The ligand-binding site of bovine beta-lactoglobulin: evidence for a function?

Ever since the fortuitous observation that beta-lactoglobulin (beta-Lg), the major whey protein in the milk of ruminants, bound retinol, the details of the binding have been controversial. beta-Lg is a lipocalin, like plasma retinol-binding protein, so that ligand association was expected to make use of the central cavity in the protein. However, an early crystallographic analysis and some of the more recent solution studies indicated binding elsewhere. We have now determined the crystal structures of the complexes of the trigonal form of beta-Lg at pH 7.5 with bound retinol (R=21.4% for 7329 reflections between 20 and 2.4 A resolution, R(free)=30.6%) and with bound retinoic acid (R=22.7% for 7813 reflections between 20 and 2.34 A resolution, R(free)=29.8%). Both ligands are found to occupy the central calyx in a manner similar to retinol binding in retinol-binding protein. We find no evidence of binding at the putative external binding site in either of these structural analyses. Further, competition between palmitic acid and retinol reveals only palmitate bound to the protein. An explanation is provided for the lack of ligand binding to the orthorhombic crystal form also obtained at pH 7.5. Finally, the possible function of beta-Lg is discussed in the light of its species distribution and similarity to other lipocalins.