The amino-acid sequence of beta-lactoglobulin II from horse colostrum (Equus caballus, Perissodactyla): beta-lactoglobulins are retinol-binding proteins.

The research paper explores the sequence and diversity of the protein beta-lactoglobulin II from horse colostrum, comparing it to other beta-lactoglobulins and drawing possible connections to its biological functions.

Research Context and Methods

• The scientists studied beta-lactoglobulin, a protein found in horse colostrum (the first form of milk produced following the delivery of a newborn). This protein is heterogeneous, meaning it contains diverse components. Specifically, the researchers looked at two components: beta-lactoglobulin I and beta-lactoglobulin II. These components differ in their electrophoretic mobilities (a measure of how fast they move under the influence of an electric field), chain lengths, and primary structures (the sequence of amino acids).
• The complete amino acid sequence of beta-lactoglobulin II was defined using automated Edman degradation, a process for sequencing proteins. The team supplemented this process through digestion with trypsin or chymotrypsin (proteolytic enzymes) and chemical cleavage using cyanogen bromide.

Key Findings

• Uniquely, horse beta-lactoglobulin II contains 166 amino acids, in comparison to other beta-lactoglobulins which typically contain 162. Four additional amino acids were discovered between positions 116 and 117, unlike any other in the beta-lactoglobulins spectrum.
• The research showed a significant degree of diversity between horse colostrum beta-lactoglobulins I and II, with 48 amino acid substitutions equating to 30% difference. This level of diversity is unprecedented within the beta-lactoglobulin gene family.
• The sequence was also compared with bovine beta-lactoglobulin A (from cows), showing 85 out of 166 amino acids replacements, accounting for over half (53%) of the residues.

Implications and Conclusions

• This research’s findings introduce a new level of complexity to our understanding of the beta-lactoglobulin gene family. The high level of diversity found between beta-lactoglobulins I and II in horse colostrum suggests more variability than previously assumed in these milk proteins.
• The researchers also note a structural homology (similarity in structure) with human retinol-binding protein, a molecule important in transporting vitamin A in the blood. They suggest this similarity might provide clues to the origin and potential biological functions of milk proteins, perhaps indicating a shared route in the evolution of these proteins.