Eukaryotic organisms are equipped with quality-control mechanisms that survey protein folding in the endoplasmic reticulum (ER) and remove non-native proteins by ER-associated degradation (ERAD). Recent research has shown that cytokinin-degrading CKX proteins are subjected to ERAD during plant development. The mechanisms of plant ERAD, including the export of substrate proteins from the ER, are not fully understood, and the molecular components involved in the ERAD of CKX are unknown. Here, we show that heavy metal-associated isoprenylated plant proteins (HIPPs) interact specifically with CKX proteins synthesized in the ER and processed by ERAD. CKX–HIPP protein complexes were detected at the ER as well as in the cytosol, suggesting that the complexes involve retrotranslocated CKX protein species. Altered CKX levels in HIPP-overexpressing and higher-order hipp mutant plants suggest that the studied HIPPs control the ERAD of CKX. Deregulation of CKX proteins caused corresponding changes in the cytokinin signaling activity and triggered typical morphological cytokinin responses. Notably, transcriptional repression of HIPP genes by cytokinin indicates a feedback regulatory mechanism of cytokinin homeostasis and signaling responses. Moreover, loss of function of HIPP genes constitutively activates the unfolded protein response and compromises the ER stress tolerance. Collectively, these results suggests that HIPPs represent novel functional components of plant ERAD.

真核生物配备了质量控制机制，可调查内质网 (ER) 中的蛋白质折叠并通过 ER 相关降解 (ERAD) 去除非天然蛋白质。最近的研究表明，细胞分裂素降解 CKX 蛋白在植物发育过程中会受到 ERAD 的影响。植物 ERAD 的机制，包括底物蛋白从 ER 的输出，尚未完全了解，参与 CKX ERAD 的分子成分未知。在这里，我们表明重金属相关异戊二烯化植物蛋白 (HIPP) 与 ER 中合成并由 ERAD 处理的 CKX 蛋白特异性相互作用。在 ER 和胞质溶胶中检测到 CKX-HIPP 蛋白复合物，表明该复合物涉及逆转录转运的 CKX 蛋白种类。改变的 CKX 水平HIPP过表达和高阶hipp突变植物表明所研究的 HIPP 控制着 CKX 的 ERAD。CKX 蛋白的失调引起细胞分裂素信号活性的相应变化，并触发典型的形态学细胞分裂素反应。值得注意的是，细胞分裂素对HIPP基因的转录抑制表明细胞分裂素稳态和信号反应的反馈调节机制。此外， HIPP基因功能的丧失组成性地激活未折叠蛋白反应并损害 ER 应激耐受性。总的来说，这些结果表明 HIPP 代表了植物 ERAD 的新功能成分。