MdKIN10-mediated phosphorylation of the E3 ubiquitin ligase MdMIEL1 leads to its autophagic degradation under cold stress

Abstract MYB30-Interacting E3 Ligase1 (MdMIEL1) plays a negative role in apple (Malus × domestica) cold tolerance by mediating the degradation of its target proteins. Its protein stability decreases under cold stress; however, how MdMIEL1 is regulated under cold conditions remains unclear. Here, we report that the α-catalytic subunit of sucrose non-fermenting-1-related protein kinase, MdKIN10, a positive regulator of cold stress, interacts with MdMIEL1, leading to its phosphorylation and subsequent degradation. Specifically, cold-activated MdKIN10 phosphorylates MdMIEL1 at serine 198, which occurs near the AIM motif; this modification further enhances the interaction between MdMIEL1 and MdATG8i, thereby promoting MdMIEL1 degradation through the autophagy pathway. Furthermore, MdBBX7, which functions as a positive regulator in apple cold stress responses by positively regulating the expression of ω-3 fatty acid desaturase (MdFAD8) and C-repeat-binding factor 2 (MdCBF2), thus promoting fatty acid desaturation and cold-responsive gene expression, is a target of MdMIEL1. In addition, MdKIN10-mediated phosphorylation of MdMIEL1 at Ser198 attenuates the ability of MdMIEL1 to inhibit apple cold tolerance and decrease MdBBX7 protein abundance in response to cold stress. Our study reveals an intricate post-translational modification cascade of MdKIN10-MdMIEL1-MdBBX7 molecular module under cold stress in apple.