SUMMARY: The amino acid sequences of myosin rod containing subfragment‐2 (S2) and light meromyosin (LMM) were determined by cDNA cloning for walleye pollack fast skeletal myosin heavy chain. While S2 and LMM were composed of 442 and 656 amino acid residues, a total of 1937 amino acid residues accounted for the whole myosin heavy chain molecule with previously determined sequence for the subfragment‐1 heavy chain region of this fish. Both regions for S2 and LMM showed a seven‐residue repeat pattern characteristic to fibrous proteins with a coiled‐coil structure of two α‐helices, displaying a, b, c, d, e, f, and g where positions a and d were frequently occupied by hydrophobic amino acids and c and g often contained charged residues. The occurrence of a 28‐residue unit with repetitive sequence was also strongly suggested, when one and three skip residues were adopted into S2 and LMM, respectively. Thus, walleye pollack S2 and LMM consisted of 17 and 24 zones with a 28‐residue repeat rearrangement. There were several amino acid substitutions which might account for a low thermal stability of walleye pollack myosin heavy chain in comparison with the sequences of higher vertebrate counterparts. However, it seemed difficult to interpret such low stability only from the comparison in the 28‐residue repeat arrangement at the primary structure.

Titin, also known as connectin, is a large filamentous protein that greatly contributes to passive myocardial stiffness. In vitro evidence suggests that one of titin's spring elements, the PEVK, interacts with actin and that this adds a viscous component to passive stiffness. Differential splicing of titin gives rise to the stiff N2B and more compliant N2BA isoforms. Here we studied the titin-isoform dependence of titin-actin interaction and studied the bovine left atrium (BLA) that expresses mainly N2BA titin, and the bovine left ventricle (BLV) that expresses a mixture of both N2B and N2BA isforms. For comparison we also studied mouse left ventricular (MLV) myocardium which expresses predominately N2B titin. Using the actin-severing protein gelsolin, we obtained evidence that titin-actin interaction contributes significantly to passive myocardial stiffness in all tissue types, but most in MLV, least in BLA, and an intermediate level in BLV. We also studied whether titin-actin interaction is regulated by S100A1/calcium and found that calcium alone or S100A1 alone did not alter passive stiffness, but that combined they significantly lowered stiffness. We propose that titin-actin interaction is a “viscous break” that is on during diastole and off during systole.

Thermodynamic properties in differential scanning calorimetry (DSC) and changes in viscoelasticity upon heating of myosins from white croaker, Alaska pollock, and rabbit fast muscles were investigated in relation to their thermal gel formation abilities. Alaska pollock myosin unfolded in a wide temperature range of 19 to 69 °C as revealed by DSC, whereas rabbit myosin unfolded in very narrow range of 32 to 56 °C. Thermal unfolding of white croaker myosin occurred in an intermediate temperature range of 30 to 60 °C. Viscoelastic properties determined as storage modulus, GЈ Ј Ј Ј Ј, and loss modulus, GЉ Љ Љ Љ Љ, reflected differences observed in DSC for the 3 myosins.

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