Protein homoeostasis in plastids is strategically regulated by the protein quality control system involving multiple chaperones and proteases, among them the Clp protease. Here, we determined the structure of the chloroplast ClpP complex from Chlamydomonas reinhardtii by cryo-electron microscopy. ClpP contains two heptameric catalytic rings without any symmetry. The top ring contains one ClpR6, three ClpP4 and three ClpP5 subunits while the bottom ring is composed of three ClpP1_C subunits and one each of the ClpR1–4 subunits. ClpR3, ClpR4 and ClpT4 subunits connect the two rings and stabilize the complex. The chloroplast Cpn11/20/23 co-chaperonin, a co-factor of Cpn60, forms a cap on the top of ClpP by protruding mobile loops into hydrophobic clefts at the surface of the top ring. The co-chaperonin repressed ClpP proteolytic activity in vitro. By regulating Cpn60 chaperone and ClpP protease activity, the co-chaperonin may play a role in coordinating protein folding and degradation in the chloroplast.

质体中的蛋白质稳态受蛋白质质量控​​制系统的战略性调节，该系统涉及多种伴侣蛋白和蛋白酶，其中包括 Clp 蛋白酶。在这里，我们通过冷冻电子显微镜确定了来自莱茵衣藻的叶绿体 ClpP 复合物的结构。ClpP 包含两个没有任何对称性的七聚体催化环。顶环包含一个 ClpR6、三个 ClpP4 和三个 ClpP5 亚基，而底环由三个 ClpP1 _C组成亚基和 ClpR1-4 亚基各一个。ClpR3、ClpR4 和 ClpT4 亚基连接两个环并稳定复合物。叶绿体 Cpn11/20/23 辅助伴侣蛋白是 Cpn60 的辅助因子，通过将移动环突出到顶环表面的疏水裂缝中，在 ClpP 的顶部形成一个帽。co-chaperonin 在体外抑制 ClpP 蛋白水解活性。通过调节 Cpn60 伴侣蛋白和 ClpP 蛋白酶活性，辅助伴侣蛋白可能在协调叶绿体中的蛋白质折叠和降解中发挥作用。