Phosphoproteomics of Arabidopsis chloroplasts reveals involvement of the STN7 kinase in phosphorylation of nucleoid protein pTAC16

Light-regulated protein kinases STN7 and STN8 phosphorylate thylakoid membrane proteins and also affect expression of several chloroplast proteins via yet unknown mechanisms. Comparative phosphoproteomics of acetic acid protein extracts of chloroplasts from Arabidopsis thaliana wild type, stn7, stn8 and stn7stn8 mutants yielded two previously unknown findings: (i) neither STN7 nor STN8 kinase was required for phosphorylation of Ser-48 in Lhcb1.1–1.3 proteins; and (ii) phosphorylation of Thr-451 in pTAC16 protein was STN7-dependent. pTAC16 was found distributed between thylakoids and nucleoid. Its knockout did not affect the nucleoid protein composition and the Thr-451 phosphorylated protein was excluded from the nucleoid. Thr-451 of pTAC16 is conserved in all studied plants and its phosphorylation may regulate membrane-anchoring functions of the nucleoid.Structured summary with protein interactionsRPOC2, RPOB, pTAC3, emb2746, RPOC1, pTAC2, pTAC12, pTAC16, pTAC5, pTAC14, RPOA, pTAC13, pTAC15 and pTAC6colocalize by mass spectrometry studies of complexes (View interaction) PsbH, Lhcb4,1, Lhcb4,2, STN7, D1, D2, CP43, TSP9, CaS, PsaP, Alb3, PsbL, Rubisco, Unknown, Unknown, OEP6, pTAC16, PGRL1A, Unknown, Unknown and PsbQcolocalize by mass spectrometry studies of complexes (View interaction)

► Comparative phosphoproteomics of Arabidopsis chloroplasts.
► Serine phosphorylation of Lhcb1 is independent of STN7 and STN8 kinases.
► Phosphorylation of Thr-451 in the pTAC16 protein is strictly STN7-dependent.
► pTAC16 is distributed between the thylakoid membrane and chloroplast nucleoid.
► pTAC16 phosphorylated at Thr-451 is excluded from the nucleoid.