Structural differences in the motor domain of temperature-associated myosin heavy chain isoforms from grass carp fast skeletal muscle

We determined coding sequences for three types of grass carp myosin subfragment-1 (S1) heavy chain by extending 5′-regions of the three known genes encoding light meromyosin isoforms (10 °C, intermediate and 30 °C types). The primary structures of these three S1 heavy chain isoforms showed 81.4%, 81.2%, and 97.8% identities between the 10 °C and intermediate types, between the 10 °C and 30 °C types, and between the intermediate and 30 °C types, respectively. Isoform-specific differences were clearly observed between the 10 °C type and the other two types in 97 amino acid residues. Furthermore, among these amino acid mutations, 51 mutations occurred at the conserved residue sites of S1 heavy chain from fish and homoiotherm. Additionally, the 10 °C type showed striking differences compared with the other two types in the two surface loops, loop 1 located near the ATP-binding pocket and loop 2, which is one of the actin-binding sites, suggesting that such structural differences possibly affect their motor functions. Interestingly, this 10 °C-type myosin heavy chain isolated from adult grass carp skeletal muscle was surprisingly similar to the embryonic fast-type myosin heavy chain from juvenile silver carp in the structure of S1 heavy chain, indicating that it may also function as embryonic fast-type myosin heavy chain in juvenile stage.