Import of membrane proteins into the ER requires a joint effort between the ribosome and the translocon to ensure that each domain of the translocating protein is targeted correctly to either the lumenal or cytoplasmic face of the ER. Transmembrane domains present an additional problem—the hydrophobic portion must move laterally past the translocon into the lipid bilayer. Current models suggest that membrane-spanning domains have very limited contacts with translocon proteins and are instead rapidly surrounded by phospholipids. But the new results show that the translocon is more than a passer-by in this process.
Johnson's group shows that imported transmembrane domains make prolonged contacts with translocon subunits. Photo-crosslinking experiments reveal that one side of a transmembrane helix contacts the translocon protein Sec61α, even up until translation is nearly complete. Exact binding sites on translocon proteins varied with the transmembrane sequence, but all sequences tested, including subsequent transmembrane domains, showed prolonged contacts with Sec61α.
As in past experiments, the sequences also showed rapid cross-linking with phospholipids. But this does not exclude translocon protein involvement. Rather, says Johnson, “this implies that phospholipids fill in holes in the translocon to avoid a vacuum as it expands to allow the transmembrane domain to move from the pore into the bilayer.”
What induces protein release from the translocon is not clear. The affinity of individual transmembrane domain–translocon interactions may dictate the timing of the release. Incoming transmembrane segments may also push out previous domains. If so, orchestrated binding and release may allow the ribosome and translocon to integrate every pair of transmembrane domains correctly in opposite orientations. ▪