Mechanisms of the assembly of actin stress fibers (SFs) have been extensively studied, while those of the disassembly—particularly cell shortening-induced ones—remain unclear. Here, we show that SFs have helical structures composed of multi-subbundles, and they tend to be delaminated upon cell shortening. Specifically, we observed with atomic force microscopy delamination of helical SFs into their subbundles. We physically caught individual SFs using a pair of glass needles to observe rotational deformations during stretching as well as ATP-driven active contraction, suggesting that they deform in a manner reflecting their intrinsic helical structure. A minimal analytical model was then developed based on the Frenet–Serret formulas with force–strain measurement data to suggest that helical SFs can be delaminated into the constituent subbundles upon axial shortening. Given that SFs are large molecular clusters that bear cellular tension but must promptly disassemble upon loss of the tension, the resulting increase in their surface area due to the shortening-induced delamination may facilitate interaction with surrounding molecules to aid subsequent disintegration. Thus, our results suggest a new mechanism of the disassembly that occurs only in the specific SFs exposed to forced shortening.

中文翻译：

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肌动蛋白应力纤维的螺旋结构及其在细胞缩短时诱导其方向选择性拆卸的可能贡献。

肌动蛋白应激纤维（SFs）的组装机制已得到广泛研究，而分解机制（尤其是细胞缩短引起的分解机制）仍不清楚。在这里，我们显示SF具有由多个子束组成的螺旋结构，并且在细胞缩短时它们倾向于分层。具体而言，我们用原子力显微镜观察到了螺旋状SF进入其子束的分层。我们使用一对玻璃针以物理方式捕获了单个SF，以观察拉伸过程中的旋转变形以及ATP驱动的主动收缩，这表明它们以反映其固有螺旋结构的方式变形。然后，基于Frenet-Serret公式和力-应变测量数据，开发了一个最小的分析模型，表明在轴向缩短时，螺旋SF可以分层为组成的子束。鉴于SF是具有细胞张力的大分子簇，但是一旦失去张力就必须立即分解，由于缩短引起的分层而导致的表面积增加可能会促进与周围分子的相互作用，从而有助于后续的崩解。因此，我们的结果提出了一种新的拆卸机制，该机制仅在暴露于强制缩短的特定SF中发生。由于缩短引起的分层而导致的表面积增加可能有助于与周围分子的相互作用，从而有助于后续的崩解。因此，我们的结果提出了一种新的拆卸机制，该机制仅在暴露于强制缩短的特定SF中发生。由于缩短引起的分层而导致的表面积增加可能有助于与周围分子的相互作用，从而有助于后续的崩解。因此，我们的结果提出了一种新的拆卸机制，该机制仅在暴露于强制缩短的特定SF中发生。