Learning to Build a β-Carboxysome.
Learning to Build a β-Carboxysome.
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学习构建β-羧基体。
DOI：
10.1021/acs.biochem.9b00199
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发表时间：
2019
期刊：
影响因子：
2.9
通讯作者：
Savage,DavidF
中科院分区：
文献类型：
--
作者：
Blikstad,Cecilia;Flamholz,AviI;Oltrogge,LukeM;Savage,DavidF
蓝藻是地球上第一个通过产氧光合作用固定二氧化碳的谱系，与当代陆地植物中发现的类型相同。今天，含氧光合作用是生物圈中几乎所有碳的最终来源，蓝藻菌负责大约10%的全球二氧化碳固定。Rubisco是Calvin - Benson - Bassham循环的主要二氧化碳固定酶，然而，它是一种相对缓慢且非特异性的酶，因此臭名昭著。除了羧基化，它还催化与O2的反应，导致一个昂贵的过程，称为光呼吸。为了克服这一问题，蓝藻、真核藻类和一些植物已经发展出不同类型的二氧化碳浓缩机制（CCMs），即增加Rubisco附近的二氧化碳水平以抑制光呼吸和加速二氧化碳固定的系统。蓝藻CCM的关键组成部分是羧酸体，在所有当代蓝藻中发现的蛋白质细菌细胞器。1羧酸体直径在100 - 400nm之间，有一个蛋白质外壳，与Rubisco一起包裹碳酸酐酶（图1a）。两种类型的羧基体α和β似乎是趋同进化的。在这两种情况下，羧酶体基因对环境CO2浓度（≈0.04%）下的生长都是必需的。由于CCMs加速Rubisco，有很大的兴趣将蓝藻CCMs移植到作物植物中以提高光合效率。最近令人兴奋的研究已经通过在烟草叶绿体中产生简化的羧基体结构来测试这种方法的可行性。尽管如此，所有蓝藻细菌中羧基体的存在激起了我们的好奇心：CCM究竟是如何工作的？为什么植物不天然表达羧基体？这种复杂的蛋白质细胞器是如何在细胞内自我组装的？在最近的一篇论文中，Hayer - Hartl， Price及其同事报告了我们对β-羧基体生物发生的理解的重大飞跃。这些发现揭示了β羧基体的主要支架蛋白CcmM如何将Rubisco招募到羧基体上。CcmM以全长和短形式存在。全长CcmM由一个n端γ-碳酸酐酶（γ-CA）样结构域和一个通过柔性连接体连接的Rubisco小亚基样结构域（3 - 5个，取决于物种）组成。缩写形式（M35）只包含SSUL结构域重复序列。γ-CA结构域与锚定在羧基体外壳上的支架蛋白CcmN相互作用，而SSUL结构域与Rubisco相互作用。Wang等人使用单颗粒低温电镜和晶体学相结合的方法重建了Rubisco - CcmM复合物，并从蓝藻模型长聚球菌（Synechococcus elongatus） PCC 7942中求解了SSUL结构域的结构。他们的结果揭示了几个惊喜，与Ryan等人的补充研究一起，4重新绘制了我们的β羧基体的生物发生和内部组织模型。由于预测的结构与Rubisco（红细胞）的小亚基相似，长期以来人们一直认为SSUL结构域通过取代红细胞与Rubisco相互作用。然而，来自两个研究小组的证据表明，这是不正确的。相反，SSUL结构域结合完全组装的杂六聚体Rubisco （L8S8，图1b）。在重组的Rubisco - CcmM复合物中，SSUL结构域结合在Rubisco大亚基（RbcL）的两个二聚体之间的间隙中。一个短的螺旋插入，在SSUL结构域高度保守，但在红细胞中不存在，与两个RbcL亚基和一个RbcL亚基形成关键的盐桥和范德华接触。
Cyanobacteria were the first lineage on Earth to fix CO2 via oxygen-producing photosynthesis, the same type found in contemporary land plants. Today, oxygenic photosynthesis is the ultimate source of nearly all carbon in the biosphere and cyanobacteria are responsible for roughly 10% of worldwide CO2 fixation. Rubisco, the primary CO2-fixing enzyme of the Calvin− Benson− Bassham cycle, is notorious, however, for being a relatively slow and nonspecific enzyme. Aside from carboxylation, it also catalyzes a reaction with O2 that results in a costly process called photorespiration. To overcome this problem, cyanobacteria, eukaryotic algae, and some plants have developed different types of CO2-concentrating mechanisms (CCMs), systems that increase CO2 levels near Rubisco to inhibit photorespiration and accelerate CO2 fixation. The key component of the cyanobacterial CCM is the carboxysome, a proteinaceous bacterial organelle found in all contemporary cyanobacteria. 1 Carboxysomes range from 100 to 400 nm in diameter, have a protein shell, and encapsulate carbonic anhydrase together with Rubisco (Figure 1 A). Two types of carboxysomes, α and β, appear to have evolved convergently. In either case, carboxysome genes are essential for growth in ambient CO2 concentrations (≈ 0.04%). Since CCMs accelerate Rubisco, there is a great interest in transplanting cyanobacterial CCMs into crop plants to increase photosynthetic efficiency. Exciting recent research has tested the feasibility of this approach by producing simplified carboxysome structures in tobacco chloroplasts. 2 Still, the presence of carboxysomes in all cyanobacteria piques our curiosity: How exactly does the CCM work? Why do plants not express carboxysomes natively? How does this complex protein organelle self-assemble inside cells? In a recent paper, Hayer− Hartl, Price, and colleagues report findings that constitute a great leap forward in our understanding of β-carboxysome biogenesis. 3 These findings shed light on how the major scaffolding protein of the βcarboxysome, CcmM, recruits Rubisco to the carboxysome. CcmM is found in a full-length and short form. Full-length CcmM consists of an N-terminal γ-carbonic anhydrase (γ-CA)-like domain followed by a repeat of Rubisco small subunit-like (SSUL) domains (between 3 and 5 dependent on species) connected via flexible linkers. The short form (M35) contains only the SSUL domain repeats. The γ-CA domain interacts with CcmN, a scaffolding protein that anchors to the carboxysome shell, while the SSUL domains interact with Rubisco. Wang et al. used a combination of single-particle cryo-EM and crystallography to reconstitute the Rubisco− CcmM complex and solve the structure of the SSUL domain from the model cyanobacteria Synechococcus elongatus PCC 7942. Their results revealed several surprises, which, together with a complementary study by Ryan et al., 4 redraw our model of the biogenesis and internal organization of the βcarboxysome.Due to the predicted structural similarity to the small subunit of Rubisco (RbcS), it has long been thought that SSUL domains interact with Rubisco by displacing an RbcS. However, evidence from both research groups show that this is incorrect. Rather, the SSUL domains bind fully assembled heterohexadecameric Rubisco (L8S8, Figure 1 B). In the reconstituted Rubisco− CcmM complex, the SSUL domains bind in a cleft between two dimers of the large subunit of Rubisco (RbcL). A short helical insertion, which is highly conserved in SSUL domains but absent in RbcS, makes critical salt bridges and van der Waals contacts with two RbcL subunits as well as one …