A NEW-TYPE OF SIGNAL PEPTIDE - CENTRAL ROLE OF A TWIN-ARGININE MOTIF IN TRANSFER SIGNALS FOR THE DELTA-PH-DEPENDENT THYLAKOIDAL PROTEIN TRANSLOCASE

The Delta pH-driven and Sec-related thylakoidal protein translocases r ecognise distinct types of thylakoid transfer signal, yet all transfer signals resemble bacterial signal peptides in structural terms, Compa rison of known transfer signals reveals a single concrete difference: signals for the Delta pH-dependent system contain a common twin-argini ne motif immediately before the hydrophobic region, We show that this motif is critical for the Delta pH-driven translocation process; subst itution of the arg-arg by gin-gin or even arglys totally blocks transl ocation across the thylakoid membrane, and replacement by lys-arg redu ces the rate of translocation by >100-fold. The targeting information in this type of signal thus differs fundamentally from that of bacteri al signal peptides, where the required positive charge can be supplied by any basic amino acid, Insertion of a twin-arg motif into a Sec-dep endent substrate does not alter the pathway followed but reduces trans location efficiency, suggesting that the motif may also repel the Sec- type system. Other information must help to specify the choice of tran slocation mechanism, but this information is unlikely to reside in the hydrophobic region because substitution by a hydrophobic section from an integral membrane protein does not affect the translocation pathwa y.