Verkhovsky A B, Svitkina T M, Borisy G G
Laboratory of Molecular Biology, University of Wisconsin, Madison 53706, USA.
J Cell Biol. 1995 Nov;131(4):989-1002. doi: 10.1083/jcb.131.4.989.
The morphogenesis of myosin II structures in active lamella undergoing net protrusion was analyzed by correlative fluorescence and electron microscopy. In rat embryo fibroblasts (REF 52) microinjected with tetramethylrhodamine-myosin II, nascent myosin spots formed close to the active edge during periods of retraction and then elongated into wavy ribbons of uniform width. The spots and ribbons initially behaved as distinct structural entities but subsequently aligned with each other in a sarcomeric-like pattern. Electron microscopy established that the spots and ribbons consisted of bipolar minifilaments associated with each other at their head-containing ends and arranged in a single row in an "open" zig-zag conformation or as a "closed" parallel stack. Ribbons also contacted each other in a nonsarcomeric, network-like arrangement as described previously (Verkhovsky and Borisy, 1993. J. Cell Biol. 123:637-652). Myosin ribbons were particularly pronounced in REF 52 cells, but small ribbons and networks were found also in a range of other mammalian cells. At the edge of the cell, individual spots and open ribbons were associated with relatively disordered actin filaments. Further from the edge, myosin filament alignment increased in parallel with the development of actin bundles. In actin bundles, the actin cross-linking protein, alpha-actinin, was excluded from sites of myosin localization but concentrated in paired sites flanking each myosin ribbon, suggesting that myosin filament association may initiate a pathway for the formation of actin filament bundles. We propose that zig-zag assemblies of myosin II filaments induce the formation of actin bundles by pulling on an actin filament network and that co-alignment of actin and myosin filaments proceeds via folding of myosin II filament assemblies in an accordion-like fashion.
通过相关荧光显微镜和电子显微镜分析了正在进行净突出的活跃薄片中肌球蛋白II结构的形态发生。在显微注射了四甲基罗丹明-肌球蛋白II的大鼠胚胎成纤维细胞（REF 52）中，新生的肌球蛋白斑点在回缩期靠近活跃边缘形成，然后伸长成宽度均匀的波浪状带。这些斑点和带最初表现为不同的结构实体，但随后以肌节样模式相互排列。电子显微镜证实，这些斑点和带由双极微丝组成，它们在含头部的末端相互关联，并以“开放”之字形构象或“封闭”平行堆叠的形式排列成单排。带也以前述的非肌节网络状排列相互接触（Verkhovsky和Borisy，1993。《细胞生物学杂志》123：637 - 652）。肌球蛋白带在REF 52细胞中特别明显，但在一系列其他哺乳动物细胞中也发现了小带和网络。在细胞边缘，单个斑点和开放带与相对无序的肌动蛋白丝相关。离边缘更远的地方，肌球蛋白丝的排列与肌动蛋白束的发育平行增加。在肌动蛋白束中，肌动蛋白交联蛋白α - 辅肌动蛋白被排除在肌球蛋白定位位点之外，但集中在每个肌球蛋白带两侧的成对位点，这表明肌球蛋白丝的关联可能启动了肌动蛋白丝束形成的途径。我们提出，肌球蛋白II丝的之字形组装通过拉动肌动蛋白丝网络诱导肌动蛋白束的形成，并且肌动蛋白和肌球蛋白丝的共同排列通过肌球蛋白II丝组装以手风琴样方式折叠而进行。
