How transmembranes tell of their approach

Nascent transmembrane sequences (TMSs) can, while still well inside the ribosome, fold into an α-helix, according to Cheryl Woolhead, Arthur Johnson, and colleagues (Texas A&M, College Station, TX). That α-helix appears to send a message through the ribosome to the bound protein translocation apparatus so that it knows when to open and close its various gates.

Making a secreted protein is easy: a ribosome docks to the ER translocon, forming a tight seal, and the protein threads through the translocon tunnel. But with transmembrane proteins, the ribosome and translocon must make space for the release of both the TMS into the membrane and any following cytoplasmic loop into the cytoplasm. Previous work suggested that an ion-tight seal is maintained by the binding of BiP to the lumenal surface of the translocon.

But...