Disulphide bonds in casein micelle from milk

Native disulphide-linked dimers facilitate amyloid fibril formation by bovine milk αS2-casein

Bovine milk α_S2-casein, an intrinsically disordered protein, readily forms amyloid fibrils in vitro and is implicated in the formation of amyloid fibril deposits in mammary tissue. Its two cysteine residues participate in the formation of either intra- or intermolecular disulphide bonds, generating monomer and dimer species. X-ray solution scattering measurements indicated that both forms of the protein adopt large, spherical oligomers at 20 °C. Upon incubation at 37 °C, the disulphide-linked dimer showed a significantly greater propensity to form amyloid fibrils than its monomeric counterpart. Thioflavin T fluorescence, circular dichroism and infrared spectra were consistent with one or both of the dimer isomers (in a parallel or antiparallel arrangement) being predisposed toward an ordered, amyloid-like structure. Limited proteolysis experiments indicated that the region from Ala⁸¹ to Lys¹¹³ is incorporated into the fibril core, implying that this region, which is predicted by several algorithms to be amyloidogenic, initiates fibril formation of α_S2-casein. The partial conservation of the cysteine motif and the frequent occurrence of disulphide-linked dimers in mammalian milks despite the associated risk of mammary amyloidosis, suggest that the dimeric conformation of α_S2-casein is a functional, yet amyloidogenic, structure.

Technical note: Quantification of caseins from a crude extract of mammary epithelial cells

En masse secretion of milk proteins, notably the caseins in the form of casein micelles, is a unique feature of the milk-secreting mammary epithelial cell. Caseins are therefore specific markers of these cells and constitute an ideal tool to monitor their differentiation, as well as functional status, during the development of the gland. To use them as such, a reliable method for quantitative analysis of the caseins from mammary cells or tissue is needed. Here we show that the caseins are heat-stable, a feature that leads to their complete extraction from a complex cellular extract by boiling. This allows for high enrichment and direct analysis of the caseins, even when they are poorly expressed in the starting material.

Isolation of Endoplasmic Reticulum Fractions from Mammary Epithelial Tissue

In the mammary glands of lactating animals, the mammary epithelial cells that surround the lumen of the acini produce and secrete copious amounts of milk. Functional differentiation of these mammary epithelial cells depends on the development of high-efficiency secretory pathways, notably for protein and lipid secretion. Protein secretion is a fundamental process common to all animal cells that involves a subset of cellular organelles, including the endoplasmic reticulum and the Golgi apparatus. In contrast, en masse secretion of triglycerides and cholesterol esters in the form of milk fat globules is a unique feature of the mammary epithelial cell. Cytoplasmic lipid droplets, the intracellular precursors of milk fat globules, originate from the endoplasmic reticulum, as do most milk-specific proteins. This organelle is therefore pivotal in the biogenesis of milk components. Fractionation of the cell into its subcellular parts is an approach that has proven very powerful for understanding organelle function and for studying the specific role of an organelle in a given cell activity. Here we describe a method for the purification of both smooth and rough microsomes, the membrane-bound endoplasmic reticulum fragments that form from endoplasmic reticulum domains when cells are broken up, from mammary gland tissue at lactation.

The membrane-associated form of α(s1)-casein interacts with cholesterol-rich detergent-resistant microdomains

Caseins, the main milk proteins, interact with colloidal calcium phosphate to form the casein micelle. The mesostructure of this supramolecular assembly markedly influences its nutritional and technological functionalities. However, its detailed molecular organization and the cellular mechanisms involved in its biogenesis have been only partially established. There is a growing body of evidence to support the concept that α(s1)-casein takes center stage in casein micelle building and transport in the secretory pathway of mammary epithelial cells. Here we have investigated the membrane-associated form of α(s1)-casein in rat mammary epithelial cells. Using metabolic labelling we show that α(s1)-casein becomes associated with membranes at the level of the endoplasmic reticulum, with no subsequent increase at the level of the Golgi apparatus. From morphological and biochemical data, it appears that caseins are in a tight relationship with membranes throughout the secretory pathway. On the other hand, we have observed that the membrane-associated form of α(s1)-casein co-purified with detergent-resistant membranes. It was poorly solubilised by Tween 20, partially insoluble in Lubrol WX, and substantially insoluble in Triton X-100. Finally, we found that cholesterol depletion results in the release of the membrane-associated form of α(s1)-casein. These experiments reveal that the insolubility of α(s1)-casein reflects its partial association with a cholesterol-rich detergent-resistant microdomain. We propose that the membrane-associated form of α(s1)-casein interacts with the lipid microdomain, or lipid raft, that forms within the membranes of the endoplasmic reticulum, for efficient forward transport and sorting in the secretory pathway of mammary epithelial cells.

Review: applications of mass spectrometry techniques in the investigation of milk proteome

The introduction of "soft" desorption/ionization methods such as electrospray ionization and matrix-assisted laser desorption/ionization has determined a breakthrough in the application of mass spectrometry to the structural analysis of proteins. The contemporary advancement of bioinformatics, together with the possibility to combine these mass spectrometric methods with electrophoretic or chromatographic separation techniques has opened up the new field of proteome analysis and, more generally, has established these approaches as indispensable tools for protein and peptide analysis in complex mixtures, such as milk and milk- derived foods. Here, a necessarily not exhaustive series of current applications of mass spectrometry-based techniques for the characterization of milk proteins will be summarized. These include the characterization of milk protein polymorphism, determination of the structural modifications induced on milk proteins by industrial processes, investigation of milk adulterations and characterization of milk allergens.

AlphaS1-casein, which is essential for efficient ER-to-Golgi casein transport, is also present in a tightly membrane-associated form

BACKGROUND

Caseins, the main milk proteins, aggregate in the secretory pathway of mammary epithelial cells into large supramolecular structures, casein micelles. The role of individual caseins in this process and the mesostructure of the casein micelle are poorly known.

RESULTS

In this study, we investigate primary steps of casein micelle formation in rough endoplasmic reticulum-derived vesicles prepared from rat or goat mammary tissues. The majority of both alphaS1- and beta-casein which are cysteine-containing casein was dimeric in the endoplasmic reticulum. Saponin permeabilisation of microsomal membranes in physico-chemical conditions believed to conserve casein interactions demonstrated that rat immature beta-casein is weakly aggregated in the endoplasmic reticulum. In striking contrast, a large proportion of immature alphaS1-casein was recovered in permeabilised microsomes when incubated in conservative conditions. Furthermore, a substantial amount of alphaS1-casein remained associated with microsomal or post-ER membranes after saponin permeabilisation in non-conservative conditions or carbonate extraction at pH11, all in the presence of DTT. Finally, we show that protein dimerisation via disulfide bond is involved in the interaction of alphaS1-casein with membranes.

CONCLUSIONS

These experiments reveal for the first time the existence of a membrane-associated form of alphaS1-casein in the endoplasmic reticulum and in more distal compartments of the secretory pathway of mammary epithelial cells. Our data suggest that alphaS1-casein, which is required for efficient export of the other caseins from the endoplasmic reticulum, plays a key role in early steps of casein micelle biogenesis and casein transport in the secretory pathway.

Proteomic quantification of disulfide-linked polymers in raw and heated bovine milk

Disulfide bond formation between milk protein molecules was quantified in raw and heated bovine milk using reducing and nonreducing two-dimensional electrophoresis. Analysis of protein profiles in raw milk indicated that 18% of alpha(S2)-casein, 25% of beta-lactoglobulin, and 46% of kappa-casein molecules were involved in disulfide-linked complexes (calculated through differences in spot volumes on two-dimensional electrophoretograms under reducing and nonreducing conditions), whereas levels of alpha(S1)- and beta-caseins were similar under both conditions. Following heat treatment at 90 degrees C for 30 min, spot volumes of serum albumin, beta-lactoglobulin, and kappa-casein decreased by 85%, 75%, and 75%, respectively, with the formation of several spots on nonreducing gels corresponding to polymers. Homopolymers and heteropolymers of kappa-casein and alpha(S2)-casein were identified by mass spectrometry in raw milk samples; polymers involving only alpha(S2)-casein or only kappa-casein accounted for 43% and 12% of the total polymers present, respectively. In addition, 45% of polymers in raw milk involved alpha(S2)-casein in association with other proteins as heteropolymers, indicating the key role of this protein in intermolecular disulfide bridging between proteins in raw milk. The intensity of monomeric kappa-casein spots decreased progressively with heating time at 90 degrees C, with greatest changes in spots with acidic isoelectric points. Interactions and association of milk proteins via disulfide bridges are discussed in relation to the proteins involved and their potential protective function against formation of fibril aggregates.

Kinetics of fibril formation of bovine kappa-casein indicate a conformational rearrangement as a critical step in the process

S-carboxymethylated (SCM) kappa-casein forms in vitro fibrils that display several characteristics of amyloid fibrils, although the protein is unrelated to amyloid diseases. In order to get insight into the processes that prevent the formation of amyloid fibrils made of kappa-caseins in milk, we have characterized in detail the reaction and the roles of its possible effectors: glycosylation and other caseins. Given that native kappa-casein occurs as a heterogeneous mixture of carbohydrate-free and carbohydrate-containing chains, kinetics of fibril formation were performed on purified glycosylated and unglycosylated SCM kappa-caseins using the fluorescent dye thioflavin T in conjunction with transmission electron microscopy and Fourier transform infrared spectroscopy for morphological and structural analyses. Both unglycosylated and glycosylated SCM kappa-caseins have the ability to fibrillate. Kinetic data indicate that the fibril formation rate increases with SCM kappa-casein concentration but reaches a plateau at high concentrations, for both the unglycosylated and glycosylated forms. Therefore, a conformational rearrangement is the rate-limiting step in fibril growth of SCM kappa-casein. Transmission electron microscopy images indicate the presence of 10- to 12-nm spherical particles prior to the appearance of amyloid structure. Fourier transform infrared spectroscopy spectra reveal a conformational change within these micellar aggregates during the fibrillation. Fibrils are helical ribbons with a pitch of about 120-130 nm and a width of 10-12 nm. Taken together, these findings suggest a model of aggregation during which the SCM kappa-casein monomer is in rapid equilibrium with a micellar aggregate that subsequently undergoes a conformational rearrangement into a more organized species. These micelles assemble and this leads to the growing of amyloid fibrils. Addition of alpha(s1)-and beta-caseins decreases the growth rate of fibrils. Their main effect was on the elongation rate, which became close to that of the limiting conformation change, leading to the appearance of a lag phase at the beginning of the kinetics.