Alternative Versions of the Myosin Relay Domain Differentially Respond to Load to Influence Drosophila Muscle Kinetics

Abstract

We measured the influence of alternative versions of the Drosophila melanogaster myosin heavy chain relay domain on muscle mechanical properties. We exchanged relay domain regions (encoded by alternative versions of exon 9) between an embryonic (EMB) isoform and the indirect flight muscle isoform (IFI) of myosin. Previously, we observed no effect of exchanging the EMB relay domain region into the flight muscle isoform (IFI-9b) on in vitro actin motility velocity or solution ATPase measurements compared to IFI. However, in indirect flight muscle fibers, IFI-9b exhibited decreased maximum power generation (P(max)) and optimal frequency of power generation (f(max)) to 70% and 83% of IFI fiber values. The decrease in muscle performance reduced the flight ability and wing-beat frequency of IFI-9b Drosophila compared to IFI Drosophila. Previously, we found that exchanging the flight muscle specific relay domain into the EMB isoform (EMB-9a) prevented actin movement in the in vitro motility assay compared to EMB, which does support actin movement. However, in indirect flight muscle fibers EMB-9a was a highly effective motor, increasing P(max) and f(max) 2.5-fold and 1.4-fold, respectively, compared to fibers expressing EMB. We propose that the oscillatory load EMB-9a experiences in the muscle fiber reduces a high activation energy barrier between two strongly bound states of the cross-bridge cycle, thereby promoting cross-bridge cycling. The IFI relay domain's enhanced sensitivity to load increases cross-bridge kinetics, whereas the EMB version is less load-sensitive.

Mapping Interactions between Myosin Relay and Converter Domains That Power Muscle Function

“…Interestingly, residue 509 varies among human muscle myosin isoforms as well, with the superfast extraocular myosin having a glutamic acid at this position compared with threonine in slower forms, suggesting a tighter linkage to the basic residue at position 759 in the faster myosin (44). We have shown that substitution of the entire relay domain of Drosophila IFI into an embryonic myosin isoform severely affects myosin function and myofibril stability (33), whereas the insertion of the embryonic relay into IFI impairs power generation in isolated fibers (45). It will be interesting to discern whether the two variable residues at the relay-converter interface play major roles in defining these functional properties.…”

The roles of troponin C isoforms in the mechanical function of Drosophila indirect flight muscle

“…Lethocerus IFMs generates high forces at relatively low muscle speeds while Drosophila takes the high speed, low force approach. From mechanical measurements on isolated fibers, we know that Lethocerus IFMs generate about 40-fold higher isometric tension than Drosophila IFMs, ~120 and ~3 mN/ mm 2 , respectively (Agianian et al 2004; Yang et al 2008). The frequency at which Drosophila IFM maximum power is generated is ~150 Hz at 15 °C, compared to ~2–4 Hz for isolated Lethocerus fibers at 23 °C (Agianian et al 2004; Linari et al 2004; Ramanath et al 2011; Wang et al 2014).…”

Alternative Relay and Converter Domains Tune Native Muscle Myosin Isoform Function in Drosophila

“…A key aspect of the current study is that it allowed the analysis of naturally occurring isoforms that contain compatible alternative domains selected through evolution; this work complements and extends previous studies of the Drosophila relay and converter domains that relied on chimeric myosin molecules. 6,7,9,18,22,30,31 …”