Homology of beta-lactoglobulin, serum retinol-binding protein, and protein HC

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Science  19 Apr 1985:
Vol. 228, Issue 4697, pp. 335-337
DOI: 10.1126/science.2580349


The milk protein beta-lactoglobulin has been extensively studied but its function has not been identified. A clue regarding the function of a protein can be obtained by discovering a genetic relationship with a protein of known function through comparisons of amino acid sequence. Such comparisons revealed that beta-lactoglobulin is similar to human serum retinol-binding protein and to another human protein of unknown function known as complex-forming glycoprotein heterogeneous in charge (protein HC). beta-Lactoglobulins from several species have been found to bind retinol, while the absorption and fluorescence properties reported for the unidentified heterogeneous prosthetic group of protein HC are retinoid-like. The role of serum retinol-binding protein in vitamin A transport in the circulation suggests that the other two homologous proteins may function in the binding and transport of retinoids; beta-lactoglobulin may facilitate the absorption of vitamin A from milk and protein HC may mediate the excretion of retinol-derived metabolites.