Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306-4380, USA.
Department of Biological Science, Florida State University, Tallahassee, FL 32306-4295, USA.
Int J Mol Sci. 2024 Oct 21;25(20):11313. doi: 10.3390/ijms252011313.
Invertebrate striated muscle myosin filaments are highly variable in structure. The best characterized myosin filaments are those found in insect indirect flight muscle (IFM) in which the flight-powering muscles are not attached directly to the wings. Four insect orders, Hemiptera, Diptera, Hymenoptera, and Coleoptera, have evolved IFM. IFM thick filaments from the first three orders have highly similar myosin arrangements but differ significantly among their non-myosin proteins. The cryo-electron microscopy of isolated IFM myosin filaments from the Dipteran described here revealed the coexistence of two distinct filament types, one presenting a tubular backbone like in previous work and the other a solid backbone. Inside an annulus of myosin tails, tubular filaments show no noticeable densities; solid filaments show four paired paramyosin densities. Both myosin heads of the tubular filaments are disordered; solid filaments have one completely and one partially immobilized head. Tubular filaments have the protein stretchin-klp on their surface; solid filaments do not. Two proteins, flightin and myofilin, are identifiable in all the IFM filaments previously determined. In , flightin assumes two conformations, being compact in solid filaments and extended in tubular filaments. Nearly identical solid filaments occur in the large water bug , which flies infrequently. The tubular filaments occur in younger flies, and the solid filaments appear in older flies, which fly less frequently if at all, suggesting that the solid filament form is correlated with infrequent muscle use. We suggest that the solid form is designed to conserve ATP when the muscle is not in active use.
无脊椎动物横纹肌肌球蛋白丝在结构上高度多变。研究得最透彻的肌球蛋白丝存在于昆虫间接飞行肌（IFM）中，这些提供飞行动力的肌肉并不直接附着在翅膀上。四个昆虫目，半翅目、双翅目、膜翅目和鞘翅目，已经进化出 IFM。前三个目的 IFM 厚丝具有高度相似的肌球蛋白排列，但它们的非肌球蛋白蛋白之间存在显著差异。本文描述的从双翅目分离的 IFM 肌球蛋白丝的低温电子显微镜显示，两种不同的丝状结构同时存在，一种呈现管状骨干，如之前的工作所示，另一种呈现实心骨干。在肌球蛋白尾部的环内，管状丝没有明显的密度；实心丝显示出四个配对的原肌球蛋白密度。管状丝的两个肌球蛋白头部都是无序的；实心丝有一个完全和一个部分固定的头部。管状丝表面有伸展蛋白-klp；实心丝没有。在以前确定的所有 IFM 丝中，都可以识别出两种蛋白质，飞行蛋白和肌球蛋白。在 中，飞行蛋白呈现两种构象，在实心丝中紧凑，在管状丝中伸展。在大型水蝽中几乎相同的实心丝很少飞行。较年轻的苍蝇中存在 管状丝，而较老的苍蝇中存在实心丝，如果苍蝇飞行频率较低，则实心丝形式出现，这表明实心丝形式与肌肉使用频率较低相关。我们认为，当肌肉不活跃时，实心形式的设计目的是为了节省 ATP。
