Serine/threonine protein kinase that specifically phosphorylates chloroplast precursor proteins in the cytosol within the cleavable presequences (transit peptides). May be part of a cytosolic regulatory network involved in chloroplast protein import. Does not phosphorylate mitochondrion precursor proteins. Specific for ATP and does not utilize other NTPs (PubMed:17090544, PubMed:16429265). Plays a role in chloroplast biogenesis and differentiation in cotyledons, possibly through phosphorylation of chloroplast preproteins (PubMed:21799034). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by autophosphorylation at Thr-445. Autophosphorylated on serine and threonine residues. Autophosphorylated at Thr-445. Plants silencing STY17 does not show visible phenotype under normal growth conditions, but plants silencing STY17 in the double mutant sty8 and sty46 background show more severe retarded growth compared to the double mutant itself. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.