The lipocalin protein family: structural and sequence overview

Lipocalins are remarkably diverse at the sequence level yet have highly conserved structures. Most lipocalins share three characteristic conserved sequence motifs – the kernel lipocalins – while others are more divergent family members – the outlier lipocalins – typically sharing only one or two. This classification is a useful tool for analysing the family, and within these large sets are smaller groups sharing much higher levels of sequence similarity. The lipocalins are also part of a larger protein superfamily: the calycins, which includes the fatty acid binding proteins, avidins, a group of metalloproteinase inhibitors, and triabin. The superfamily is characterised by a similar structure (a repeated +1 topology β-barrel) and by the conservation of a remarkable structural signature.