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

Article

DOI: 10.1007/s11103-004-7848-y

Cite this article as:
Chaal, B.K. & Green, B.R. Plant Mol Biol (2005) 57: 333. doi:10.1007/s11103-004-7848-y

Abstract

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.

Keywords

chloroplast importHeterocapsa triquetraHeterosigma akashiwoPorphyra yezoensisPsbOstromal processing peptidase

Abbreviations

CER

chloroplast endoplasmic reticulum

ER

endoplasmic reticulum

IEM and OEM

inner and outer chloroplast envelope membranes

OEC

oxygen evolving complex

PPM

periplastid membrane

PsbO

33 kDa oxygen-enhancer 1 protein

SPase

signal peptidase

SPP

stromal processing peptidase

STD

stromal targeting domain

TLCK

tosyl-lysylchloromethane

TPCK

tosyl-phenylalanylchloromethane

TPP

thylakoidal processing peptidase

TTD

thylakoid targeting domain

Copyright information

© Springer 2005

Authors and Affiliations

  1. 1.Department of BotanyUniversity of British ColumbiaVancouverCanada