A chloroplast ABC1-like kinase regulates vitamin E metabolism in Arabidopsis

In bacteria and mitochondria, ABC1 (for Activity of bc1 complex)-like kinases regulate ubiquinone synthesis, mutations causing severe respiration defects, including neurological disorders in humans. Little is known about plant ABC1-like kinases; in Arabidopsis (Arabidopsis thaliana), five are predicted in mitochondria but, surprisingly, six are located at lipid droplets in chloroplasts. These are a known site of prenylquinone (including tocopherol [vitamin E], phylloquinone [vitamin K] and plastoquinone) metabolism and contain a large proportion of the tocopherol cyclase (VTE1) required for vitamin E synthesis and recycling. Therefore, ABC1-like kinases may be involved in the regulation of chloroplast prenylquinone metabolism. Using a nontargeted lipidomics approach, we demonstrate that plants lacking the plastoglobule ABC1-like kinase ABC1K3 are defective both for the production of plastochromanol-8 (a plastoquinone-derived lipid antioxidant) and the redox recycling of α-tocopherol, whereas tocopherol production is not affected. All of these pathways require VTE1 activity. However, in the abc1k3 mutant, VTE1 levels are strongly reduced posttranscriptionally. We provide evidence that the ABC1-like kinase ABC1K3 phosphorylates VTE1, possibly stabilizing it at plastoglobules. However, ABC1K3 may also have other targets and be involved in a wider chloroplast regulatory network.