Chaperone-Assisted Post-Translational Transport of Plastidic Type I Signal Peptidase 1 [Membrane Biology]

October 7th, 2015 by Endow, J. K., Singhal, R., Fernandez, D. E., Inoue, K.

Type I signal peptidase (SPase I) is an integral membrane Ser/Lys-protease with one or two transmembrane domains (TMDs), cleaving transport signals off translocated precursor proteins. The catalytic domain of SPase I folds to form a hydrophobic surface and inserts into the lipid bilayers at the trans-side of the membrane. In bacteria, SPase I is targeted co-translationally and the catalytic domain remains unfolded until it reaches the periplasm. By contrast, SPases I in eukaryotes are targeted post-translationally, requiring an alternative strategy to prevent premature folding. Here we demonstrate that two distinct stromal components are involved in posttranslational transport of plastidic SPase I (Plsp1) from Arabidopsis thaliana which contains a single TMD. During import into isolated chloroplasts, Plsp1 was targeted to the membrane via a soluble intermediate in an ATP-hydrolysis dependent manner. Insertion of Plsp1 into isolated chloroplast membranes, by contrast, was found to occur by two distinct mechanisms. The first mechanism requires ATP hydrolysis and the protein conducting channel cpSecY1, and was strongly enhanced by exogenously-added cpSecA1. The second mechanism was independent of nucleoside triphosphates and proteinaceous components but with a high frequency of mis-orientation. This unassisted insertion was inhibited by urea and stroma extract. During import-chase assays using intact chloroplasts, Plsp1 was incorporated into a soluble 700-kD complex that co-migrated with the CPN60 complex before inserting into the membrane. The TMD within Plsp1 was required for the cpSecA1-dependent insertion but was dispensable for association with the 700-kD complex and also for unassisted membrane insertion. These results indicate cooperation of Cpn60 and cpSecA1 for proper membrane insertion of Plsp1 by cpSecY1.