Sec-independent protein translocation in chloroplasts and bacteria

Abstract

The majority of chloroplast proteins are encoded in the nuclear genome and are synthesized in the cytoplasm. This means that successful assembly of the photosynthetic complexes requires the import of numerous proteins into the organelle. Thylakoid lumen proteins are initially synthesized as larger precursors with N-terminal bipartite presequences consisting of envelope transfer signals and thylakoid transfer signals in tandem. Thylakoid transfer signals resemble bacterial signal peptides in comprising of three distinct domains: an N-terminal basic domain, a hydrophobic core domain and a polar C-terminal domain ending in short chain −3 and −1 residues prior to the cleavage site for the thylakoidal processing peptidase (TPP). The development of in vitro protein import assays into isolated thylakoids revealed that lumenal proteins were translocated across the thylakoid membrane by either a Sec-dependent or a ΔpH-dependent translocase [1]. One group of proteins, including plastocyanin and 33K, require stromal SecA and ATP for import into the lumen; whilst the translocation of other lumenal proteins, including 23K and 16K are totally dependent on a ΔpH gradient across the thylakoid membrane. Despite their overall similarity, thylakoid transfer signals specify the translocation pathway adopted. Signals for the ΔpH-dependent system always contain a twin-arginine motif (Figure 1), which was shown by Chaddock et al. [2] to be essential, but not sufficient, for targeting by this system. In this study we propose that additional information in the H-domain is equally important in determining the mechanism of translocation adopted by a protein. We also show that a conserved charge distribution around the hydrophobic domain of 23K transfer peptide stops it from being recognized as a substrate for the Sec system.