The structure of β-lactoglobulin and its similarity to plasma retinol-binding protein

Since its first isolation1, bovine β-lactoglobulin (BLG) has been an enigma: although it is abundant in the whey fraction of milk, its function is still not clear. The results of the many physicochemical studies on the protein need a structural interpretation. We report here the structure of the orthorhombic crystal form of cow BLG at pH 7.6, at a resolution of 2.8 Å. It has an unusual protein fold, composed of two slabs of antiparallel β-sheet, which shows a remarkable similarity to plasma retinol-binding protein. A possible binding site for retinol in BLG has been identified by model-building. This suggests a role for BLG in vitamin A transport and we have discovered specific receptors for the BLG–retinol complex in the intestine of neonate calves.