Academic journal article Genetics

The Chloroplast Protein Translocation Complexes of Chlamydomonas Reinhardtii: A Bioinformatic Comparison of Toc and Tic Components in Plants, Green Algae and Red Algae

Academic journal article Genetics

The Chloroplast Protein Translocation Complexes of Chlamydomonas Reinhardtii: A Bioinformatic Comparison of Toc and Tic Components in Plants, Green Algae and Red Algae

Article excerpt

ABSTRACT

The recently completed genome of Chlamydomonas reinhardtii was surveyed for components of the chloroplast protein translocation complexes. Putative components were identified using reciprocal BlastP searches with the protein sequences of Arabidopsis thaliana as queries. As a comparison, we also surveyed the new genomes of the bryophyte Physcomitrella patens, two prasinophyte green algae (Ostreococcus lucimarinus and Ostreococcus tauri), the red alga Cyanidioschizon merolae, and several cyanobacteria. Overall, we found that the components of the import pathway are remarkably well conserved, particularly among the Viridiplantae lineages. Specifically, C. reinhardtii contained almost all the components found in A. thaliana, with two exceptions. Missing from C. reinhardtii are the C-terminal ferredoxin-NADPH-reductase (FNR) binding domain of Tic62 and a full-length, TPR-bearing Toc64. Further, the N-terminal domain of C. reinhardtii Toc34 is highly acidic, whereas the analogous region in C. reinhardtii Toc159 is not. This reversal of the vascular plant model may explain the similarity of C. reinhardtii chloroplast transit peptides to mitochondrial-targeting peptides. Other findings from our genome survey include the absence of Tic22 in both Ostreococcus genomes; the presence of only one Toc75 homolog in C. merolae; and, finally, a distinctive propensity for gene duplication in P. patens.

THE completion of the Chlamydomonas reinhardtii genome presents an opportunity for a genomewide survey of components making up its chloroplast protein translocation complexes. This first glimpse of the Chlamydomonas chloroplast protein importmachinery provides an important new perspective on our models of chloroplast protein translocators, which until recently have relied heavily upon studies of vascular plants, particularly Arabidopsis thaliana and Pisum sativum.

Despite significant variation in plastid morphology and function, all plastids derive from a single endosymbiosis (MARTIN and HERRMANN 1998; CAVALIER-SMITH 2000; LÓPEZ-JUEZ 2007), which occurred >930 million years ago (BERNEY and PAWLOWSKI 2006) and possibly >1200million years ago (BUTTERFIELD 2000). Although the endosymbiont retained its prokaryotic doublemembrane architecture along with its thylakoid membranes, it ceded control of the majority of its genetic blueprint, withmost of its genomebeinglostor transferred to the host nucleus (MARTIN et al. 1998). Currently, plastid genomes contain only 50-200 protein-encoding genes, a fraction of the original number of genes that would have been possessed by the cyanobacterium-like endosymbiont(MARTIN et al. 2002; LEISTER 2003; TIMMIS et al. 2004). However, this loss of genes from nascent plastids was not accompanied by a cognate reduction in plastid metabolic function or activity. Indeed, while plastids have abandoned some activities common to the cyanobacterial forebears, they still practice a diverse retinue ofmetabolism and contain an estimated 1000-2000 proteins. Most of these plastid proteins are encoded by nuclear genes and imported post-translationally from the cytosol.Thus, one fundamental requirement ofplastid evolution is a protein translocation system to facilitate the post-translational return of endosymbiont proteins back to the organelle. It is likely that at least a rudimentary form of such a translocation system existed soon after the initial endosymbiotic event, since the majority of the gene transfer from the endosymbiont had already occurred at this time (MARTIN et al. 1998; TIMMIS et al. 2004).

Our current understanding of plastid protein import complexes stems from two decades of elegant cell biological and genetic studies focused on vascular plants (reviewed in SOLL and SCHLEIFF 2004). A host of proteins have been identified as components of two quasiindependent translocons resident in the outer and inner membranes of the plant plastid, known as Toc (translocon at the outer chloroplast envelope) and Tic (translocon at the inner chloroplast envelope). …

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