Sec/SRP-independent insertion of two thylakoid membrane proteins bearing cleavable signal peptides

Two imported thylakoid membrane proteins, PSII-X and PSII-W, are synthesised with cleavable N-terminal signal peptides that closely resemble those of Sec-dependent lumenal proteins. In this report we have reconstituted the insertion of pre-PSII-X and pre-PSII-W into isolated thylakoids. We show that insertion does not require either nucleoside triphosphates or stromal extracts, both of which are required for Sec- and signal recognition particle (SRP)-dependent targeting mechanisms. Insertion is furthermore unaffected by protease treatments that destroy the known protein translocation apparatus in the thylakoid membrane. We conclude that these membrane proteins are inserted by an unusual Sec/SRP-independent mechanism that probably resembles that used by CFoII, and we discuss possible parallels with the biogenesis of phage M13 procoat.