Biochim Biophys Acta Proteins Proteom 2019 Jun 3;1867(6):627-636. Epub 2019 Jan 3.
Department of Molecular Cell Biology of Plants, Goethe University, Max-von-Laue Str. 9, D-60438 Frankfurt, Germany; Frankfurt Institute for Advanced Studies, Ruth-Moufang-Straße 1, D-60438 Frankfurt, Germany; Cluster of Excellence Frankfurt, Goethe University, D-60438 Frankfurt, Germany; Buchmann Institute for Molecular Life Sciences, Goethe University, Max-von-Laue Str. 15, D-60438 Frankfurt, Germany. Electronic address:
The GTPases Toc159 and Toc34 of the translocon of the outer envelope of chloroplasts (TOC) are involved in recognition and transfer of precursor proteins at the cytosolic face of the organelle. Both proteins engage multiple interactions within the translocon during the translocation process, including dimeric states of their G-domains. The units of the Toc34 homodimer are involved in the recognition of the transit peptide representing the translocation signal of precursor proteins. This substrate recognition is part of the regulation of the GTPase cycle of Toc34. The Toc159 monomer and the Toc34 homodimer recognize the transit peptide of the small subunit of Rubisco at the N- and at the C-terminal region, respectively. Analysis of the transit peptide interaction by crosslinking shows that the heterodimer between both G-domains binds pSSU most efficiently. While substrate recognition by Toc34 homodimer was shown to regulate nucleotide exchange, we provide evidence that the high activation energy of the GTPase Toc159 is lowered by substrate recognition. The nucleotide affinity of Toc34 homodimer and Toc159 monomer are distinct, Toc34 homodimer recognizes GDP and Toc159 GTP with highest affinity. Moreover, the analysis of the nucleotide association rates of the monomeric and dimeric receptor units suggests that the heterodimer has an arrangement distinct from the homodimer of Toc34. Based on the biochemical parameters determined we propose a model for the order of events at the cytosolic side of TOC. The molecular processes described by this hypothesis range from transit peptide recognition to perception of the substrate by the translocation channel.
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