Dynamic membrane topology in an unassembled membrane protein

Helical membrane proteins constitute roughly a quarter of all proteomes and perform diverse biological functions. To avoid aggregation, they undergo cotranslational membrane insertion and are typically assumed to attain stable transmembrane topologies immediately upon insertion. To what extent post-translational changes in topology are possible in-vivo and how they may affect biogenesis is incompletely understood. Here, we show that monomeric forms of Small Multidrug Resistance (SMR) proteins display topological dynamics, where the N-terminal transmembrane helix equilibrates between membrane-inserted and non-inserted states. We characterize the kinetics of the process and show how the composition of the helix regulates the topological dynamics. We further show that topological dynamics is a property of the unassembled monomeric protein, as the N-terminal helix becomes fixed in a transmembrane disposition upon dimerization. Membrane protein topology can thus remain dynamic long after cotranslational membrane insertion, and can be regulated by later assembly processes.