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A role of Toc33 in the protochlorophyllide‐dependent plastid import pathway of NADPH:protochlorophyllide oxidoreductase (POR) A †
Author(s) -
Reinbothe Steffen,
Pollmann Stephan,
Springer Armin,
James Richard Johari,
Tichtinsky Gabrielle,
Reinbothe Christiane
Publication year - 2005
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1111/j.1365-313x.2005.02353.x
Subject(s) - protochlorophyllide , plastid , chloroplast , biology , oxidoreductase , biochemistry , translocon , cytosol , arabidopsis , thylakoid , microbiology and biotechnology , mutant , membrane protein , enzyme , gene , membrane
Summary NADPH:protochlorophyllide oxidoreductase (POR) A is a key enzyme of chlorophyll biosynthesis in angiosperms. It is nucleus‐encoded, synthesized as a larger precursor in the cytosol and imported into the plastids in a substrate‐dependent manner. Plastid envelope membrane proteins, called protochlorophyllide‐dependent translocon proteins, Ptcs, have been identified that interact with pPORA during import. Among them are a 16‐kDa ortholog of the previously characterized outer envelope protein Oep16 (named Ptc16) and a 33‐kDa protein (Ptc33) related to the GTP‐binding proteins Toc33 and Toc34 of Arabidopsis. In the present work, we studied the interactions and roles of Ptc16 and Ptc33 during pPORA import. Radiolabeled Ptc16/Oep16 was synthesized from a corresponding cDNA and imported into isolated Arabidopsis plastids. Crosslinking experiments revealed that import of 35 S‐Oep16/Ptc16 is stimulated by GTP. 35 S‐Oep16/Ptc16 forms larger complexes with Toc33 but not Toc34. Plastids of the ppi1 mutant of Arabidopsis lacking Toc33, were unable to import pPORA in darkness but imported the small subunit precursor of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (pSSU), precursor ferredoxin (pFd) as well as pPORB which is a close relative of pPORA. In white light, partial suppressions of pSSU, pFd and pPORB import were observed. Our results unveil a hitherto unrecognized role of Toc33 in pPORA import and suggest photooxidative membrane damage, induced by excess Pchlide accumulating in ppi1 chloroplasts because of the lack of pPORA import, to be the cause of the general drop of protein import.