Self-assembly is a hallmark of Biology. We are far from a complete understanding of this natural assembly, which in turn limits our ability to mimic biological construction in the bioengineering of tuneable synthetic systems.This proposal addresses the major challenge of membrane protein folding. Here, I intend to make a step change to my pioneering biophysical studies and investigate co-translational membrane protein folding. A central feature will be the creation of synthetic systems to probe key events in co-translational folding, as the protein folds in the membrane whilst emerging from the ribosome. An ambitious target is to make these systems tuneable, which will provide a new tool for the fabrication of membrane proteins and artificial cells in Synthetic Biology. The assembly of a tuneable artificial module that affords control over membrane protein synthesis is unprecedented.I focus on the ubiquitous superfamily of major facilitator proteins, namely the best studied family member, lactose permease, LacY. This proposal has state of the art biophysical mechanistic studies at its core, which interleave into Cell and Synthetic Biology. There are two themes:Theme 1. Determination of fundamental membrane protein folding parameters: folding transition states and lipid controlPhi-value analysis will be used to probe the folding transition state of LacY; the first such analysis of a multi-domain membrane protein. Lipid parameters that control LacY folding will be quantified, including bilayer asymmetry using novel droplet interface bilayer methods.Theme 2: construction of tuneable synthetic co-translational folding systemsEngineered ribosomes and translocon insertion machinery will be incorporated and LacY folding will be controlled. Translation will be regulated or halted using mutant ribosomes, arrest sequences, altered codon usage and controlling tRNA addition. Trapped LacY folding intermediates will be studied using biophysical methods.