Inversion of Signal Sequence Topology during Membrane Integration

Abstract

In many cases, the topology of membrane proteins is established during co-translational membrane integration. This process involves the Sec translocon, a heterotrimeric protein-conducting channel that allows for both the translocation of secreted domains across the membrane through the central pore and the integration of membrane domains directly into the lipid bilayer through a lateral opening. For many proteins, the N-terminus is retained on the cytosolic side of the membrane and forms an inverted (type II) topology that threads the C-terminus through the channel. This inverted topology is hypothesized to involve a head-first intermediate which then undergoes a step-wise inversion process to its final topology. We use a newly developed coarse-grained model to simulate the integration of the signal sequence during the elongation of the nascent chain on the minute-long timescales that are relevant to the biological process. This coarse-grained simulation method enables direct comparisons to experimentally measured energetics of ribosome-nascent chain to translocon interactions. We observe a series of pulling and pushing forces on the ribosome-nascent chain as translation proceeds and identify a head-first intermediate whose inversion is driven by the entropic confinement of nascent chain residues in the ribosome-translocon junction.