Proteins which transport small hydrophobic molecules such as steroids, bilins,
retinoids, and lipids share limited regions of sequence homology and a common
tertiary structure architecture [1,2,3,4,5]. This is an eight stranded
antiparallel β-barrel with a repeated + 1 topology enclosing a internal
ligand binding site [1,3]. The name 'lipocalin' has been proposed  for
this protein family. Proteins known to belong to this family are listed below
(references are only provided for recently determined sequences).
α-1-microglobulin (protein HC), which seems to bind porphyrin.
α-1-acid glycoprotein (orosomucoid), which can bind a remarkable array
of natural and synthetic compounds .
Aphrodisin which, in hamsters, functions as an aphrodisiac pheromone.
Apolipoprotein D, which probably binds heme-related compounds.
β-lactoglobulin, a milk protein whose physiological function appears to
Complement component C8 γ chain, which seems to bind retinol .
Crustacyanin , a protein from lobster carapace, which binds astaxanthin,
Epididymal-retinoic acid binding protein (E-RABP)  involved in sperm
Insectacyanin, a moth bilin-binding protein, and a related butterfly bilin-
binding protein (BBP).
Late Lactation protein (LALP), a milk protein from tammar wallaby .
The sequences of most members of the family, the core or kernal lipocalins,
are characterized by three short conserved stretches of residues [3,18].
Others, the outlier lipocalin group, share only one or two of these [3,18]. A
signature pattern was built around the first, common to all outlier and kernal
lipocalins, which occurs near the start of the first β-strand.
It is suggested, on the basis of similarities of structure, function,
and sequence, that this family forms an overall superfamily, called the
calycins, with the avidin/streptavidin <PDOC00499> and the cytosolic fatty-
acid binding proteins <PDOC00188> families [3,19].
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