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Insertion and topology of the Toc159 protein import receptor of chloroplasts
Author(s) -
Schnell Danny,
Phan TheUyen,
Lee Jeonghwa,
Wang Fei,
Rounds Caleb
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.4.a518-c
Subject(s) - microbiology and biotechnology , hamp domain , gtpase , transmembrane domain , biology , cytoplasm , intermembrane space , bacterial outer membrane , egf like domain , transmembrane protein , biophysics , chemistry , binding domain , receptor , biochemistry , binding site , escherichia coli , gene
The multimeric Toc complex of the outer envelope membrane initiates the import of nucleus‐encoded preproteins into chloroplasts. Two receptor GTPases, Toc159 and Toc33/34, and a beta‐barrel membrane channel, Toc75, form the core of the Toc complex. The Toc159 GTPase plays at least two critical roles in the import process by serving as the primary preprotein receptor at the chloroplast surface and participating with Toc75 in forming the preprotein translocation channel. Toc159 is an unconventional membrane protein that is tightly associated with the outer envelope membrane but lacks apparent transmembrane regions. As a result, the topology and integration of the receptor at the Toc complex remains largely undefined. Toc159 has a tripartite domain structure composed of an N‐terminal acidic domain (A‐domain), a central GTPase domain (G‐domain) and a C‐terminal domain (M‐domain). The A‐ and G‐domains are proposed to face the cytoplasm with the M‐domain anchoring the receptor to the outer membrane. In this study, we have examined the targeting and integration of Toc159 into the outer membrane using a combination of deletion mutagenesis and protease protection assays. We show that Toc159 targeting is strictly dependent on its intrinsic GTPase activity. Furthermore, we show that integration of the receptor results in the translocation of its G‐domain into the intermembrane space, whereas the bulk of the A‐ and M‐domains are exposed to the cytoplasm. A short segment of the M‐domain that lies adjacent to the G‐domain is required for tight association of the receptor with the membrane. These results have dramatic implications for both the structure of the Toc complex and the function of the Toc159 receptor in the import process.