β-Lactoglobulin Identified in Marsupial Milk. The Primary Structure, Binding Site and possible Function of β-Lactoglobulin from Eastern Grey Kangaroo(Macropus giganteus)

Abstract

.beta.-Lactoglobulin has been isolated in the milk of the Eastern Grey Kangaroo (Macropus giganteus). This is the first time this protein has been reported to be in the milk of marsupials. The complete amino-acid sequence has been determined by spinning cup and pulsed liquid phase microsequencing of the protein and peptides after enzymatic or cyanogen bromide cleavages. The 155-residue protein is the shortest .beta.-lactoglobulin so far sequenced. When the kangaroo protein is included in a comparison of the members of the .beta.-lactoglobulin family, the percentage of residues common to all members is reduced from 33% to 13%. Despite the large number of accumulated amino-acid exchanges the protein exists as a dimer and shows higher homology to the usually very conservative dimeric, ruminant .beta.-lactoglobulins than to the monomeric protein from monogastrics. Half-cystine residues that form disulphide bridges are conserved. The Eastern Grey Kangaroo .beta.-lactoglobulin possesses significant homology in several characteristic segments thought to be important for a functional trait common to the .beta.-lactoglobulin family and retinol-binding proteins. Structural similarity to the retinol-binding protein is indicated by 22% of identical residues. Homology to the .beta.-lactoglobulins and retinol-binding proteins, the binding site and possible function based on comparative structural studies are discussed.