Upstream charged and hydrophobic residues impact the timing of membrane insertion of transmembrane helices

During SecYEG‐mediated cotranslational insertion of membrane proteins, transmembrane helices (TMHs) first make contact with the membrane when their N‐terminal end is ~ 45 residues away from the peptidyl transferase centre. However, we recently uncovered instances where the first contact is delayed by up to ~ 10 residues. Here, we recapitulate these effects using a model TMH fused to two short segments from the Escherichia coli inner membrane protein BtuC: a positively charged loop and a re‐entrant loop. We show that the critical residues are two Arg residues in the positively charged loop and four hydrophobic residues in the re‐entrant loop. Thus, both electrostatic and hydrophobic interactions involving sequence elements that are not part of a TMH can impact the way the latter behaves during membrane insertion.