It is widely accepted in eukaryotes that the cleavage furrow only initiates after mitosis completion. In fission yeast, cytokinesis requires the synthesis of a septum tightly coupled to cleavage furrow ingression. The current cytokinesis model establishes that simultaneous septation and furrow ingression only initiate after spindle breakage and mitosis exit. Thus, this model considers that although Cdk1 is inactivated at early-anaphase, septation onset requires the long elapsed time until mitosis completion and full activation of the Hippo-like SIN pathway. Here, we studied the precise timing of septation onset regarding mitosis by exploiting both the septum-specific detection with the fluorochrome calcofluor and the high-resolution electron microscopy during anaphase and telophase. Contrarily to the existing model, we found that both septum and cleavage furrow start to ingress at early anaphase B, long before spindle breakage, with a slow ingression rate during anaphase B, and greatly increasing after telophase onset. This shows that mitosis and cleavage furrow ingression are not concatenated but simultaneous events in fission yeast. We found that the timing of septation during early anaphase correlates with the cell size and is regulated by the corresponding levels of SIN Etd1 and Rho1. Cdk1 inactivation was directly required for timely septation in early anaphase. Strikingly the reduced SIN activity present after Cdk1 loss was enough to trigger septation by immediately inducing the medial recruitment of the SIN kinase complex Sid2-Mob1. On the other hand, septation onset did not depend on the SIN asymmetry establishment, which is considered a hallmark for SIN activation. These results recalibrate the timing of key cytokinetic events in fission yeast; and unveil a size-dependent control mechanism that synchronizes simultaneous nuclei separation with septum and cleavage furrow ingression to safeguard the proper chromosome segregation during cell division.

在真核生物中，卵裂沟仅在有丝分裂完成后才开始，被广泛接受。在裂变酵母中，胞质分裂需要紧密结合裂沟进入的隔膜的合成。当前的胞质分裂模型确定同步分裂和犁沟进入仅在纺锤体破裂和有丝分裂退出后才开始。因此，该模型认为，尽管Cdk1在早期晚期被灭活，但分隔开始需要较长的时间，直到有丝分裂完成和Hippo样SIN途径的完全激活。在这里，我们通过利用荧光色素calcofluor进行隔垫特异性检测以及在后期和末期使用高分辨率电子显微镜研究了有丝分裂分隔发作的精确时机。与现有模型相反，我们发现隔膜和卵裂沟都在后期B早期开始进入，早在纺锤体破裂之前，后期B进入速度较慢，在末期开始后大大增加。这表明在裂殖酵母中，有丝分裂和分裂沟的进入不是串联的，而是同时发生的。我们发现，后期后期的分隔时间与细胞大小相关，并受SIN Etd1和Rho1的相应水平调节。Cdk1失活是后期后期及时分离的直接要求。引人注目的是，Cdk1丢失后SIN活性降低，足以通过立即诱导SIN激酶复合物Sid2-Mob1的内侧募集而触发分离。另一方面，分隔发作并不取决于SIN不对称性的建立，这被认为是SIN激活的标志。这些结果重新校准了裂变酵母中关键细胞动力学事件的时间。并揭示了一种大小相关的控制机制，该机制使同时发生的细胞核分离与隔膜和卵裂沟进入同步化，以确保细胞分裂过程中染色体的正确分离。