Milk whey glycoproteins: Interspecies comparison
- Published: Apr 15, 2017
- Author: Steve Down
- Channels: Proteomics & Genomics / Proteomics
Milk is one of the staple foodstuffs around the world and is renowned for its nutritional properties. The milk proteins that afford much of the nutrition and biological activity are classified into two groups: caseins and whey proteins, the latter comprising globular proteins such as serum albumin, haptoglobin and α-lactalbumin. There have been many studies characterising and comparing the proteins in milk from humans and dairy animals but one new approach by Chinese scientists has yielded more extensive results.
Jiaqi Wang and coresearchers from the Ministry of Agriculture-Milk Risk Assessment Laboratory, Beijing, the Institute of Animal Science and Veterinary Medicine, Hefei, and the Qingdao Agricultural University, were familiar with published studies which used proteomics techniques to identify proteins and glycoproteins in milk. They too had carried out the analysis of milk proteins from various dairy animals.
However, in new work they have targeted the N-glycoproteome of milk whey, those proteins which are modified at nitrogen atoms by an oligosaccharide. This was accomplished by isolating and analysing those peptides from the protein chain that were glycosylated and was applied to expand current knowledge of the milk proteome in the cow, buffalo, yak, goat, camel and horse, while comparing the interspecies variations.
Milk from multiple animals of each species was pooled to avoid individual variations and the whey proteins were separated out by centrifugation. To isolate the N-glycopeptides, the proteins in each whey sample were digested with trypsin and the mixture was added to an affinity column containing a mixture of lectins which trapped the N-glycopeptides.
While they were still trapped, the N-glycopeptides were treated with a peptide N-glycosidase enzyme that separated the glyco functions from the peptides, then the peptides were washed from the column for analysis by liquid chromatography/mass spectrometry on an instrument with high resolving power.
The data were searched against a database that contained peptide sequences from Bovidae (a family that includes bison, buffalo, wildebeest, cattle, antelope, sheep and goat), Camelidae (camel, llama, alpaca) and Equidae (horse, donkey, ass, zebra). From these unique peptide sequences, the parent proteins were identified.
Novel N-glycosylation sites
An impressive 233 glycosylation sites were identified, corresponding to 147 glycoproteins across the species. Most of them had just one attached sugar chain but a small number, up to 2%, had more than three N-glycosylation sites. About 50% of the sites were already known in the cow whey proteins but, for the other species, the majority of the sites were unknown with no records in the reference database.
The Holstein and Jersey cows had the most glycoproteins combined with more than 90% shared between the two species. In contrast, a mere 12 proteins were common to the Holsteins and the camels and horses. Only four glycosylated proteins, sharing five N-glycosylation sites, were common across all of the species studied.
A few novel milk glycoproteins were also identified. They included tolloid-like protein 1 which was present only in goat milk, torsin A interacting protein 2 which was found only in camel milk and WAP four-disulfide core domain protein 2 from buffalo milk. In addition, novel N-glycosylation sites were also found in the various species.
When the functions of the N-glycoproteins were clarified, many were found to be involved in the response to stimulus, with abundant proteins also associated with localisation and protein and lipid metabolism. The functions and biological pathways implicated from the whey proteins were also analysed and compared across the different species.
So, these findings provide an important boost to the knowledge of the composition of milk whey and allow the interspecies variations to be examined more accurately. The results will help to clarify the various glycoprotein biosynthetic pathways that exist in the different animals and to differentiate their biological functions, leading to a better understanding of the beneficial factors in milk.
Proteomics 2017, 17, online: "N-glycosylation proteomic characterization and cross-species comparison of milk whey proteins from dairy animals"
Article by Steve Down
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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