Premium
Polyphenol oxidase can cross thylakoids by both the Tat and the Sec‐dependent pathways: a putative role for two stromal processing sites
Author(s) -
Koussevitzky Shai,
Ne’eman Emma,
Peleg Smadar,
Harel Eitan
Publication year - 2008
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.2008.01074.x
Subject(s) - thylakoid , twin arginine translocation pathway , chloroplast , chromosomal translocation , biochemistry , biology , signal peptide , periplasmic space , transit peptide , atp synthase , escherichia coli , microbiology and biotechnology , enzyme , gene , peptide sequence , plastid
Polyphenol oxidase (PPO; EC 1.10.3.2 or EC 1.14.18.1), a thylakoid‐lumen protein encoded by a nuclear gene, plays a role in the defense of plants against both herbivores and pathogens. Although previously reported to be a Tat ( t win‐ a rginine‐dependent t ranslocation) protein, the import of PPO by isolated chloroplasts was inhibited by azide, a diagnostic inhibitor of the Sec‐dependent pathway. Import of PPO inhibited thylakoid translocation of a Tat protein and did not affect translocation of Sec‐dependent proteins. In contrast, a pre‐accumulated iPPO competed with Sec‐dependent but not with Tat proteins. A previously reported second processing step in the stroma removes a twin‐Arg that is part of a ‘Sec‐avoidance’ motif in the thylakoid targeting domain of PPO. When the second processing site was mutated, the import of the resulting precursor showed Sec‐dependent characteristics. The PPO transit peptide could drive thylakoid translocation of a Tat protein in the dark. Azide inhibited the secretion of a PPO intermediate that lacks a twin‐Arg to the periplasm of Escherichia coli , but had no effect on the export of the intermediate containing the twin‐Arg. PPO is synthesized in plants in response to wound and pathogen‐related signals and it is possible that when the Tat pathway is unable to translocate adequate amounts of newly synthesized PPO, translocation is diverted to the Sec‐dependent pathway by processing the intermediate at the second site and removing the twin‐Arg.