Phycobilisome (PBS) is the main light-harvesting antenna in cyanobacteria and red algae. How PBS transfers the light energy to photosystem II (PSII) remains to be elucidated. Here we report the in situ structure of the PBS–PSII supercomplex from Porphyridium purpureum UTEX 2757 using cryo-electron tomography and subtomogram averaging. Our work reveals the organized network of hemiellipsoidal PBS with PSII on the thylakoid membrane in the native cellular environment. In the PBS–PSII supercomplex, each PBS interacts with six PSII monomers, of which four directly bind to the PBS, and two bind indirectly. Additional three ‘connector’ proteins also contribute to the connections between PBS and PSIIs. Two PsbO subunits from adjacent PSII dimers bind with each other, which may promote stabilization of the PBS–PSII supercomplex. By analyzing the interaction interface between PBS and PSII, we reveal that α^LCM and ApcD connect with CP43 of PSII monomer and that α^LCM also interacts with CP47' of the neighboring PSII monomer, suggesting the multiple light energy delivery pathways. The in situ structures illustrate the coupling pattern of PBS and PSII and the arrangement of the PBS–PSII supercomplex on the thylakoid, providing the near-native 3D structural information of the various energy transfer from PBS to PSII.

中文翻译：

---

红藻藻胆体-光系统 II 超复合物的原位冷冻电子断层扫描结构

藻胆体（PBS）是蓝细菌和红藻中主要的光捕获天线。PBS 如何将光能转移到光系统 II (PSII) 仍有待阐明。在这里，我们使用冷冻电子断层扫描和亚断层扫描平均报告了来自Porphyridium purpureum UTEX 2757的 PBS-PSII 超复合物的原位结构。我们的工作揭示了天然细胞环境中类囊体膜上半椭圆体 PBS 与 PSII 的组织网络。在 PBS-PSII 超复合物中，每个 PBS 与六个 PSII 单体相互作用，其中四个直接与 PBS 结合，两个间接结合。另外三种“连接器”蛋白也有助于 PBS 和 PSII 之间的连接。相邻 PSII 二聚体的两个 PsbO 亚基相互结合，这可能会促进 PBS-PSII 超复合物的稳定。通过分析 PBS 和 PSII 之间的相互作用界面，我们发现 α ^LCM和 ApcD 与 PSII 单体的 CP43 连接，并且 α ^LCM还与邻近 PSII 单体的 CP47' 相互作用，表明存在多种光能传递途径。原位结构说明了 PBS 和 PSII 的耦合模式以及 PBS-PSII 超复合物在类囊体上的排列，提供了从 PBS 到 PSII 的各种能量转移的近乎天然的 3D 结构信息。