Biogenesis of polytopic membrane proteins: membrane segments of P-glycoprotein sequentially translocate to span the ER membrane.

The initial steps in the biogenesis of membrane proteins parallel those of secretory proteins. However, membrane proteins contain a signal to stop translocation across the membrane. For polytopic membrane proteins, those with multiple transmembrane segments, little is known of the temporal sequence or relationship between synthesis of the nascent proteins, translocation, folding, and integration of the membrane segments into the bilayer. Here we demonstrate that latent membrane segments translocate sequentially as they emerge from the ribosome and do not accumulate on the cytosolic side to form loops, or larger structures, prior to translocation across the membrane.