The cytosolic kinases STY8, STY17, and STY46 are involved in chloroplast differentiation in Arabidopsis.

In Arabidopsis transit peptides for chloroplast destined preproteins can be phosphorylated by the protein kinases STY8, STY17 and STY46. In the present study we have investigated the in vitro properties of these plant specific kinases. Characterization of the mechanistic functioning of STY8 lead to the identification of an essential threonine in the activation segment, which is phosphorylated by an intramolecular mechanism. STY8 is inhibited by specific tyrosine kinase inhibitors albeit it lacked the ability to phosphorylate tyrosine residues in vitro. In vivo analysis of sty8, sty17 and sty46 Arabidopsis knockout/knockdown mutants revealed a distinct function of the three kinases in the greening process and in the efficient differentiation of chloroplasts. Mutant plants displayed not only a delayed accumulation of chlorophyll, but also a reduction of nuclear encoded chloroplast proteins and an retarded establishment of the photosynthetic capacity during the first 6 hours of de-etiolation, supporting a role of cytosolic STY kinases in chloroplast differentiation.