Department of Genetics, University of Cambridge, Cambridge CB2 3EH, United Kingdom.
Mol Biol Cell. 2010 Aug 1;21(15):2685-95. doi: 10.1091/mbc.e10-03-0197. Epub 2010 Jun 9.
In Saccharomyces cerevisiae, Kar9p, one player in spindle alignment, guides the bud-ward spindle pole by linking astral microtubule plus ends to Myo2p-based transport along actin cables generated by the formins Bni1p and Bnr1p and the polarity determinant Bud6p. Initially, Kar9p labels both poles but progressively singles out the bud-ward pole. Here, we show that this polarization requires cell polarity determinants, actin cables, and microtubules. Indeed, in a bud6 Delta bni1 Delta mutant or upon direct depolymerization of actin cables Kar9p symmetry increased. Furthermore, symmetry was selectively induced by myo2 alleles, preventing Kar9p binding to the Myo2p cargo domain. Kar9p polarity was rebuilt after transient disruption of microtubules, dependent on cell polarity and actin cables. Symmetry breaking also occurred after transient depolymerization of actin cables, with Kar9p increasing at the spindle pole engaging in repeated cycles of Kar9p-mediated transport. Kar9p returning to the spindle pole on shrinking astral microtubules may contribute toward this bias. Thus, Myo2p transport along actin cables may support a feedback loop by which delivery of astral microtubule plus ends sustains Kar9p polarized recruitment to the bud-ward spindle pole. Our findings also explain the link between Kar9p polarity and the choice setting aside the old spindle pole for daughter-bound fate.
在酿酒酵母中，Kar9p 是纺锤体对准的参与者之一，通过将星体微管的正极与 Myo2p 基运输连接起来，引导芽向纺锤体极，该运输沿着由formin Bni1p 和 Bnr1p 以及极性决定因子 Bud6p 产生的肌动蛋白电缆进行。最初，Kar9p 标记两个极，但逐渐挑出芽向极。在这里，我们表明这种极化需要细胞极性决定因素、肌动蛋白电缆和微管。事实上，在 bud6Δ bni1Δ 突变体中，或直接解聚肌动蛋白电缆时，Kar9p 的对称性增加。此外，Myo2 等位基因的选择性诱导也导致了对称性，从而阻止了 Kar9p 与 Myo2p 货物结构域的结合。微管短暂失稳后，Kar9p 极性得以重建，这依赖于细胞极性和肌动蛋白电缆。在肌动蛋白电缆短暂解聚后也会发生对称性破坏，Kar9p 在参与重复 Kar9p 介导的运输的纺锤体极上增加。Kar9p 可能在收缩的星体微管上回到纺锤体极，这可能有助于这种偏向。因此，Myo2p 沿着肌动蛋白电缆的运输可能支持一个反馈回路，通过该回路，星体微管正极的输送维持了 Kar9p 向芽向纺锤体极的极化招募。我们的发现也解释了 Kar9p 极性与选择为女儿绑定命运预留旧纺锤体极之间的联系。
