Changes in myofibrillar structure and function produced by N-terminal deletion of the regulatory light chain in Drosophila

The similarity of amino acid sequence and motifs of the N-terminal extensions of certain class II myosin light chains, found throughout the animal kingdom, suggest a common functional role. One possible role of the N-terminal extension is to enhance oscillatory work and power production in striated muscles that normally operate in an oscillatory mode. We conducted small-angle X-ray diffraction experiments and small-length-perturbation analysis to examine the structural and functional consequences of deleting the N-terminal extension of the myosin regulatory light chain (RLC) in Drosophila flight muscle. The in vivo lattice spacing of dorsal longitudinal muscle (DLM) of flies lacking the RLC N-terminal extension (Dmlc2Δ2–46) was ∼1 nm less than that of wild type (48.56 ± 0.02 nm). The myofilament lattice of detergent-treated, demembranated DLM swelled, with the Dmlc2Δ2–46 lattice expanding more than wild type and requiring roughly twice the concentration of Dextran T500 to restore its lattice to in vivo spacing (9–10% vs. 4% w/v). The calcium sensitivity and maximum amplitude of net oscillatory work near the in vivo lattice spacing was significantly lower in Dmlc2Δ2–46 compared to wild type (pCa50 shifted by approximately one-third of a pCa unit; amplitude reduced by approximately one-half). These changes were in contrast to the lack of effect reported in a previous study carried out in the absence of Dextran T500. The results are consistent with the N-terminal extension interacting with actin to increase the probability that crossbridges form during stretch-activated oscillatory work and power production, especially at submaximal levels of calcium activation.