Cotranslational folding and assembly of the dimeric Escherichia coli inner membrane protein EmrE

In recent years, it has become clear that many homo- and heterodimeric cytoplasmic proteins in both prokaryotic and eukaryotic cells start to dimerize cotranslationally (i.e., while at least one of the two chains is still attached to the ribosome). Whether this is also possible for integral membrane proteins is, however, unknown. Here, we apply force profile analysis (FPA)-a method where a translational arrest peptide (AP) engineered into the polypeptide chain is used to detect force generated on the nascent chain during membrane insertion-to demonstrate cotranslational interactions between a fully membrane-inserted monomer and a nascent, ribosome-tethered monomer of the Escherichia coli inner membrane protein EmrE. Similar cotranslational interactions are also seen when the two monomers are fused into a single polypeptide. Further, we uncover an apparent intrachain interaction between E¹⁴ in transmembrane helix 1 (TMH1) and S⁶⁴ in TMH3 that forms at a precise nascent chain length during cotranslational membrane insertion of an EmrE monomer. Like soluble proteins, inner membrane proteins thus appear to be able to both start to fold and start to dimerize during the cotranslational membrane insertion process.