Proteomic characterization and comparison of mammalian milk fat globule proteomes by iTRAQ analysis

Milk fat globule membrane (MFGM) proteins are known to be involved in many biological functions; however, their components and inter-species complexity have not yet been completely elucidated. We investigated the protein composition of the MFGM-enriched fraction from Holstein, Jersey, yak, buffalo, goat, camel, horse, and human. Extracted proteins from the MFGM-enriched fractions were identified and quantified by an iTRAQ proteomic approach. We identified 520 protein species categorized as biological processes, cellular components and molecular function according to their annotation. Cellular process, localization, transport, signal transduction, and response to stimulus were the most common biological processes; binding and catalytic activities were the most prevalent molecular functions. Pathway analysis revealed several pathways, including glycolysis/gluconeogenesis, peroxisome proliferator-activated receptor signaling, and fatty acid biosynthesis. Quantified MFGM-enriched proteins were subjected to discriminative proteomic profiling by principal component analysis and a hierarchical clustering method, and then organized into four major clusters: (1) Holstein, Jersey, and yak milk; (2) buffalo and goat milk; (3) Holstein, Jersey, buffalo, yak, and goat milk; and (4) camel, horse, and human milk. These novel quantitative data provide insight into the protein composition of the MFGM and their potential physiological functions, and highlight the significant differences in the MFGM fractions among mammalian species.

Milk fat globule membrane (MFGM) proteins have exhibited a relatively larger diversity than other milk fractions, and implicated health beneficial effects. Proteomic analysis of MFGM protein was mainly focused on human, bovine and goat in previous studies. Recently, there is an increasing demand for natural milk from minor dairy animals. Differences in protein components were not yet elucidated that required the integration of this information across multiple species. Thus, iTRAQ analysis of the proteins in MFGM fractions from Holstein, Jersey, yak, buffalo, goat, camel, horse, and human was performed in this study. A total of 520 proteins were identified and quantified in the MFGM fractions. The results were contributed to a comprehensive overview and discriminative profiling of the MFGM proteome across species.