Determinants of membrane protein integration mediated by the Sec translocon

Abstract

The cellular membrane is a soft biol. interface that surrounds the cell interior and regulates interactions with the outside environment. Embedded within the membrane are proteins that are essential for cellular functions including signal transduction, material transport, and energy conversion. Performing these functions requires the proteins to correctly integrate within the membrane. The integration of most membrane proteins proceeds via the Sec translocon, a conserved protein-conducting channel that allows a nascent protein chain to access the membrane while being fed into the channel during protein synthesis. Previous studies have established that Sec-facilitated protein integration depends on amino-acid sequence properties, including nascent chain hydrophobicity and charge, and is also influenced by the dynamics of protein synthesis on a second-minute timescale. Correct membrane protein integration thus depends on physicochem. factors that span a wide range of time- and length-scales, and is further coupled to interactions with the membrane itself; however, mechanistic details of this process are still largely unknown. Here, we present a simulation strategy to investigate the Sec-facilitated integration of membrane proteins on realistic biol. timescales. We employ a novel coarse-grained simulation model that enables access to a timescale of minutes while retaining sufficient chem. accuracy to capture the forces that drive membrane integration. In particular, the model is able to describe membrane integration processes that are governed either by thermodn. or kinetics, and it provides a means of understanding the competition between such effects. We show that the model can predict the integration of both single- and multi-spanning membrane proteins in strong agreement with available expts. In particular, we provide mechanistic insight into the ability of certain proteins to integrate in two possible orientations with respect to the membrane (1), or to integrate despite having marginally hydrophobic transmembrane domains. These studies highlight the power of the model to connect sequence-level determinants of membrane protein integration to the mesoscale behavior obsd. exptl.