Major coat proteins of bacteriophage Pf3 and M13 as model systems for Sec-independent protein transport

The membrane insertion of bacteriophage coat proteins occurs independent of the Sec-translocase of Escherichia coli. Detailed study of the Pf3 and M13 coat proteins has elucidated two fundamental mechanisms of how proteins invade the membrane, most likely by direct interaction with the lipid bilayer. The Sec-independent translocation of amino-terminal regions across the inner membrane is limited to a short length and a small number of charged residues. Protein regions that contain several charged residues are efficiently translocated across the membrane when these regions are flanked by two adjacent hydrophobic segments interacting synergistically. The relevance of these findings for the membrane insertion mechanism of multispanning membrane proteins is discussed.