Bovine Milk Fat Globule Membrane Proteome

Milk fat globule membranes (MFGM) were isolated from the milk of mid-lactation Holstein cows. The purified MFGM were fractionated using 1-dimensional SDS gels. Tryptic peptides from gel slices were further fractionated on a micro-capillary high performance liquid chromatograph connected to a nanospray-tandem mass spectrometer. Analysis of the data resulted in 120 proteins being identified by two or more unique peptide sequences. Of these 120 proteins, 71% are membrane associated proteins with the remainder being cytoplasmic or secreted proteins. Only 15 of the proteins identified in the cow MFGM were the same as proteins identified in previous mouse or human MFGM proteomic studies. Thus, the bulk of the proteins identified are new for bovine MFGM proteomics. The proteins identified were associated with membrane/protein trafficking (23%), cell signalling (23%), unknown functions (21%), fat transport/metabolism (11%), transport (9%), protein synthesis/folding (7%), immune proteins (4%) and milk proteins (2%). The proteins associated with cell signalling or membrane/protein trafficking may provide insights into MFGM secretion mechanisms. The finding of CD14, toll like receptor (TLR2), and TLR4 on MFGM suggests a direct role for the mammary gland in detecting an infection.