Calycins form a large protein superfamily that share a common beta-barrel core structure. Calycins include several protein families such as lipocalins, fatty acid binding proteins PMID:16700541, avidins, triabin, thrombin inhibitor PMID:9342325 and metalloprotease inhibitors (MPIs). This superfamily entry represents the beta-barrel domain found in a variety of protein structures within the calycin superfamily, in particular in lipocalins. Lipocalins INTERPRO:IPR002345 are largest family of calycins, having a highly conserved core yet exhibiting great structural and functional variation. Usually lipocalins display unusually low levels of overall sequence conservation.

The common structure of the lipocalin protein fold is a single eight-stranded (A-H) antiparallel beta-sheet linked by seven beta-hairpin loops (L1-L7), forming a continuously hydrogen-bonded beta-barrel. Between strand H and the short terminal strand I there is an alpha-helix which is often found in proteins of the lipocalin fold. The diversity of the ligand-binding site within the beta-barrel structure gives rise to a variety of different binding modes each capable of accommodating ligands of different size, shape, and chemical character.

Lipocalins are extracellular proteins that share several common recognition properties such as ligand binding, receptor binding and the formation of complexes with other macromolecules. Functions of these proteins include transport of nutrients, control of cell regulation, pheromone transport, cryptic colouration, and the enzymatic synthesis of prostaglandins. Lipocalins include the retinol binding protein, lipocalin allergen, aphrodisin (a sex hormone), alpha-2U-globulin, prostaglandin D synthase, beta-lactoglobulin, bilin-binding protein, nitrophorins INTERPRO:IPR023613, luciferase, bacterial YodA from E. coli PMID:12909634 and YwiB from Bacillus subtilis. Most lipocalins share three characteristic conserved sequence motifs - the kernel lipocalins - while other more divergent family members - the outlier lipocalins - typically share only one or two.

The FABPs are a family of predominantly intracellular proteins involved in lipid metabolism, including intracellular lipid binding, transport of lipids through the cytosol, protection of polyunsaturated fatty acids, regulation of the postprandial lipid metabolism, influences on insulin resistance and atherosclerosis. The avidins are proteins with a remarkable affinity for the vitamin biotin. Both MPIs and triabin function as proteinase inhibitors, as do some lipocalins.

INTERPRO:IPR012674, Pfam clan [PfamClan:CL0116],PMID:11058743,PMID:15234551