Characterization of two tropomyosin isoforms from the fast skeletal muscle of bluefin tuna Thunnus thynnusorientalis

► Fast skeletal muscle tropomyosin (TM) of tunas including bluefin tuna Thunnus thynnus orientalis is composed of nearly equimolar amount of two isoforms, α-TM and β-TM unlike TMs from the other fish species and mammals (only α-type). ► The amino acid sequence of β-TM could be obtained by cDNA cloning for the first time in this study. The sequence of β-TM showed high similarity to those of other vertebrate α-type TMs including tuna α-TM. This profile was quite unlike the relation of mammalian α- and β-TMs. It was suggested that tuna TM isoform are the products of different genes. ► The isoforms form a heterodimer, in contrast to other fish which have homodimers only. ► Thermodynamic analysis of native and reconstituted TMs demonstrated that β-TM is less thermostable than α-TM. Proteolytic digestion also revealed the lower stability of β-TM. ► All these results showed the uniqueness of tuna TM.