Hybrid troponin reconstituted from vertebrate and arthropod subunits

Abstract

TROPONIN, a regulatory protein modifying muscular contraction is found in all the chordate striated muscles as well as in many of the invertebrate muscles which have been studied^1,3. The complex of troponin and tropomyosin is bound to the actin containing thin filaments, and both components are required for thin filament-linked regulation^1–3,10. In the absence of calcium the troponin–tropomyosin complex blocks the myosin combining sites, inhibiting ATPase activity and therefore contraction. When present in micromolar amounts, calcium binds to troponin and releases the inhibition. Vertebrate troponin consists of three subunits: TN-I, TN-C, and TN-T (ref. 4). TN-1 by itself (molecular weight 24,000) inhibits the actin–myosin interaction irrespective of calcium ion concentration. TN-C (molecular weight 18,000) binds calcium ions and this releases the TN-I imposed inhibition. TN-T (molecular weight 37,000) attaches the subunit complex to tropomyosin. The troponin found on arthropod thin filaments is also composed of subunits^3,5,6. Components with chain weights of 29,000 and 18,000 have features analogous to those of vertebrate TN-I and TN-C respectively⁶. A third component is also found in arthropod troponin preparations (chain weight 55,000–59,000), but its function is not yet fully understood^5,6. For convenience, this chain will be referred to here as TN-T. In spite of the overall similarity of vertebrate and arthropod troponin, invertebrate troponin differs from vertebrate troponin in calcium binding properties, and binds one-half to one-quarter of the calcium in conditions where the thin filament is maximally switchedon^3,6. Moreover, the thin filament-based regulation in invertebrates is usually coupled with a myosin-linked regulatory system which is apparently not present in chordate striated muscle^3,10.