Antibodies to the major insoluble milk fat globule membrane-associated protein: specific location in apical regions of lactating epithelial cells.

Abstract

Milk lipid globules of various species are surrounded by a membrane structure that is separated from the triglyceride core of the globule by a densely staining fuzzy coat layer of 10-to 50-nm thickness. This internal coat structure remains attached to the membrane during isolation and extraction with low-and high-salt buffers, is insoluble in nondenaturing detergents, and is enriched in an acidic glycoprotein (butyrophilin) with an apparent Mr of 67,000 . Guinea pig antibodies against this protein, which show cross-reaction with the corresponding protein in some (goat) but not other (human, rat) species, have been used for localization of butyrophilin on frozen sections of various tissues from cow by immunofluorescence and electron microscopy . Significant reaction is found only in milk-secreting epithelial cells and not in other cell types of mammary gland and various epithelial tissues. In milk-secreting cells, the staining is restricted to the apical cell surface, including budding milk lipid globules, and to the periphery of the milk lipid globules contained in the alveolar lumina . These findings indicate that butyrophilin, which is constitutively secreted by surface budding in coordination with milk lipid production, is located at the apical surface and is not detected at basolateral surfaces, in endoplasmic reticulum, and in the Golgi apparatus. This protein structure represents an example of a cell type-specific cytoskeletal component in a cell apex . It is suggested that this antigen provides a specific marker for the apical surface of milk-secreting cells and that butyrophilin is involved in the vectorial discharge of milk lipid globules .Lactating mammary gland has received increasing attention as a model for studies of secretory mechanisms and membrane dynamics. Two major secretory mechanisms operate in milksecreting mammary epithelial cells. Lactose and milk proteins such as a-lactalbumin and caseins appear to be packaged together in Golgi apparatus-derived secretory vesicles and exit from the cell by the process of exocytotic fusion of secretory vesicle membrane with the apical plasma membrane (see references 13, 28, and 41, and references cited therein) . In contrast, morphological and biochemical evidence indicates that milk lipid globules are enveloped in apical plasma membrane during their extrusion from the cells (2; for reviews, see references 26,

Butyrophilin, a Milk Protein, Modulates the Encephalitogenic T Cell Response to Myelin Oligodendrocyte Glycoprotein in Experimental Autoimmune Encephalomyelitis

“…Of particular interest was the finding that the highest level of sequence identity, ϳ50%, was with a homologous extracellular Ig domain of butyrophilin (BTN) (24), a major protein of the milk fat globule membrane (MFGM) (25,26). This observation led us to speculate that immunological cross-reactivity or "molecular mimicry" with BTN may influence the function of the MOG-reactive autoimmune repertoire.…”

Composition, Properties and Nutritional Aspects of Milk Fat Globule Membrane – A Review

“…This and other observed morphological differences, such as a lack of intramembranous particles (probably membrane proteins; [Peixoto de Menezes & Pinto da Silva, 1978;Hui & Boni, 1991] in the plasma membrane at locations where fat globules are budding [Peixoto de Menezes & Pinto da Silva, 1978], suggest that a re-arrangement of constituents within the apical plasma membrane/MFGM occurs upon secretion of the fat globule by the mammary secretory cell. The clearing of intramembranous particles in the membrane during budding may increase local membrane elasticity [Freudenstein et al, 1979], but the phenomenon is not consistent, as the MFGM of some extra cellular fat globules does contain intramembranous particles, albeit at a reduced density [Banghart et al, 1998]. The reason for the inconsistency is not known.…”

“…The origin and the secretion of milk fat globules have been covered in many reviews [Bargmann & Knoop, 1959;Patton & Jensen, 1975;Franke et al, 1981;Keenan et al, 1983;Mather & Keenan, 1983, 1998Mather, 1987;Keenan et al, 1988;Aoki et al, 1994;Keenan & Dylewski, 1995;Keenan & Patton, 1995;Keenan, 2001;Heid & Keenan, 2005].…”

Developmental Changes of Cardiac and Slow Skeletal Muscle Troponin T Expression in Chicken Cardiac and Skeletal Muscles

“…After 2D SDS-PAGE, proteins in gels were transferred onto nitrocellulose membranes at 300 mA for 3 hr according to the methods of Towbin et al . (1979) and Franke et al . (1981).…”