Determination of the phosphorylation level and deamidation susceptibility of equine beta-casein.

This research aims to investigate the degree of phosphorylation and susceptibility to deamidation of beta-casein, a protein derived from Haflinger mare’s milk. The resulting protein was displayed as a variant with 226 amino acid residues and composed of multi-phosphorylated isoforms. After 96 hours, approximately 80% of the protein deamidated under physiological conditions.

Methodology

• The protein beta-casein was isolated from Haflinger mare’s milk using the technique known as Reversed Phase High-Performance Liquid Chromatography (RP-HPLC).
• The researchers observed microheterogeneity (diversity in composition) in the isolated protein via urea-electrophoresis and two-dimensional electrophoresis (2-DE), which implied variable phosphorylation levels.

Phosphorylation Investigation

• To examine the extent of phosphorylation, researchers determined the primary structure of the equine beta-casein by tryptic hydrolysis and mass spectrometry (MS) of the released peptides. This was also done by using MS on the protein post alkaline phosphatase treatment.
• The molecular mass of the apo-form of Haflinger mare’s beta-casein was found to be similar to the theoretical mass of a sequence reported previously, modified by inserting a region encoded by an exon that is occasionally out-spliced. This determined that the beta-casein variant consisted of 226 amino acid residues.

Results

• The beta-casein variant was found to be composed of highly multi-phosphorylated isoforms, which possessed three to seven phosphate groups.
• The isoelectric points (pIs), as determined by 2-DE, ranged from 4.74 to 5.30, indicating a difference in the charge of the protein under neutral pH environments.
• The equine beta-casein demonstrated the ability to spontaneously deamidate at the Asn level in the potential deamidation motif.
• Approximately 80% of the beta-casein was found to be deamidated after 96 h of incubation under physiological conditions.

Implications

• The findings of the study provide critical insights into the structure and behavior of beta-casein, a protein derived from Haflinger mare’s milk.
• A higher understanding of the phosphorylation degree and deamidation susceptibility of this protein could potentially inform its function and role in biological processes.