ABSTRACT
Determination of the insertion energetics of transmembrane (TM) α-helices into membranes has proved difcult. The chief challenge is to overcome the tendency of nonpolar helices to aggregate and precipitate out of aqueous solution.1,2 So far this has been unsuccessful. Numerous alternative experimental and computational approaches have been presented over the last decades to obtain closely related transfer properties.3 Several approaches in particular have provided estimates of the energetics of protein insertion and stability (Figure 5.1). The rst is based on recent in vitro experiments using the Sec61 translocon (Figure 5.1a). Cells have conquered aggregation by means of the translocon machinery, consisting primarily of the SecY complex of membrane proteins in bacteria and archaea and the highly homologous Sec61 complex in eukaryotes. The SecY/Sec61 translocons receive nascent membrane chains directly from the ribosome and guide their insertion into the membrane cotranslationally. All available evidence suggests that the TM segments partition between the translocon complex and the lipid bilayer following physicochemical principles.4−6 The code, in the form of a biological hydrophobicity scale, is highly correlated with physical hydrophobicity scales determined, for example, from measurements of the partitioning of amino acids between water and n-octanol.7 Another recent set of experiments has used the folding and refolding capability of outer membrane phospholipase A (OmpLA) as a scaffold to determine the relative transfer free energies of amino acids into lipid bilayers (Figure 5.1b).8
