Insertion of membrane proteins into the ER - an update

by Gunnar von Heijne (DBB)

Wednesday 17 Mar 2010, 16:00 → 17:00 Europe/Stockholm

RB35 (RB35)

Despite their impressive structural and functional diversity, most membrane proteins share a common mechanism of membrane insertion. The Sec61 translocon in the ER (and its bacterial homologue, the SecYEG translocon) not only channels secreted proteins across the membrane, but also mediates the co-translational insertion of transmembrane proteins into the lipid bilayer. Even though an X-ray structure of an archaeal Sec61 complex is known, the actual process whereby incipient transmembrane alpha-helices in a nascent polypeptide are recognized by the translocon and shunted into the lipid bilayer remains a mystery. We have attacked this problem using a 'substrate engineering' approach, in which we challenge the Sec61 translocon with systematically designed potential transmembrane helices built on a very simple Ala-Leu framework. Analysis of a large panel of such designed segments has made it possible to derive a matrix that describes the position-specific contribution of each of the 20 amino acids to the overall free energy of insertion of a transmembrane helix. This experimentally derived matrix is surprisingly similar to the matrix of statistical free energy contributions that one can calculate from the position-specific frequencies of the different amino acids in the known high-resolution membrane protein structures. To a first approximation, the 'molecular code' for transmembrane-helix recognition by the Sec61 translocon appears to be surprisingly simple, suggesting that the translocon allows the nascent polypeptide chain to sample the surrounding bilayer as it passes through the translocation channel.