A comprehensive understanding of structure-function relationships of protei
ns requires their structures to be elucidated to high resolution. With most
membrane proteins this has not been accomplished so far, mainly because of
their notoriously poor crystallizability, Here we present a completely det
ergent-free procedure for the incorporation of a native purple membrane int
o a monoolein-based lipidic cubic phase, and subsequent crystallization of
three-dimensional bacteriorhodopsin crystals therein, These crystals exhibi
t comparable X-ray diffraction quality and mosaicity, and identical crystal
habit and space group to those of bacteriorhodopsin crystals that are grow
n from detergent-solubilized protein in cubic phase. (C) 1999 Federation of
European Biochemical Societies.