The chloroplast protein import receptors Toc34 and Toc159 are phosphorylated by distinct protein kinases

The molecular composition of chloroplast outer and inner envelope translocons is fairly well established, but little is known about mechanisms and elements involved in import regulation. After synthesis in the cytosol, chloroplast targeted precursor proteins are recognized by outer envelope receptors Toc34 and Toc159. Phosphorylation plays an important role in regulation of Toc34 activity and preprotein binding. Using kinase renaturation assays, we have identified an ATP-dependent 98-kDa outer envelope kinase which is able to selectively phosphorylate Toc34 at a specific site. A 70-kDa outer envelope polypeptide phosphorylating Toc159 was identified by the same strategy. Antiserum against the 98-kDa kinase inhibits phosphorylation of Toc34, whereas labeling of Toc159 remains unaffected. Both kinases do not autophosphorylate in vitro and are unable to utilize myelin basic protein as substrate. We propose that distinct kinases are involved in regulation of chloroplast import via desensitization of preprotein receptors.