Bacterial proteins carrying twin-R signal peptides are specifically targeted by the Delta pH-dependent transport machinery of the thylakoid membrane system

Glucose-fructose oxidoreductase (GFOR), a periplasmic protein of Zymomonas mobilis, is synthesized as a precursor polypeptide with a twin-R signal pep tide for Sec-independent protein export in bacteria. In higher plant chloro plasts, twin-R signal peptides are specific targeting signals for the Sec-i ndependent Delta pH pathway of the thylakoid membrane system. In agreement with the assumed common phylogenetic origin of the two protein transport me chanisms, GFOR can be efficiently translocated by the Delta pH-dependent pa thway when analyzed with isolated thylakoid membranes. Transport is sensiti ve to the ionophore nigericin and competes with specific substrates for the Delta pH-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 invol ved in this process. Mutagenesis of the twin-R motif in the GFOR signal pep tide prevents membrane translocation of the protein emphasizing the importa nce of these residues for the transport process. (C) 1999 Federation of Eur opean Biochemical Societies.