SEC SRP-INDEPENDENT INSERTION OF 2 THYLAKOID MEMBRANE-PROTEINS BEARING CLEAVABLE SIGNAL PEPTIDES/

Two imported thylakoid membrane proteins, PSII-X and PSII-W, are synth esised with cleavable N-terminal signal peptides that closely resemble those of Sec-dependent lumenal proteins. In this report we have recon stituted the insertion of pre-PSII-X and pre-PSII-W into isolated thyl akoids. We show that insertion does not require either nucleoside trip hosphates or stromal extracts, both of which are required for Sec- and signal recognition particle (SRP)-dependent targeting mechanisms. Ins ertion is furthermore unaffected by protease treatments that destroy t he 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 me discuss possible parallels with the biogenesis of phage M13 pro coat. (C) 1998 Federation of European Biochemical Societies.