Proteasomal control of division in Archaea In eukaryotes, proteasome-mediated degradation of cell cycle factors triggers mitotic exit, DNA segregation, and cytokinesis, a process that culminates in abscission dependent on the protein ESCRT-III. By studying cell division in an archaeal relative of eukaryotes, Tarrason Risa et al. identified a role for the proteasome in triggering cytokinesis by an archaeal ESCRT-III homolog. Cell division in this archaeon was driven by stepwise remodeling of a composite ESCRT-III–based division ring, where rapid proteasome-mediated degradation of one ESCRT-III subunit triggered the constriction of the remaining ESCRT-III–based copolymer. These data strengthen the case for the eukaryotic cell division machinery having its origins in Archaea. Science, this issue p. eaaz2532 ESCRT-III–mediated membrane remodeling and the proteasome play a role in archaeal cell cycle control. INTRODUCTION Eukaryotes likely arose from a symbiotic partnership between an archaeal host and an alpha-proteobacterium, giving rise to the cell body and the mitochondria, respectively. Because of this, a number of proteins controlling key events in the eukaryotic cell division cycle have their origins in archaea. These include ESCRT-III proteins, which catalyze the final step of cytokinesis in many eukaryotes and in the archaeon Sulfolobus acidocaldarius. However, to date, no archaeon has been found that harbors homologs of cell cycle regulators, like cyclin-dependent kinases and cyclins, which order events in the cell cycle across all eukaryotes. Thus, it remains uncertain how key events in the archaeal cell cycle, including division, are regulated. RATIONALE An exception to this is the 20S proteasome, which is conserved between archaea and eukaryotes and which regulates the eukaryotic cell cycle through the degradation of cyclins. To explore the function of the 20S proteasome in the archaeon S. acidocaldarius, we determined its structure by crystallography and carried out in vitro biochemical analyses of its activity with and without inhibition. The impact of proteasome inhibition on cell division and cell cycle progression was examined in vivo by flow cytometry and super-resolution microscopy. Following up with mass spectrometry, we identified proteins degraded by the proteasome during division. Finally, we used molecular dynamics simulations to model the mechanics of this process. RESULTS Here, we present a structure of the 20S proteasome of S. acidocaldarius to a resolution of 3.7 Å, which we used to model its sensitivity to the eukaryotic inhibitor bortezomib. When this inhibitor was added to synchronous cultures, it was found to arrest cells mid-division, with a stable ESCRT-III division ring positioned at the cell center between the two separated and prereplicative nucleoids. Proteomics was then used to identify a single archaeal ESCRT-III homolog, CdvB, as a key target of the proteasome that must be degraded to enable division to proceed. Examining the localization patterns of CdvB and two other archaeal ESCRT-III homologs, CdvB1 and CdvB2, by flow cytometry and super-resolution microscopy revealed the sequence of events that leads to division. First, a CdvB ring is assembled. This CdvB ring then templates the assembly of the contractile ESCRT-III homologs, CdvB1 and CdvB2, to form a composite division ring. Cell division is then triggered by proteasome-mediated degradation of CdvB, which allows the CdvB1:CdvB2 copolymer to constrict, pulling the membrane with it. During constriction, the CdvB1:CdvB2 copolymer is disassembled, thus vacating the membrane neck to drive abscission, yielding two daughter cells with diffuse CdvB1 and CdvB2. CONCLUSION This study reveals a role for the proteasome in driving structural changes in a composite ESCRT-III copolymer, enabling the stepwise assembly, disassembly, and contraction of an ESCRT-III–based division ring. Although it is not yet clear how proteasomal inhibition prevents S. acidocaldarius cells from resetting the cell cycle to initiate the next S phase, these data strengthen the case for the eukaryotic cell cycle regulation having its origins in archaea. Model of ESCRT-III–mediated cell division in the archaeon S. acidocaldarius. The model shows the sequential stages of the division process in S. acidocaldarius (labeled 1 to 6), together with the corresponding stage-specific images. DNA is in blue, CdvB in purple, and CdvB1 and CdvB2 in green. The broken arrow represents an extended period where cells progress through G1, S, and G2. Note that Vps4 (not shown) is likely required for ESCRT-III polymer disassembly. Sulfolobus acidocaldarius is the closest experimentally tractable archaeal relative of eukaryotes and, despite lacking obvious cyclin-dependent kinase and cyclin homologs, has an ordered eukaryote-like cell cycle with distinct phases of DNA replication and division. Here, in exploring the mechanism of cell division in S. acidocaldarius, we identify a role for the archaeal proteasome in regulating the transition from the end of one cell cycle to the beginning of the next. Further, we identify the archaeal ESCRT-III homolog, CdvB, as a key target of the proteasome and show that its degradation triggers division by allowing constriction of the CdvB1:CdvB2 ESCRT-III division ring. These findings offer a minimal mechanism for ESCRT-III–mediated membrane remodeling and point to a conserved role for the proteasome in eukaryotic and archaeal cell cycle control.

中文翻译：

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蛋白酶体控制古细菌中 ESCRT-III 介导的细胞分裂


古细菌中蛋白酶体对分裂的控制在真核生物中，蛋白酶体介导的细胞周期因子降解会触发有丝分裂退出、DNA 分离和胞质分裂，这一过程最终导致依赖于蛋白质 ESCRT-III 的脱落。通过研究真核生物古细菌的细胞分裂，Tarrason Risa 等人。确定了蛋白酶体在古细菌 ESCRT-III 同源物触发胞质分裂中的作用。该古细菌中的细胞分裂是由基于 ESCRT-III 的复合分裂环的逐步重塑驱动的，其中一个 ESCRT-III 亚基的快速蛋白酶体介导的降解触发了剩余的基于 ESCRT-III 的共聚物的收缩。这些数据强化了真核细胞分裂机制起源于古细菌的论据。科学，本期第 14 页。 eaaz2532 ESCRT-III 介导的膜重塑和蛋白酶体在古细菌细胞周期控制中发挥作用。引言真核生物可能起源于古细菌宿主和α-变形菌之间的共生伙伴关系，分别产生细胞体和线粒体。正因为如此，许多控制真核细胞分裂周期关键事件的蛋白质都起源于古细菌。其中包括 ESCRT-III 蛋白，它在许多真核生物和古细菌酸热硫化叶菌中催化胞质分裂的最后一步。然而，迄今为止，尚未发现古细菌具有细胞周期调节因子的同源物，例如细胞周期蛋白依赖性激酶和细胞周期蛋白，它们对所有真核生物的细胞周期事件进行排序。因此，古细菌细胞周期中的关键事件（包括分裂）是如何受到调节的仍然不确定。 基本原理 20S 蛋白酶体是一个例外，它在古细菌和真核生物之间是保守的，并且通过细胞周期蛋白的降解来调节真核细胞周期。为了探索古细菌 S.acidocaldarius 中 20S 蛋白酶体的功能，我们通过晶体学确定了其结构，并对其有抑制和无抑制的活性进行了体外生化分析。通过流式细胞术和超分辨率显微镜在体内检查蛋白酶体抑制对细胞分裂和细胞周期进展的影响。通过质谱分析，我们鉴定出了分裂过程中被蛋白酶体降解的蛋白质。最后，我们使用分子动力学模拟来模拟该过程的力学。结果在这里，我们展示了酸热链球菌 20S 蛋白酶体的结构，分辨率为 3.7 Å，我们用它来模拟其对真核抑制剂硼替佐米的敏感性。当将该抑制剂添加到同步培养物中时，发现它会阻止细胞分裂中期，稳定的 ESCRT-III 分裂环位于两个分离的预复制核之间的细胞中心。然后使用蛋白质组学来鉴定单个古细菌 ESCRT-III 同源物 CdvB，它是蛋白酶体的关键目标，必须将其降解才能进行分裂。通过流式细胞术和超分辨率显微镜检查 CdvB 和其他两种古菌 ESCRT-III 同源物 CdvB1 和 CdvB2 的定位模式，揭示了导致分裂的事件序列。首先，组装 CdvB 环。然后，该 CdvB 环以收缩性 ESCRT-III 同系物 CdvB1 和 CdvB2 的组装为模板，形成复合分裂环。 然后，蛋白酶体介导的 CdvB 降解触发细胞分裂，从而使 CdvB1:CdvB2 共聚物收缩，从而拉动膜。在收缩过程中，CdvB1:CdvB2 共聚物被分解，从而腾出膜颈以驱动脱落，产生两个具有弥漫性 CdvB1 和 CdvB2 的子细胞。结论 这项研究揭示了蛋白酶体在驱动复合 ESCRT-III 共聚物结构变化中的作用，从而实现基于 ESCRT-III 的分割环的逐步组装、拆卸和收缩。尽管尚不清楚蛋白酶体抑制如何阻止酸热链球菌细胞重置细胞周期以启动下一个 S 期，但这些数据强化了真核细胞周期调节起源于古细菌的情况。 ESCRT-III 介导的古细菌 S. Acidocaldarius 细胞分裂模型。该模型显示了酸热链球菌分裂过程的连续阶段（标记为 1 至 6），以及相应的特定阶段图像。 DNA 为蓝色，CdvB 为紫色，CdvB1 和 CdvB2 为绿色。虚线箭头代表细胞经历 G1、S 和 G2 的较长时期。请注意，ESCRT-III 聚合物拆卸可能需要 Vps4（未显示）。酸热硫化叶菌是实验上最易处理的真核生物近亲，尽管缺乏明显的细胞周期蛋白依赖性激酶和细胞周期蛋白同源物，但其具有有序的真核细胞周期，具有不同的 DNA 复制和分裂阶段。在这里，在探索酸热链球菌细胞分裂的机制时，我们确定了古菌蛋白酶体在调节从一个细胞周期结束到下一个细胞周期开始的转变中的作用。 此外，我们将古菌 ESCRT-III 同源物 CdvB 确定为蛋白酶体的关键靶标，并表明其降解通过允许 CdvB1:CdvB2 ESCRT-III 分裂环收缩来触发分裂。这些发现为 ESCRT-III 介导的膜重塑提供了一个最小的机制，并指出蛋白酶体在真核和古细菌细胞周期控制中的保守作用。