Characterization of Subfragment-2 Regions of Myosins from Invertebrate and Vertebrate Striated Muscles

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Type II myosin, the major contractile component of muscle, is a hexamer composed of two heavy chains and four light chains (two essential and two regulatory light chains). The N terminal globular head portion of heavy chain subunit (called subfragment-1, S1) is connected to rod portion through subfragment-2 (S2) region, which causes the bends in the rod portion of myosin and might make S1 lift off the thick filament to facilitate its reaction with actin. In the present study, S2 regions from various striated muscle myosins were compared including recently sequenced ones from cephalopods to shed light on the uniqueness of these stretches. The sequences of myosins from eleven species were aligned, and the α-helical and coiled-coil formations of S2 regions were estimated. As a result, all the S2 regions were considered to have reduced propensity of coiled-coil structure, especially at around the hinge regions, suggesting these regions are structurally flexible. One skip residues was found for all the species, though glutamine was replaced by threonine for fly and vertebrate counterparts. Prediction of secondary structure revealed uniqueness of cephalopod sequence in the N terminal region of S2. This was also confirmed by coiled-coil propensity prediction. Phylogenetic tree drawn based on the amino acid sequence showed that both of scallop and cephalopod S2s formed clear clusters, and were clearly distant from the other invertebrate (fly, worm) and vertebrate counterparts. From the results obtained, it was suggested that S2 regions of muscle myosins are ingeniously tuned so as to adapt to the contraction speed of each myosin or contractile performance of each muscle.