The current model for spindle positioning requires attachment of the microtubule (MT) motor cytoplasmic dynein to the cell cortex, where it generates pulling force on astral MTs to effect spindle displacement. How dynein is anchored by cortical attachment machinery to generate large spindle-pulling forces remains unclear. Here, we show that cortical clustering of Num1, the yeast dynein attachment molecule, is limited by its assembly factor Mdm36. Overexpression of Mdm36 results in an overall enhancement of Num1 clustering but reveals a population of dim Num1 clusters that mediate dynein anchoring at the cell cortex. Direct imaging shows that bud-localized, dim Num1 clusters containing around only six Num1 molecules mediate dynein-dependent spindle pulling via a lateral MT sliding mechanism. Mutations affecting Num1 clustering interfere with mitochondrial tethering but do not interfere with the dynein-based spindle-pulling function of Num1. We propose that formation of small ensembles of attachment molecules is sufficient for dynein anchorage and cortical generation of large spindle-pulling forces.

This article has an associated First Person interview with the first author of the paper.

当前的纺锤体定位模型需要将微管 (MT) 运动细胞质动力蛋白附着到细胞皮层，在细胞皮层上它对星体 MT 产生拉力以影响纺锤体位移。动力蛋白如何通过皮质附着机制锚定以产生巨大的纺锤体拉力仍不清楚。在这里，我们表明 Num1（酵母动力蛋白附着分子）的皮质聚类受到其组装因子 Mdm36 的限制。 Mdm36 的过度表达导致 Num1 聚类的整体增强，但揭示了介导动力蛋白锚定在细胞皮层的暗淡 Num1 簇的群体。直接成像显示，芽定位的暗淡 Num1 簇仅包含大约 6 个 Num1 分子，通过横向 MT 滑动机制介导动力蛋白依赖性纺锤体牵引。影响 Num1 聚类的突变会干扰线粒体束缚，但不会干扰 Num1 基于动力蛋白的纺锤体拉动功能。我们认为，附着分子小群的形成足以实现动力蛋白锚定和皮质产生大纺锤体拉力。

本文有对该论文第一作者的相关第一人称采访。