Characterization of sow milk N-linked glycoproteome over the course of lactation

Milk proteins serve as nutrition and affect neonate development and immunity through their bioactivity. Post-translational modifications of proteins affect their bioactivity. Glycosylation is the attachment of sugar moieties to proteins, with attachment of glycans to asparagine indicated as N-linked glycosylation. Our objective was to characterize N-linked glycosylated proteins in homogenate swine milk samples collected from sows (n = 5/6) during farrowing to represent colostrum and on days 3 and 14 post-farrowing to represent transitional and mature milk, respectively. Glycopeptides were isolated with lectin-based extraction and treated with Peptide N-glycosidase F (PNGase F) to identify N-linked glycosylation sites. Purified glycopeptides were analyzed by label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS). MaxQuant software was used to align spectra to Sus scrofa Uniport database to identify proteins and measure their relative abundances. Analysis of variance and Welch's t-test analysis identified glycoproteins differentially abundant between colostrum, transitional, and mature milk (false discovery rate <0.05). Shotgun proteome analysis identified 545 N-linked and glutamine, Q, -linked, glycosylation (P > 0.75 for deamidation) sites on 220 glycoproteins in sow milk. Glycoproteins were found across all three phases of swine milk production and varied by number of glycosylation sites (1-14) and in abundance and distribution between colostrum, transitional, and mature milk. Polymeric immunoglobulin receptor was the most glycosylated protein with 14 sites identified. Also highly glycosylated were casein and mucin proteins. These data are described and the relevance of glycosylated milk proteins in neonate development, such as protection against pathogens, is discussed.