Structural and functional properties of αß-heterodimers of tropomyosin with myopathic mutations Q147P and K49del in the ß-chain.

Tropomyosin (Tpm) is an α-helical coiled-coil actin-binding protein that plays a key role in the Ca2+-regulated contraction of striated muscles. Two Tpm isoforms, α (Tpm 1.1) and ß (Tpm 2.2), are expressed in fast skeletal muscles. These Tpm isoforms can form either αα and ßß homodimers, or αß heterodimers. However, only αα-Tpm and αß-Tpm dimers are usually present in most of fast skeletal muscles, because ßß-homodimers are relatively unstable and cannot exist under physiologic conditions. Nevertheless, the most of previous studies of myopathy-causing mutations in the Tpm ß-chains were performed on the ßß-homodimers. In the present work, we applied different methods to investigate the effects of two myopathic mutations in the ß-chain, Q147P and K49del (i.e. deletion of Lys49), on structural and functional properties of Tpm αß-heterodimers and to compare them with the properties of ßß-homodimers carrying these mutations in both ß-chains. The results show that the properties of αß-Tpm heterodimers with these mutations in the ß-chain differ significantly from the properties of ßß-homodimers with the same substitutions in both ß-chains. This indicates that the αß-heterodimer is a more appropriate model for studying the effects of myopathic mutations in the ß-chain of Tpm than the ßß-homodimer which virtually does not exist in human skeletal muscles.

Thermal unfolding of homodimers and heterodimers of different skeletal-muscle isoforms of tropomyosin.

We applied differential scanning calorimetry (DSC) to investigate the structural properties of three isoforms of tropomyosin (Tpm), α, ß, and γ, expressed from different genes in human skeletal muscles. We compared specific features of the thermal unfolding of αα, ßß, and γγ Tpm homodimers, as well as of αß and Î³ß Tpm heterodimers. The results show that the thermal stability of γγ homodimer is much higher than that of αα homodimer which, in turn, is much more thermostable than the ßß homodimer. The stability of the Î³ß Tpm heterodimer is much lower than that of the γγ homodimer, and its thermal unfolding is quite different from that for γγ and ßß homodimers, whereas the unfolding of the αß heterodimer is roughly similar to that of the αα homodimer.