The major protein import receptor of plastids is essential for chloroplast biogenesis

Abstract

Light triggers the developmental programme in plants that leads to the production of photosynthetically active chloroplasts from non-photosynthetic proplastids. During this chloroplast biogenesis, the photosynthetic apparatus is rapidly assembled, mostly from nuclear-encoded imported proteins, which are synthesized in the cytosol as precursors with cleavable amino-terminal targeting sequences called transit sequences. Protein translocon complexes at the outer (Toc complex) and inner (Tic complex) envelope membranes recognize these transit sequences, leading to the precursors being imported. The Toc complex in the pea consists of three major components, Toc75, Toc34 and Toc159 (formerly termed Toc86). Toc159, which is an integral membrane GTPase, functions as a transit-sequence receptor. Here we show that Arabidopsis thaliana Toc159 (atToc159) is essential for the biogenesis of chloroplasts. In an Arabidopsis mutant (ppi2) that lacks atToc159, photosynthetic proteins that are normally abundant are transcriptionally repressed, and are found in much smaller amounts in the plastids, although ppi2 does not affect either the expression or the import of less abundant non-photosynthetic plastid proteins. These findings indicate that atToc159 is required for the quantitative import of photosynthetic proteins. Two proteins that are related to atToc159 (atToc120 and atToc132) probably help to maintain basal protein import in ppi2, and so constitute components of alternative, atToc159-independent import pathways.

AtToc90, a New GTP-Binding Component of the Arabidopsis Chloroplast Protein Import Machinery

“…The PCR product was then ligated into the NcoI/EcoRI site of pET21d resulting in pET21d-atToc90. pET21d-atToc159 has been described (Bauer et al, 2000). pPCR-ScriptatToc159 1-731 , which contains the A-domain of atToc159 under the control of the T7 promoter, has been described (Bauer et al, 2002).…”

“…AtToc33 and atToc34, the two homologues of pea Toc34, consist of a soluble GTP-binding domain (G-domain), containing GTP-binding motifs, followed by a C-terminal hydrophobic transmembrane helix anchoring the proteins in the outer membrane, while the G-domain faces the cytosol (Jarvis et al, 1998). AtToc159, atToc132 and atToc120, highly homologous to pea Toc159, have a tripartite structure (Bauer et al, 2000). In this group of proteins, the G-domain is flanked by an N-terminal A-domain (acidic domain) of unknown function.…”

“…The initial characterization of the last, atToc90, is presented in this study. Disruption of atToc159 in the ppi2 mutant causes a lethal block in chloroplast biogenesis resulting in albino seedlings lacking thylakoids and starch granules from their undifferentiated plastids (Bauer et al, 2000). Furthermore, ppi2 plastids fail to normally accumulate highly abundant photosynthetic proteins (Bauer et al, 2000).…”

“…Disruption of atToc159 in the ppi2 mutant causes a lethal block in chloroplast biogenesis resulting in albino seedlings lacking thylakoids and starch granules from their undifferentiated plastids (Bauer et al, 2000). Furthermore, ppi2 plastids fail to normally accumulate highly abundant photosynthetic proteins (Bauer et al, 2000). Nevertheless, ppi2 plants have basal import activity as residual amounts of photosynthetic proteins as well as wild type levels of non-photosynthetic proteins are imported into the undifferentiated plastids (Bauer et al, 2000).…”

“…Furthermore, ppi2 plastids fail to normally accumulate highly abundant photosynthetic proteins (Bauer et al, 2000). Nevertheless, ppi2 plants have basal import activity as residual amounts of photosynthetic proteins as well as wild type levels of non-photosynthetic proteins are imported into the undifferentiated plastids (Bauer et al, 2000). Thus, atToc159 is not required for the import of constitutive proteins and even its function in the import of photosynthetic proteins is partially substituted for, presumably by the activity of its homologues atToc132, -120 and -90 (Bauer et al, 2001).…”