Flip to Insert: Conformational Freedom of C-terminal Transmembrane Helices

Helical membrane proteins make up a quarter of the proteome in every living organism, performing a huge array of essential functions. The transmembrane helices (TMHs) of most of these proteins are inserted co-translationally, giving each protein its unique membrane topology. This topology is assumed to be constrained by the membrane and remain unchanged after translation. However, our results suggest that some TMHs are surprisingly dynamic, and can flip across the membrane. This dynamic behavior is dependent on the hydrophilicity of the TMHs and their flanking loop.

Bioinformatic analysis of the sequences of E. coli membrane proteins demonstrates that C-terminal TMHs might have a higher propensity to flip across the membrane. Interestingly, mutations designed to decrease the ability of these potential “flippers” to flip, perturbed their ability to insert correctly into the membrane. We suggest that helix flipping might have a functional role in the insertion of a subset of TMHs that are unable to insert co-translationally. In such instances, a dynamic helix might be able to insert spontaneously after the protein is released from the ribosome, resulting in a mature active protein.