Folding and Assembly of Integral Membrane Proteins: An Introduction

As recently as 1980, the structure of integral membrane proteins was largely terra incognita None of them had yielded three-dimensional crystals, and determination of the amino acid sequence of bacteriorhodopsin (BR) by the groups of Yu. A. Ovchinnikov (Ovchinnikov et al., 1979) and H. G. Khorana (Khorana et al., 1979) had been a biochemical tour de force The most detailed three-dimensional information available was—and was to remain until 1985—the medium-resolution structure of BR that R. Henderson and N. Unwin (1975) had established by electron microscopy. The electron density map showed the bulk of BR to be made up of a bundle of transmembrane α-helices. Since spectroscopic data on bacterial outer membrane porins (Rosenbusch, 1974) indicated that these integral proteins were primarily made up of β-sheets, it was clear that the model offered by BR was not universally transposable.