BIOGENESIS OF POLYTOPIC MEMBRANE-PROTEINS - MEMBRANE SEGMENTS ASSEMBLE WITHIN TRANSLOCATION CHANNELS PRIOR TO MEMBRANE INTEGRATION

The initial steps in the biogenesis of membrane proteins parallel that of secretory proteins. The translocation of membrane proteins, howeve r, must be interrupted prior to the complete traversal of the membrane . This is followed by their folding and integrating into the lipid bil ayer. We have previously shown that as each latent transmembrane segme nt (TMS) in a polytopic membrane protein emerges from the ribosome, it sequentially translocates across the membrane. Here we demonstrate th at these translocated TMSs can be extracted from the membrane with ure a. This suggests that nascent TMSs do not integrate into the bilayer a s they achieve a transmembrane topography. The integration is delayed until after the protein is synthesized and released from the ribosome. Prior to insertion into the bilayer, these TMSs appear to be stabiliz ed by salt-sensitive electrostatic bonds within an aqueous-accessible compartment.