Protein translocation within chloroplast is similar in Euglena and higher plants

It is currently thought that chloroplasts of higher plants were derived from endosymbiont oxygenic photosynthetic bacteria (primary endosymbiosis), while Euglena, a photosynthetic protista, gained chloroplasts by secondary endosymbiosis (i.e., incorporation of a photosynthetic eukaryote into heterotrophic eukaryotic host). To examine if the protein transport inside chloroplasts is similar between these organisms, we carried out heterologous protein import experiments with Euglena precursor proteins and spinach chloroplasts. The precursor of a 30-kDa subunit of the oxygen-evolving complex (OEC30) from the thylakoid lumen of Euglena chloroplasts contained the N-terminal signal, stroma targeting, and thylakoid transfer domains. Truncated preOEC30s lacking the N-terminal domain were post-translationally imported into spinach chloroplasts, transported into the thylakoid lumen, and processed to a mature protein. These results showed that protein translocations within chloroplasts in Euglena and higher plants are similar and supported the hypothesis that Euglena chloroplasts are derived from the ancestral Chlorophyta.