Membrane protein folding and oligomerization: the two-stage model.

We discuss the view that the folding of many, perhaps most, integral membrane proteins can be considered as a two-stage process. In stage I, hydrophobic alpha-helices are established across the lipid bilayer. In stage II, they interact to form functional transmembrane structures. This model is suggested by the nature of transmembrane segments in known structures, refolding experiments, the assembly of integral membrane protein from fragments, and the existence of very small integral membrane protein subunits. It may extend to proteins with a variety of functions, including the formation of transmembrane aqueous channels. The model is discussed in the context of the forces involved in membrane protein folding and the interpretation of sequence data.