The chloroplast protein import system: From algae to trees

Abstract

Chloroplasts are essential organelles in the cells of plants and algae. The functions of these specialized plastids are largely dependent on the ~3000 proteins residing in the organelle. Although chloroplasts are capable of a limited amount of semiautonomous protein synthesis - their genomes encode ~100 proteins - they must import more than 95% of their proteins after synthesis in the cytosol. Imported proteins generally possess an N-terminal extension termed a transit peptide. The importing translocons are made up of two complexes in the outer and inner envelope membranes, the so-called Toc and Tic machineries, respectively. The Toc complex contains two precursor receptors, Toc159 and Toc34, a protein channel, Toc75, and a peripheral component, Toc64/OEP64. The Tic complex consists of as many as eight components, namely Tic22, Tic110, Tic40, Tic20, Tic21 Tic62, Tic55 and Tic32. This general Toc/Tic import pathway, worked out largely in pea chloroplasts, appears to operate in chloroplasts in all green plants, albeit with significant modifications. Sub-complexes of the Toc and Tic machineries are proposed to exist to satisfy different substrate-, tissue-, cell- and developmental requirements. In this review, we summarize our understanding of the functions of Toc and Tic components, comparing these components of the import machinery in green algae through trees. We emphasize recent findings that point to growing complexities of chloroplast protein import process, and use the evolutionary relationships between proteins of different species in an attempt to define the essential core translocon components and those more likely to be responsible for regulation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Non-native, N-terminal Hsp70 Molecular Motor Recognition Elements in Transit Peptides Support Plastid Protein Translocation

“…Unfortunately, without any additional data, these existing prediction tools are the best we have to predict Hsp70 binding. Multiple Chaperones May Drive Translocation-Although the mechanism of protein translocation into plastids is still poorly understood (4,(62)(63)(64)(65), most current models illustrate a vectorial transfer of the TP into the organelle with the N-ter entering the stroma first and the C-ter and the mature domain remaining behind. It has also been suggested that the TP enters and is translocated via the TOC and TIC translocons as an extended linear peptide devoid of secondary structure (66 -68).…”

Plastid-bearing sea slugs fix CO2in the light but do not require photosynthesis to survive

“…The predicted proteome of Arabidopsis plastids ranges from 1000 to approximately 3500 proteins [11], but plastid genomes only encode for 60 (higher plants) to 200 (red algae) protein-coding genes in the organelle's DNA [12]. The remaining plastid proteins (more than 90%) are encoded in the nucleus, synthesized as precursor proteins on cytosolic ribosomes and imported from the cytosol through the plastid-specific protein translocon machinery (reviewed in [13][14][15]). Because plakobranchids do not sequester algal nuclei, which can however be ingested for a short time during feeding [16], and because some proteins in higher plant chloroplasts can have turnover rates on the order of 30-120 min [17][18][19], it has been widely assumed that sequestered plastids of plakobranchids also require imported proteins to remain photosynthetically active.…”

Structural characterizations of chloroplast translocon protein Tic110

“…Tic110 is conserved from glaucophytes, red algae to flowering plants and is almost always encoded by a single gene, except in organisms like Physcomitrella patens in which the entire genome was recently duplicated (Kalanon and McFadden, 2008;Shi and Theg, 2013). We cloned and expressed Tic110 homologues from several flowering plants for crystallography studies, but without success.…”