Integral membrane proteins adopt diverse structures with different stabilites, dynamics and oligomeric states. It is unknown how much of their folding is dictated by the amino acid sequence and how much by the membrane environment. Membrane proteins account for about 25-30% of cellular proteins and their structures are largely hydrophobic and dominated by transmembrane helical bundles. Successful approaches to fold helical membrane proteins have been developed together with advances in kinetic studies in vitro. We have combined kinetic, thermodynamic and mutageneis in a study of folding which reveals the reaction free energy and a means to probe the transition state and demonstrates correlations with cellular folding. We also develop in vitro lipid bicelle and bilayer folding systems for membrane proteins. Bicelle properties, as well as the stored curvature elastic stress of model bilayers can be used to optimise the rate, yield and stability of folded protein. We have shown that events such as transmembrane helix insertion, as well as tertiary and quaternary structure formation are altered by the stored curvature stress of the bilayer. We are also progressing our studies to more complex, larger and multi-subunit proteins.1. Di Bartolo ND, Hvorup RN, Locher KP, Booth PJ: In vitro folding and assembly of the Escherichia coli ATP binding cassette transporter, BtuCD. J Biol Chem 2011, 286:18807-18815.2. Curnow P, Di Bartolo ND, Moreton KM, Ajoje OO, Saggese NP, Booth PJ: Stable folding core in the folding transition state of an alpha-helical integral membrane protein. Proc Natl Acad Sci U S A 2011, 108:14133-14138.