Carboxysomes are proteinaceous bacterial microcompartments that sequester the key enzymes for carbon fixation in cyanobacteria and some proteobacteria. They consist of a virus-like icosahedral shell, encapsulating several enzymes, including ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), responsible for the first step of the Calvin-Benson-Bassham cycle. Despite their significance in carbon fixation and great bioengineering potentials, the structural understanding of native carboxysomes is currently limited to low-resolution studies. Here, we report the characterization of a native α-carboxysome from a marine cyanobacterium by single-particle cryoelectron microscopy (cryo-EM). We have determined the structure of its RuBisCO enzyme, and obtained low-resolution maps of its icosahedral shell, and of its concentric interior organization. Using integrative modeling approaches, we have proposed a complete atomic model of an intact carboxysome, providing insight into its organization and assembly. This is critical for a better understanding of the carbon fixation mechanism and toward repurposing carboxysomes in synthetic biology for biotechnological applications.

羧基体是蛋白质细菌微区室，可隔离蓝藻和一些变形菌中固碳的关键酶。它们由类似病毒的二十面体外壳组成，封装了多种酶，包括核酮糖 1,5-二磷酸羧化酶/加氧酶 (RuBisCO)，负责卡尔文-本森-巴沙姆循环的第一步。尽管它们在碳固定方面具有重要意义并且具有巨大的生物工程潜力，但对天然羧基体的结构理解目前仅限于低分辨率研究。在这里，我们报告了通过单颗粒冷冻电子显微镜（cryo-EM）对海洋蓝藻天然α-羧基体的表征。我们已经确定了其 RuBisCO 酶的结构，并获得了其二十面体外壳及其同心内部组织的低分辨率图。使用综合建模方法，我们提出了完整羧基体的完整原子模型，提供了对其组织和组装的深入了解。这对于更好地理解碳固定机制以及在合成生物学中重新利用羧基体用于生物技术应用至关重要。