Plant Molecular Biology

, Volume 57, Issue 3, pp 333–342

Protein import pathways in ‘complex’ chloroplasts derived from secondary endosymbiosis involving a red algal ancestor



Heterokont algae such as diatoms and the raphidophyte Heterosigmaakashiwo and peridinin-containing dinoflagellates such as Heterocapsa triquetra originally acquired their chloroplasts via secondary endosymbiosis involving a red algal endosymbiont and a eukaryote host, resulting in ‘complex’ chloroplasts surrounded by four and three membranes, respectively. The precursors of both heterokont and dinoflagellate chloroplast-targeted proteins are first inserted into the ER with removal of an N-terminal signal peptide, but how they traverse the remaining membranes is unclear. Using a nuclear-encoded thylakoid lumen protein, PsbO, from the heterokont alga Heterosigma akashiwo, the dinoflagellate Heterocapsa triquetra and the red alga Porphyra yezoensis we show that precursors without the ER signal peptide can be imported into pea chloroplasts. In the case of the H. triquetra and Porphyra PsbO, the precursors were processed to their predicted mature size and localized within the thylakoid lumen, using the Sec-dependent pathway. We report for the first time a stromal processing peptidase (SPP) activity from an alga of the red lineage. The enzyme processes the Heterosigma PsbO precursor at a single site and appears to have different substrate and reaction specificities from the plant SPP. In spite of the fact that we could not find convincing homologs of the plant chloroplast import machinery in heterokont (diatom) and red algal genomes, it is clear that these three very different lines of algae use similar mechanisms to import chloroplast precursors.


chloroplast import Heterocapsa triquetra Heterosigma akashiwo Porphyra yezoensis PsbO stromal processing peptidase 



chloroplast endoplasmic reticulum


endoplasmic reticulum


inner and outer chloroplast envelope membranes


oxygen evolving complex


periplastid membrane


33 kDa oxygen-enhancer 1 protein


signal peptidase


stromal processing peptidase


stromal targeting domain






thylakoidal processing peptidase


thylakoid targeting domain


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Copyright information

© Springer 2005

Authors and Affiliations

  1. 1.Department of BotanyUniversity of British ColumbiaVancouverCanada

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