Timely Endocytosis of Cytokinetic Enzymes Prevents Premature Spindle Breakage during Mitotic Exit

Cytokinesis requires the spatio-temporal coordination of membrane deposition and primary septum (PS) formation at the division site to drive acto-myosin ring (AMR) constriction. It has been demonstrated that AMR constriction invariably occurs only after the mitotic spindle disassembly. It has also been established that Chitin Synthase II (Chs2p) neck localization precedes mitotic spindle disassembly during mitotic exit. As AMR constriction depends upon PS formation, the question arises as to how chitin deposition is regulated so as to prevent premature AMR constriction and mitotic spindle breakage. In this study, we propose that cells regulate the coordination between spindle disassembly and AMR constriction via timely endocytosis of cytokinetic enzymes, Chs2p, Chs3p, and Fks1p. Inhibition of endocytosis leads to over accumulation of cytokinetic enzymes during mitotic exit, which accelerates the constriction of the AMR, and causes spindle breakage that eventually could contribute to monopolar spindle formation in the subsequent round of cell division. Intriguingly, the mitotic spindle breakage observed in endocytosis mutants can be rescued either by deleting or inhibiting the activities of, CHS2, CHS3 and FKS1, which are involved in septum formation. The findings from our study highlight the importance of timely endocytosis of cytokinetic enzymes at the division site in safeguarding mitotic spindle integrity during mitotic exit.

胞质分裂（Cytokinesis）需要在分裂位点对膜沉积与初生隔膜（primary septum，PS）的形成进行时空协调，以驱动肌动蛋白-肌球蛋白环（acto-myosin ring，AMR）的收缩。已有研究证实，AMR收缩始终仅发生在有丝分裂纺锤体解聚之后。同时研究也已明确，在有丝分裂退出过程中，几丁质合酶II（Chitin Synthase II，Chs2p）的颈部定位先于有丝分裂纺锤体解聚。由于AMR收缩依赖于PS的形成，由此产生的核心问题是：细胞如何调控几丁质沉积，以避免AMR过早收缩与有丝分裂纺锤体断裂？本研究提出，细胞可通过对胞质分裂相关酶Chs2p、Chs3p与Fks1p进行及时的内吞调控，来协调纺锤体解聚与AMR收缩的过程。抑制内吞作用会导致有丝分裂退出阶段胞质分裂相关酶的过度积累，进而加速AMR的收缩，并引发纺锤体断裂，该现象最终可能在后续细胞分裂周期中导致单极纺锤体的形成。有趣的是，在内吞作用突变体中观察到的有丝分裂纺锤体断裂现象，可通过敲除或抑制参与隔膜形成的CHS2、CHS3及FKS1的活性得以挽救。本研究的发现凸显了在分裂位点及时对胞质分裂相关酶进行内吞调控，对于在有丝分裂退出过程中保障有丝分裂纺锤体完整性的重要意义。