Protein translocon at the Arabidopsis outer chloroplast membrane

Abstract

Chloroplasts are organelles essential for the photoautotrophic growth of plants. Their biogenesis from undifferentiated proplastids is triggered by light and requires the import of hundreds of different precursor proteins from the cytoplasm. Cleavable N-terminal transit sequences target the precursors to the chloroplast where translocon complexes at the outer (Toc complex) and inner (Tic complex) envelope membranes enable their import. In pea, the Toc complex is trimeric consisting of two surface-exposed GTP-binding proteins (Toc159 and Toc34) involved in precursor recognition and Toc75 forming an aequeous protein-conducting channel. Completion of the Arabidopsis genome has revealed an unexpected complexity of predicted components of the Toc complex in this plant model organism: four genes encode homologs of Toc159, two encode homologs of Toc34, but only one encodes a likely functional homolog of Toc75. The availability of the genomic sequence data and powerful molecular genetic techniques in Arabidopsis set the stage to unravel the mechanisms of chloroplast protein import in unprecedented depth.

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

“…Ectopic expression of atToc34 rescues ppi1 indicating that atToc34 and atToc33 are functionally equivalent in vivo (Jarvis et al, 1998). There are four homologues of pea Toc159 in Arabidopsis, atToc159, -132, -120 and -90 (Hiltbrunner et al, 2001a;Jackson-Constan and Keegstra, 2001). The initial characterization of the last, atToc90, is presented in this study.…”

“…Apart from atToc159, -132 and -120 (Bauer et al, 2000) the Arabidopsis databases contain a fourth and previously uncharacterized homologue of Toc159 which, based on the sequence similarity, we designate a Toc protein (Hiltbrunner et al, 2001a). A full length ESTclone (H4C12T7) with a 2379 bp open reading frame (ORF) encoding the protein is available (Kieber et al, 1993), indicating that the gene is expressed.…”

“…D1 is highly conserved throughout the Arabidopsis family of Toc-GTPases (Figure 1c). Thus, conceivably all of these GTPases may be able to form homodimers as well as heterodimers in all permutations (Hiltbrunner et al, 2001a). We have recently demonstrated that soluble synthetic atToc159 binds directly to the purified recombinant G-domain of atToc33 (Hiltbrunner et al, 2001b) and that the interaction contributes to targeting of atToc159 to the Toc-complex.…”

“…Sequencing of the Arabidopsis genome revealed an unexpected complexity of Toc-components (The Arabidopsis Genome Initiative, 2000; Jackson-Constan and Hiltbrunner et al, 2001a). All of the known pea Tic-and Tocproteins have Arabidopsis homologues (JacksonConstan and .…”

“…All of the known pea Tic-and Tocproteins have Arabidopsis homologues (JacksonConstan and . Strikingly the Arabidopsis homologues of the Toc34 and Toc159 GTP-binding proteins are encoded by a small gene family (Jackson-Constan and Hiltbrunner et al, 2001a). 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).…”