Preprotein recognition by the Toc complex

Abstract

The Toc core complex consists of the pore-forming Toc75 and the GTPases Toc159 and Toc34. We confirm that the receptor form of Toc159 is integrated into the membrane. The association of Toc34 to Toc75/Toc159 is GTP dependent and enhanced by preprotein interaction. The N-terminal half of the pSSU transit peptide interacts with high affinity with Toc159, whereas the C-terminal part stimulates its GTP hydrolysis. The phosphorylated C-terminal peptide of pSSU interacts strongly with Toc34 and therefore inhibits binding and translocation of pSSU into Toc proteoliposomes. In contrast, Toc159 recognises only the dephosphorylated forms. The N-terminal part of the pSSU presequence does not influence binding to the Toc complex, but is able to block import into proteoliposomes through its interaction with Toc159. We developed a model of differential presequence recognition by Toc34 and Toc159.

Chloroplast Omp85 proteins change orientation during evolution

“…Note that we only observed a slight reduction of Toc75 when treated with thermolysin (lane α-75 B ). This phenomenon was observed before and ascribed to the high abundance of the protein (43,44). The protease sensitivity of the receptors clearly showed the right-side-out orientation of the isolated OEVs.…”

Chloroplast protein import: solve the GTPase riddle for entry

“…It has long been known that GTP hydrolysis is required for protein import [2], which implicates the nucleotide-binding and hydrolysis activities of Toc159 and Toc34 as being key regulators of the import reaction. Becker et al [3] have now provided evidence to support a model for GTP-dependent preprotein recognition and translocation by the coordinated activities of Toc159 and Toc34.…”

A Pentapeptide Motif Related to a Pigment Binding Site in the Major Light-Harvesting Protein of Photosystem II, LHCII, Governs Substrate-Dependent Plastid Import of NADPH:Protochlorophyllide Oxidoreductase A

“…The driving force for the actual translocation across the membrane can be provided by the ribosome itself (cotranslational import of secretory proteins into the ER) or ATPpowered molecular chaperones on the trans-side of the target membrane (posttranslational transport into the ER, the mitochondrial matrix, and the chloroplast stroma; Schatz and Dobberstein, 1996;Wickner and Schekman, 2005;Rapoport, 2007). A TOC159-mediated, GTP-powered ''push'' of translocation of cytoslic precursors into chloroplasts has been proposed (Becker et al, 2004) but is highly disputed (Kessler and Schnell, 2004).…”