The thylakoid Delta pH-dependent pathway machinery facilitates RR-independent N-tail protein integration

The thylakoidal Delta pH-dependent and bacterial twin arginine transport sy stems are structurally and functionally related protein export machineries. These recently discovered systems have been shown to transport folded prot eins brit are not known to assemble integral membrane proteins. We determin ed the translocation pathway of a thylakoidal FtsH homologue, plastid fusio n/protein translocation factor, which is synthesized with a chloroplast-tar geting peptide, a hydrophobic signal peptide, and a hydrophobic membrane an chor. The twin arginine motif in its signal peptide and its sole integratio n requirement of a Delta pH suggested that plastid fusion/protein transloca tion factor employs the Delta pH pathway. Surprisingly, changing the twin a rginine to twin lysine or deleting the signal peptide did not abrogate inte gration capability or characteristics. Nevertheless, three criteria argue t hat all three forms require the Delta pH pathway for integration. First, in tegration was competed by an authentic Delta pH pathway precursor. Second, antibodies to Delta pH pathway component Hcf106 specifically inhibited inte gration. Finally, chloroplasts from the hcf106 null mutant were unable to i ntegrate Pftf into their thylakoids. Thus, Delta pH pathway machinery facil itates both signal peptide-directed and N-tail-mediated membrane integratio n and does not strictly require the twin arginine motif.