科学家报告说，分裂的细胞靠构造障碍来留下它们完成分裂为所需要的生化物质。这个被称为“胞质分裂”的过程是身体产生新细胞的关键。它涉及复杂的但人们还很不了解的一系列事件。Jeroen Dobbelaere和同事报告说，由蛋白质septin组成的环在细胞分裂处子细胞之间构成障碍。环阻碍了控制细胞分裂最后事件的因子的扩散，包括封住新细胞的细胞膜和将新产生的子细胞分开。如果没有septin形成的障碍，这些因子可能在正常细胞分裂完成前从分裂处游移开。
Spatial Coordination of Cytokinetic Events by Compartmentalization of the Cell Cortex
During cytokinesis, furrow ingression and plasma membrane fission irreversibly separate daughter cells. How actomyosin ring assembly and contraction, vesicle fusion, and abscission are spatially coordinated was unknown. We found that during cytokinesis septin rings, located on both sides of the actomyosin ring, acted as barriers to compartmentalize the cortex around the cleavage site. Compartmentalization maintained diffusible cortical factors, such as the exocyst and the polarizome, to the site of cleavage. In turn, such factors were required for actomyosin ring function and membrane abscission. Thus, a specialized cortical compartment ensures the spatial coordination of cytokinetic events.
Fig. 1. Protein dynamics during cytokinesis. (A) Localization of Spa2-GFP and the septin CFP-Cdc3 during G2/M (upper lane) and cytokinesis (lower lane). Enlargements of the bud neck are shown (insets). (B) Localization of Iqg1-, Hof1-, Myo1-, Spa2-, Sec3-, and Chs2-GFP (green) and the septin CFP-Cdc3 (red). (C) Analysis of Spa2, Sec3, and Cdc12 dynamics at the bud neck using FRAP. The photobleached region is indicated (orange box). Fluorescence intensities are shown over time for the bleached (light green), and unbleached area (dark). (D and E) Analysis of Spa2 and Myo2 diffusion out of the cytokinetic area using fluorescence loss in photobleaching (FLIP). A region of the cell cortex (orange circle) was repetitively bleached, and fluorescence intensity at the bud neck was monitored. (F and G) Actomyosin rings of diploid cells, coexpressing Myo1-CFP and Myo1-YFP, were subjected to FRAP. Only YFP was bleached. The overlay between CFP and YFP (green), the signal in the CFP channel (red) are shown during cytokinesis (F) and G2 (G). Elapsed time is indicated starting right before bleaching. YFP intensities over time are shown as in (C). Scale bars, 2 µm. All movies and imaging methods are available (17).
Fig. 2. Septin rings maintain diffusible bud neck proteins at the cleavage area. (A) Spa2 localization in wild-type and cdc12-6 cytokinetic cells after shift from permissive (22°C) to restrictive temperature (35°C). GFP-labeled dots moving away from the neck are indicated (open arrows). Elapsed time since the temperature shift is indicated. (B) Localization of Cdc12, Spa2, Sec3, and Myo1 in rts1- (top) and wild-type cells (bottom) after 3 hours at 37°C. Coexpression of GFP-Nop1 (arrow), a nucleolar marker, and CFP-tubulin (asterisk) facilitated the identification of post-anaphase cells. (C) Myo1 localization and contraction in wild-type and cdc12-6 cells shifted to 35°C. Kymographs of ring contraction are shown. Genotypes and cell cycle stages are indicated. Where appropriate, an asterisk indicates the beginning of contraction.
Fig. 3. Septins are required for the completion of cytokinesis. (A and B) Septin ring splitting and cell separation were monitored in wild-type (A) and cdc12-6 (B) cells shifted from 22°C to 30°C. The elapsed time since ring splitting is indicated. (C to E) Role of septins in neck closure, assayed by FLIP. Cells coexpressed cytosolic GFP and Myo1-GFP (arrow), which indicates cytokinetic progression. The photobleaching area is indicated (orange circle). (C and D) Wild-type cell before (C) and after (D) Myo1 contraction. (E) cdc12-6 cell after contraction. In (C), t = 0 was set to the end of the movie. In (D and E), t = 0 corresponds to actomyosin ring disassembly.
Fig. 4. The exocyst is required for actomyosin ring contraction and completion of cytokinesis. (A) Wild-type, sec3-4, and sec3-2 cells were arrested before mitosis with hydroxy urea (HU) (left) and released at 37°C for 3 hours (right). The percentage of cells in the indicated cycle stages is shown. (B) Septin localization (GFP-Cdc3) in cytokinetic cells of the indicated genotype. (C) Actomyosin localization and contraction in sec3-4 and wild type as in Fig. 2B.
