Rigor crossbridge structure in tilted single filament layers and flared-X formations from insect flight muscle.
Rigor crossbridge structure in tilted single filament layers and flared-X formations from insect flight muscle.
复制标题
倾斜的单丝层和昆虫飞行肌肉的喇叭形 X 结构中的严格横桥结构。
DOI：
10.1016/0022-2836(85)90188-3
复制
发表时间：
1985
影响因子：
5.6
通讯作者：
Reedy,MC
中科院分区：
文献类型：
--
作者：
Reedy,MK;Reedy,MC
昆虫飞行肌中严格横桥的平均结构已在滤波图像中进行了研究。它们的三维结构是通过将 25 nm 纵向截面中的单丝层（myac 层和肌动蛋白层）的倾斜视图与显示单个横桥水平的 15 nm 横截面中的喇叭形 X 外观相关联来推断的。围绕丝轴倾斜 myac 或肌动蛋白层使横桥显示两种模式之一。珠状密度要么单独出现在细丝上（“中心珠状”），要么出现在双侧细丝上（“跨珠状”）。这些表达了跨桥肌动蛋白复合物的两种不同投影，如在喇叭形 X 结构中端部看到的那样。单宁酸/戊二醛固定改善了肌动蛋白的保存，在 15 nm 横截面中显示，长螺距螺旋链在双 V 形龙之间的间隙中呈一致方位角的“双点”轮廓。然后根据晶格关系推断出张开的 X 臂的方位角，因为它不是直接看到的。逗号形横桥与推断的肌动蛋白二元体的切向连接符合最近肌动蛋白亚片段 1 复合物重建中的结合几何形状。然而，与装饰肌动蛋白上的均匀头部不同，双 V 形的前导和后部成员之间的平均横桥结构不同。在 myac 层的过滤图像中，前导桥密集且角度陡峭；后面的人字形被视为细丝上的密集珠子，带有微弱的、角度较小的横向延伸条。肌动蛋白层图像还表明后桥和前桥的角度不同。左、右喇叭形 X 臂分别是前桥和后 V 形桥的端视图，并且形状不同。改进的单宁酸/戊二醛固定使我们能够区分张开的 X 臂中的三个横桥结构域：（1）密集的灯泡状头部合并到细丝中； (2) 与肌动蛋白相切的致密但较细的颈部； (3) 一根微弱的细茎将颈部与肌球蛋白丝连接起来。头颈肌动蛋白复合体之间前导和后部成员的形状差异由名称“L sigmoid”和“Rdogleg”表示。在横桥内，头颈轴和头肌动蛋白头轴之间的内角在乙状结肠和狗腿之间相差约 30°，这意味着桥头和桥颈之间存在柔性连接。肌动蛋白上前桥和后桥轴向相距至少 13 nm；预期的 60° 方位角差异由头颈部分表示，但头-肌动蛋白-头轴仅旋转 30°。因此，如果假定肌动蛋白-亚片段 1 复合物的结合几何形状恒定，则肌动蛋白扭曲必须在 38.7 nm 重复范围内变化。低密度茎必须以不同方式弯曲，以从后桥和前桥到达肌球蛋白。因此，后桥茎沿着头颈线笔直延伸，而前桥茎则从该线急剧向后弯曲 70° 或更多。最后，myac 层和喇叭状 X 结构中的平均横桥足够大，足以容纳两个肌球蛋白头。
The averaged structure of rigor crossbridges in insect flight muscle has been studied in filtered images. Their three-dimensional structure has been deduced by relating tilt views of single filament layers in 25 nm longitudinal sections (myac layers and actin layers) to the flared-X appearance in 15 nm cross-sections showing single crossbridge levels. Tilting myac or actin layers around the filament axis makes crossbridges show one of two patterns. Bead-like densities appear either singly over thin filaments (“center-beading”) or doubled and flanking thin filaments (“straddle-beading”). These express two different projections from the crossbridge-actin complexes as seen end-on in flared-X formations.Tannic acid/glutaraldehyde fixation gave improved actin preservation, showing, in 15 nm cross-sections, the long-pitch helical strands as “two-dot” profiles of consistent azimuth in the gaps between double chevrons. The azimuth in the flared-X arms was then inferred from lattice relationships, since it was not seen directly. The tangential attachment of comma-shaped crossbridges to the inferred actin dyad fits the binding geometry in recent actin-subfragment 1 complex reconstructions. However, averaged crossbridge structure differs between lead and rear members of double chevrons, unlike the uniform heads on decorated actin.In filtered images of myac layers, the lead bridges are dense and steeply angled; the rear chevron is seen as a dense bead over the thin filament with faint, less angled bars extending laterally. Actin layer images also suggest that rear and lead bridges differ in angle.Left and right flared-X arms are end-on views of lead and rear chevron bridges, respectively, and differ in shape. Improved fixation with tannic acid/glutaraldehyde allows us to distinguish three crossbridge domains in flared-X arms: (1) a dense bulb-like head merged into the thin filament; (2) a dense but thinner neck tangential to actin; and (3) a faint thin stem joining the necks to myosin filaments. Shape differences in lead and rear members between the head-neck-actin complexes are indicated by the names “L sigmoid” and “R dogleg”. Within crossbridges, internal angles between the head-neck axis and the head-actin-head axis differ between sigmoid and dogleg by about 30 °, implying a flexible junction between bridge-head and bridge-neck.Lead and rear bridges are axially at least 13 nm apart on actin; the expected 60 ° difference in azimuth is expressed by head-neck portions, but the head-actin-head axis rotates by only 30 °. Therefore, if the binding geometry of the actin-subfragment 1 complex is presumed constant, then actin twist must vary within the 38.7 nm repeat.The low-density stems must bend differently to reach myosin from the rear and lead bridges. Thus rear bridge stems extend straight along the head-neck line, while lead bridge stems are bent back sharply 70 ° or more off this line.Finally, averaged crossbridges in myac layers and in flared-X formations are large enough to accommodate two myosin heads.