Cotranslational folding and assembly of the dimeric <i>E. coli</i> 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 possible also for integral membrane proteins is unknown, however. 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 E. 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 E 14 in TMH1 and S 64 in TMH3 that forms at a precise nascent chain length during cotranslational membrane insertion of an EmrE monomer. Like soluble proteins, inner membrane proteins can thus both start to fold and start to dimerize during the cotranslational membrane-insertion process. Significance statement Many water-soluble proteins are known to fold and even dimerize cotranslationally, i . e ., when still attached to the ribosome. Here, we show that an E. coli inner membrane protein can also start to fold and dimerize cotranslationally, establishing the generality of these cotranslational maturation processes.