Bacterial proteins carrying twin‐R signal peptides are specifically targeted by the ΔpH‐dependent transport machinery of the thylakoid membrane system

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Abstract
Glucose-fructose oxidoreductase (GFOR), a periplasmic protein of Zymomonas mobilis, is synthesized as a precursor polypeptide with a twin-R signal peptide for Sec-independent protein export in bacteria. In higher plant chloroplasts, twin-R signal peptides are specific targeting signals for the Sec-independent ΔpH pathway of the thylakoid membrane system. In agreement with the assumed common phylogenetic origin of the two protein transport mechanisms, GFOR can be efficiently translocated by the ΔpH-dependent pathway when analyzed with isolated thylakoid membranes. Transport is sensitive to the ionophore nigericin and competes with specific substrates for the ΔpH-dependent transport route. In contrast, neither sodium azide nor enzymatic destruction of the nucleoside triphosphates in the assays affects thylakoid transport of GFOR indicating that the Sec apparatus is not involved in this process. Mutagenesis of the twin-R motif in the GFOR signal peptide prevents membrane translocation of the protein emphasizing the importance of these residues for the transport process.