Quantitative Phosphoproteomics of Milk Fat Globule Membrane in Human Colostrum and Mature Milk: New Insights into Changes in Protein Phosphorylation during Lactation

Phosphorylation
is a widespread posttranslational protein modification
and is important in various biological processes. However, milk fat
globule membrane (MFGM) phosphoproteins have not been explored systematically
in human milk. Here, we used quantitative phosphoproteomics to analyze
phosphorylation sites in human MFGM proteins and their differences
at different stages of lactation; 305 phosphorylation sites on 170
proteins and 269 phosphorylation sites on 170 proteins were identified
in colostrum and mature MFGM, respectively. Among these, 71 phosphorylation
sites on 48 proteins were differentially expressed between the different
stages of lactation. Osteopontin in human MFGM was the most heavily
phosphorylated protein, with a total of 39 identified phosphorylation
sites. Our results shed light on phosphorylation sites, composition,
and biological functions of MFGM phosphoproteins in human colostrum
and mature milk, and provide novel insights into the crucial roles
of protein phosphorylation during infant development.