An ancient glaucophyte c6-like cytochrome related to higher plant cytochrome c6A is imported into muroplasts

Cytochrome c(6) is a redox carrier in the thylakoid lumen of cyanobacteria and some eukaryotic algae. Although the isofunctional plastocyanin is present in land plants and the green alga Chlamydomonas reinhardtii, these organisms also possess a cytochrome c(6)-like protein designated as cytochrome c(6A). Two other cytochrome c(6)-like groups, c(6B) and c(6C), have been identified in cyanobacteria. In this study, we have identified a novel c(6)-like cytochrome called PetJ2, which is encoded in the nuclear genome of Cyanophora paradoxa, a member of the glaucophytes- the basal branch of the Archaeplastida. We propose that glaucophyte PetJ2 protein is related to cyanobacterial c(6B) and c(6C) cytochromes, and that cryptic green algal and land plant cytochromes c(6A) evolved from an ancestral archaeplastidial PetJ2 protein. In vitro import experiments with isolated muroplasts revealed that PetJ2 is imported into plastids. Although it harbors a twin-arginine motif in its thylakoid-targeting peptide, which is generally indicative of thylakoid import via the Tat import pathway, our import experiments with isolated muroplasts and the heterologous pea thylakoid import system revealed that PetJ2 uses the Sec pathway instead of the Tat import pathway.

Automated summary

In this study, a novel cytochrome c(6)-like protein called PetJ2 was identified in the nuclear genome of Cyanophora paradoxa, a member of the glaucophytes. It is proposed that the glaucophyte PetJ2 protein is related to cyanobacterial c(6B) and c(6C) cytochromes, and that green algal and land plant cytochromes c(6A) may have evolved from an ancestral PetJ2 protein. Import experiments showed that PetJ2 is imported into plastids using the Sec pathway instead of the Tat import pathway.