The "dark side" of β-lactoglobulin: unedited structural features suggest unexpected functions

The in-depth characterization of water buffalo (WB) whey proteins based on chromatographic and mass spectrometric techniques revealed unexpected structural co- and post-translational modifications for β-lactoglobulin (β-Lg). The residues Lys⁴⁷ and Lys⁶⁹ of β-Lg were found to be lactosylated early, at the time of milking. Thiol groups of β-Lg underwent a dynamic sulfhydryl/disulfide exchange that is probably essential in accomplishing specific physiological requirements in which proteins may alternatively act either as a trigger or as a target. In this sense, the free sulfhydryl group of β-Lg established a glutathionylation/deglutathionylation equilibrium, which could be functional in conveying and delivering glutathione. Furthermore, the N-lauroylated β-Lg occurring exclusively in WB milk has been characterized for the first time. N-acylation could be an evolutionary remnant of ancestral lipocalins. Combined with the known aptitude of β-Lg to interact with phospholipid bilayers, this suggests that the protein could also be involved in the membrane translocation of small molecules, in addition to targeting, trafficking or the maintenance of membrane integrity. This structural characterization of β-Lg adds to the currently existing data and expands our understanding of the possible biological roles of this enigmatic protein.