Targeting of thylakoid proteins by the ΔpH‐driven twin‐arginine translocation pathway requires a specific signal in the hydrophobic domain in conjunction with the twin‐arginine motif

Superficially similar cleavable targeting signals specify whether lumenal proteins are transported across the thylakoid membrane by a Sec‐ or ΔpH‐dependent pathway. A twin‐arginine motif is essential but not sufficient to direct ΔpH‐dependent targeting; here we show that a second determinant is located in the hydrophobic region. A highly hydrophobic amino acid is found either two or three residues C‐terminal to the twin‐arginine in all known transfer peptides for the ΔpH‐dependent system, and substitution of this residue in the 23‐kDa (23K) peptide markedly inhibits translocation. Further, whereas the insertion of twin‐arginine in a Sec‐dependent precursor does not permit efficient ΔpH‐dependent targeting, the simultaneous presence of a leucine at the +3 position (relative to the RR) enables the peptide to function as efficiently as an authentic transfer peptide. RRNVL, RRAAL and RRALA within a Sec targeting signal all support efficient ΔpH‐dependent targeting, RRNVA is less effective and RRNAA/RRNAG are totally ineffective. We conclude that the core signal for this pathway is a twin‐arginine together with an adjacent hydrophobic determinant.

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