulator of muscle contraction, in striated and smooth muscles of protochordates

Troponin is well known as a Ca2+-dependent regulator of striated muscle contraction and it has been generally accepted that troponin functions as an inhibitor of muscle contraction or actin–myosin interaction at low Ca2+ concentrations, and Ca2+ at higher concentrations removes the inhibitory action of troponin. Recently, however, troponin became detectable in non-striated muscles of several invertebrates and in addition, unique troponin that functions as a Ca2+-dependent activator of muscle contraction has been detected in protochordate animals, although troponin in vertebrate striated muscle is known as an inhibitor of the contraction in the absence of a Ca2+. Further studies on troponin in invertebrate muscle, especially in non-striated muscle, would provide new insight into the evolution of regulatory systems for muscle contraction and diverse function of troponin and related proteins. The methodology used for preparation and characterization of functional properties of protochordate striated and smooth muscles will be helpful for further studies of troponin in other invertebrate animals.