Molecular cloning of troponin I expressed in fast white muscle of a teleost fish, the Atlantic herring (Clupea harengus L.).

By cDNA cloning and 5'-RACE analysis we characterised a Clupea harengus troponin I mRNA expressed in larvae and in adult white (fast) muscle, but not in red (slow) or cardiac muscle. The mRNA encodes a TnI protein of the short chain length (176 residues) N-terminally truncated type previously observed only in tetrapod skeletal muscles. Despite its expression specificity the herring TnI does not particularly resemble the tetrapod fast skeletal muscle isoform in sequence but appears to be outside of the tetrapod TnIfast/Tnslow/TnIcardiac isoform family. Surprisingly, the actin/TnC-binding sequence resembles that of arthropod, rather than tetrapod vertebrate, troponin I's and has, besides, unique features not seen in any other troponin I's, vertebrate or invertebrate.