InterPro domain: IPR012674

Calycins form a large protein superfamily that share similar beta-barrel structures. Calycins can be divided into families that include lipocalins, fatty acid binding proteins, triabin, and thrombin inhibitor [ 1 ]. Of these families, the lipocalin family ( IPR002345 ) is the largest and functionally the most diverse. Lipocalins are extracellular proteins that share several common recognition properties such as ligand binding, receptor binding and the formation of complexes with other macromolecules. Lipocalins include the retinol binding protein, lipocalin allergen, aphrodisin (a sex hormone), alpha-2U-globulin, prostaglandin D synthase, beta-lactoglobulin, bilin-binding protein, and the nitrophorins [ 2 , 3 , 4 , 5 ]. Bacterial hypothetical proteins YodA from Escherichia coli and YwiB from Bacillus subtilis share a similar calycin beta-barrel structure. Part of the YodA hypothetical protein has a calycin-like structure [ 6 ].

1. The lipocalin protein family: structural and sequence overview. Biochim. Biophys. Acta 1482, 9-24
2. Lipocalin-type and hematopoietic prostaglandin D synthases as a novel example of functional convergence. Prostaglandins Other Lipid Mediat. 68-69, 375-82
3. Major urinary proteins, alpha(2U)-globulins and aphrodisin. Biochim. Biophys. Acta 1482, 218-28
4. Immunocalins: a lipocalin subfamily that modulates immune and inflammatory responses. Biochim. Biophys. Acta 1482, 284-97
5. The core lipocalin, bovine beta-lactoglobulin. Biochim. Biophys. Acta 1482, 136-48
6. YodA from Escherichia coli is a metal-binding, lipocalin-like protein. J. Biol. Chem. 278, 43728-35