The translocon at the outer chloroplast membrane (TOC) is the gateway for chloroplast protein import and so is vital for photosynthetic establishment and plant growth. Chloroplast-associated protein degradation (CHLORAD) is a ubiquitin-dependent proteolytic system that regulates TOC. In CHLORAD, cytosolic Cdc48 provides motive force for the retrotranslocation of ubiquitinated TOC proteins to the cytosol but how Cdc48 is recruited is unknown. Here, we identify plant UBX-domain protein PUX10 as a component of the CHLORAD machinery. We show that PUX10 is an integral chloroplast outer membrane protein that projects UBX and ubiquitin-associated domains into the cytosol. It interacts with Cdc48 via its UBX domain, bringing it to the chloroplast surface, and with ubiquitinated TOC proteins via its ubiquitin-associated domain. Genetic analyses in Arabidopsis revealed a requirement for PUX10 during CHLORAD-mediated regulation of TOC function and plant development. Thus, PUX10 coordinates ubiquitination and retrotranslocation activities of CHLORAD to enable efficient TOC turnover.

叶绿体外膜 (TOC) 的易位子是叶绿体蛋白输入的门户，因此对于光合作用的建立和植物生长至关重要。叶绿体相关蛋白降解 (CHLORAD) 是一种调节 TOC 的泛素依赖性蛋白水解系统。在 CHLORAD 中，胞质 Cdc48 为泛素化 TOC 蛋白逆转录至胞质溶胶提供动力，但 Cdc48 是如何招募的尚不清楚。在这里，我们将植物 UBX 结构域蛋白 PUX10 鉴定为 CHLORAD 机制的一个组成部分。我们证明 PUX10 是一种完整的叶绿体外膜蛋白，它将 UBX 和泛素相关结构域投射到细胞质中。它通过其 UBX 结构域与 Cdc48 相互作用，将其带到叶绿体表面，并通过其泛素相关结构域与泛素化 TOC 蛋白相互作用。拟南芥遗传分析揭示了 CHLORAD 介导的 TOC 功能和植物发育调节过程中对 PUX10 的需求。因此，PUX10 协调 CHLORAD 的泛素化和逆转录转位活性，以实现有效的 TOC 周转。