Dickinson M H, Hyatt C J, Lehmann F O, Moore J R, Reedy M C, Simcox A, Tohtong R, Vigoreaux J O, Yamashita H, Maughan D W
Department of Integrative Biology, University of California, Berkeley 94720, USA.
Biophys J. 1997 Dec;73(6):3122-34. doi: 10.1016/S0006-3495(97)78338-3.
We examine how the structure and function of indirect flight muscle (IFM) and the entire flight system of Drosophila melanogaster are affected by phosphorylation of the myosin regulatory light chain (MLC2). This integrated study uses site-directed mutagenesis to examine the relationship between removal of the myosin light chain kinase (MLCK) phosphorylation site, in vivo function of the flight system (flight tests, wing kinematics, metabolism, power output), isolated IFM fiber mechanics, MLC2 isoform pattern, and sarcomeric ultrastructure. The MLC2 mutants exhibit graded impairment of flight ability that correlates with a reduction in both IFM and flight system power output and a reduction in the constitutive level of MLC2 phosphorylation. The MLC2 mutants have wild-type IFM sarcomere and cross-bridge structures, ruling out obvious changes in the ultrastructure as the cause of the reduced performance. We describe a viscoelastic model of cross-bridge dynamics based on sinusoidal length perturbation analysis (Nyquist plots) of skinned IFM fibers. The sinusoidal analysis suggests the high power output of Drosophila IFM required for flight results from a phosphorylation-dependent recruitment of power-generating cross-bridges rather than a change in kinetics of the power generating step. The reduction in cross-bridge number appears to affect the way mutant flies generate flight forces of sufficient magnitude to keep them airborne. In two MLC2 mutant strains that exhibit a reduced IFM power output, flies appear to compensate by lowering wingbeat frequency and by elevating wingstroke amplitude (and presumably muscle strain). This behavioral alteration is not seen in another mutant strain in which the power output and estimated number of recruited cross-bridges is similar to that of wild type.
我们研究了果蝇间接飞行肌（IFM）的结构和功能以及整个飞行系统如何受到肌球蛋白调节轻链（MLC2）磷酸化的影响。这项综合研究使用定点诱变来研究去除肌球蛋白轻链激酶（MLCK）磷酸化位点、飞行系统的体内功能（飞行测试、翅膀运动学、代谢、功率输出）、分离的IFM纤维力学、MLC2同工型模式和肌节超微结构之间的关系。MLC2突变体表现出飞行能力的分级损伤，这与IFM和飞行系统功率输出的降低以及MLC2磷酸化组成水平的降低相关。MLC2突变体具有野生型IFM肌节和横桥结构，排除了超微结构的明显变化是性能降低的原因。我们基于对去表皮IFM纤维的正弦长度扰动分析（奈奎斯特图）描述了一个横桥动力学的粘弹性模型。正弦分析表明，果蝇IFM飞行所需的高功率输出源于磷酸化依赖性的产生动力的横桥募集，而不是产生动力步骤动力学的变化。横桥数量的减少似乎影响了突变果蝇产生足够大小的飞行动力以使其在空中飞行的方式。在两个表现出IFM功率输出降低的MLC2突变体菌株中，果蝇似乎通过降低振翅频率和提高冲程幅度（以及推测的肌肉应变）来进行补偿。在另一个突变体菌株中未观察到这种行为改变，该菌株的功率输出和估计的募集横桥数量与野生型相似。
