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A helix–turn–strand structural motif common in α–β proteins

By exhaustive structural comparisons, we have found that about one‐third of the α‐helix–turn–β–strand polypeptides in α–β barrel domains share a common structural motif. The chief characteristics of this motif are that first, the geometry of the turn between the α‐helix and the β‐strand is somewhat constrained, and second, the β‐strand contains a hydrophobic patch that fits into a hydrophobic pocket on the α‐helix. The geometry of the turn does not seem to be a major determinant of the α–β unit, because the turns vary in length from four to six residues. However, the motif does not occur when there are few constraints on the geometry of the turn–for instance, when the turns between the α‐helix and the β‐strands are very long. It also occurs much less frequently in flat‐sheet α–β proteins, where the topology is much less regular and the amount of twist on the sheet varies considerably more than in the barrel proteins. The motif may be one of the basic building blocks from which α–β barrels are constructed.