Investigation into the asymmetric distribution of proteins in human milk-fat-globule-membranes.

The proteins from human milk-fat-globule-membrane were radioiodinated, solubilized and analyzed by SDS-polyacrylamide gel electrophoresis. The solubilized milk-fat-globule-membrane preparations contained six major size classes of components with apparent molecular weights of 155, 70, 58, 52, 42, and 39 kilodaltons. The membrane proteins were significantly more accessible to lactoperoxidase-125I in isolated membrane compared with that of whole cream. Major proteins of apparent molecular weights of 155, 70, 58, 52, 42, and 39 kilodaltons were labeled in whole cream and were extracted from the fat-globules membrane with magnesium chloride. Residual cream (after being extracted with MgCl2) showed the loss of the above proteins components. Using an indirect immunoperoxidase staining method and the antibodies to MFGM which immunoprecipitated all the six major glycoprotein components of MFGM, demonstrated their presence on the apical plasma membrane of mammary epithelial cells lining the breast duct in tissue sections. The asymmetric arrangements of proteins in the human milk-fat-globule-membranes, after secretion, is discussed.