Myosin Stiffness and Cross-Bridge Kinetics are Influenced by Five Drosophila Alternative Converters

Abstract

The converter has been proposed to be the myosin elastic element that sets cross-bridge stiffness. According to most cross-bridge models, the elastic element's stiffness should influence muscle fiber type force and power generation levels. We tested these hypotheses by exploiting the natural variation in converter regions found in Drosophila muscle types. In Drosophila, myosin isoforms are generated by alternative splicing from a single myosin heavy chain gene. The converter region is encoded by five alternative versions. We forced the four non-native versions to be expressed in Drosophila indirect flight muscle (IFM) and found that two of the three versions tested to date, 11b and 11e (found in larval and adult body wall muscles) significantly decreased IFM rigor stiffness by 63% and 43%, respectively. This supports the hypothesis that the converter is at least part of the elastic element of myosin. The converters influenced more than myosin stiffness, as all three converters slowed the super fast IFM fiber kinetics. 11b, 11d, and 11e decreased the frequency at which maximum power was generated (fmax) by 28%, 40%, and 53%, respectively. All three decreased apparent rate constant 2πb (set by myosin attachment and Pi release kinetics). However, 11d and 11e increased 2πc (set by detachment kinetics). Differences in force generation are minimal, likely due to myosin stiffness changes being offset by changes in duty ratio. With little to no force differences, power almost directly correlated with fmax, and the optimal percent ML amplitude for power inversely correlated with fmax. All three lines showed reduced wing beat frequency, with flight performance better correlating with fiber fmax than rigor stiffness. Thus, the converter is critical for muscle fiber type diversity as it helps set myosin stiffness, cross-bridge kinetics and power generation.