Folding patterns of membrane proteins: diversity and the limitations of their prediction

Significantly more high resolution structures of membrane proteins, obtained either by X-ray analysis, electron crystallographic methods or, in the future, by NMR spectroscopy, will be required for reliable structure predictions. Aberrations from the motifs of alpha-helical bundles and beta-barrels occur that are not easily identified by algorithms unless structural homologies exist in the data banks. The coexistence of secondary structure motifs, originally proposed for a neurotransmitter receptor, has now been confirmed for a bacterial iron-siderophore translocating protein (FhuA) by X-ray analysis to 2.7 A resolution. This protein contains a plug domain that has both alpha- and beta-structures.