Human alpha s1-casein: purification and characterization

A novel proteomic approach for the identification and relative quantification of disulfide-bridges in the human milk proteome

This study explores the disulfide bridges present in the human milk proteome by a novel approach permitting both positional identification and relative quantification of the disulfide bridges. Human milk from six donors was subjected to trypsin digestion without reduction. The digested human milk proteins were analyzed by nanoLC-timsTOF Pro combined with data analysis using xiSEARCH. A total of 85 unique disulfide bridges were identified in 25 different human milk proteins. The total relative abundance of disulfide bridge-containing peptides constituted approximately 5% of the total amount of tryptic-peptides. Seven inter-molecular disulfide bridges were identified between either α-lactalbumin and lactotransferrin (5) or αS1-casein and κ-casein (2). All cysteines involved in the observed disulfide bridges of α-lactalbumin and lactotransferrin were mapped onto protein models using AlphaFold2 Multimer to estimate the length of the observed disulfide bridges. The lengths of the disulfide bridges of lactotransferrin indicate a potential for multi- or poly-merization of lactotransferrin. The high number of intramolecular lactotransferrin disulfide bridges identified, suggests that these are more heterogeneous than previously presumed. SIGNIFICANCE: Disulfide-bridges in the human milk proteome are an often overseen post-transaltional modification. Thus, mapping the disulfide-bridges, their positions and relative abundance, are valuable new knowledge needed for an improved understanding of human milk protein behaviour. Although glycosylation and phosphorylation have been described, even less information is available on the disulfide bridges and the disulfide-bridge derived protein complexes. This is important for future work in precision fermentation for recombinant production of human milk proteins, as this will highlight which disulfide-bridges are naturally occouring in human milk proteins. Further, this knowledge would be of value for the infant formula industry as it provides more information on how to humanize bovine-milk based infant formula. The novel method developed here can be broadly applied in other biological systems as the disulfid-brigdes are important for the structure and functionality of proteins.

Structural Analysis of Breast-Milk αS1-Casein: An α-Helical Conformation Is Required for TLR4-Stimulation

Breast-milk αS1-casein is a Toll-like receptor 4 (TLR4) agonist, whereas phosphorylated αS1-casein does not bind TLR4. The objective of this study was to analyse the structural requirements for these effects. In silico analysis of αS1-casein indicated high α-helical content with coiled-coil characteristics. This was confirmed by CD-spectroscopy, showing the α-helical conformation to be stable between pH 2 and 7.4. After in vitro phosphorylation, the α-helical content was significantly reduced, similar to what it was after incubation at 80 °C. This conformation showed no in vitro induction of IL-8 secretion via TLR4. A synthetic peptide corresponding to V77-E92 of αS1-casein induced an IL-8 secretion of 0.95 ng/mL via TLR4. Our results indicate that αS1-casein appears in two distinct conformations, an α-helical TLR4-agonistic and a less α-helical TLR4 non-agonistic conformation induced by phosphorylation. This is to indicate that the immunomodulatory role of αS1-casein, as described before, could be regulated by conformational changes induced by phosphorylation.

Variation in milk fat globule size and composition: A source of bioactives for human health

Milk fat globules (MFGs) are secreted from the mammalian gland and are composed of a triacylglycerol core surrounded by a triple membrane structure, the milk fat globule membrane (MFGM). The MFGM contains complex lipids and proteins reported to have nutritional, immunological, neurological and digestive functions. Human and ruminant milk are shown to share a similar MFG structure but with different size, profile and abundance of protein and polar lipids. This review summarizes the reported data on human, bovine, caprine and ovine MFG composition and concentration of bioactive components in different MFG-size fractions. A comprehensive understanding of compositional variations between milk from different species and MFG size fractions may help promote various milk sources as targeted supplements to improve human development and health. MFG size and MFGM composition are species-specific and affected by lactation, diet and breed (or maternal origin). Purification and enrichment methods for some bioactive proteins and lipids present in the MFGM have yet to be established or are not scaled sufficiently to be used to supplement human diets. To overcome this problem, MFG size selection through fractionation or herd selection may provide a convenient way to pre-enrich the MFG fraction with specific protein and lipid components to fulfill human dietary and health requirements.

Phenotypic diversity and metabolic specialization of renal endothelial cells

Complex multicellular life in mammals relies on functional cooperation of different organs for the survival of the whole organism. The kidneys play a critical part in this process through the maintenance of fluid volume and composition homeostasis, which enables other organs to fulfil their tasks. The renal endothelium exhibits phenotypic and molecular traits that distinguish it from endothelia of other organs. Moreover, the adult kidney vasculature comprises diverse populations of mostly quiescent, but not metabolically inactive, endothelial cells (ECs) that reside within the kidney glomeruli, cortex and medulla. Each of these populations supports specific functions, for example, in the filtration of blood plasma, the reabsorption and secretion of water and solutes, and the concentration of urine. Transcriptional profiling of these diverse EC populations suggests they have adapted to local microenvironmental conditions (hypoxia, shear stress, hyperosmolarity), enabling them to support kidney functions. Exposure of ECs to microenvironment-derived angiogenic factors affects their metabolism, and sustains kidney development and homeostasis, whereas EC-derived angiocrine factors preserve distinct microenvironment niches. In the context of kidney disease, renal ECs show alteration in their metabolism and phenotype in response to pathological changes in the local microenvironment, further promoting kidney dysfunction. Understanding the diversity and specialization of kidney ECs could provide new avenues for the treatment of kidney diseases and kidney regeneration.

Caseoperoxidase, mixed β-casein-SDS-hemin-imidazole complex: a nano artificial enzyme

A novel peroxidase-like artificial enzyme, named "caseoperoxidase", was biomimetically designed using a nano artificial amino acid apo-protein hydrophobic pocket. This four-component nano artificial enzyme containing heme-imidazole-β-casein-SDS exhibited high activity growth and k(cat) performance toward the native horseradish peroxidase demonstrated by the steady state kinetics using UV-vis spectrophotometry. The hydrophobicity and secondary structure of the caseoperoxidase were studied by ANS fluorescence and circular dichroism spectroscopy. Camel β-casein (Cβ-casein) was selected as an appropriate apo-protein for the heme active site because of its innate flexibility and exalted hydrophobicity. This selection was confirmed by homology modeling method. Heme docking into the newly obtained Cβ-casein structure indicated one heme was mainly incorporated with Cβ-casein. The presence of a main electrostatic site for the active site in the Cβ-casein was also confirmed by experimental methods through Wyman binding potential and isothermal titration calorimetry. The existence of Cβ-casein protein in this biocatalyst lowered the suicide inactivation and provided a suitable protective role for the heme active-site. Additional experiments confirmed the retention of caseoperoxidase structure and function as an artificial enzyme.

Development of a surface display ELISA to detect anti-IgG antibodies against bovine αS1-casein in human sera

The aim of the present study was to develop a surface display ELISA (SD-ELISA) for IgG-serum reaction against bovine casein αS1 (CSN1S1). In a SD-ELISA, the antigen is displayed on the surface of Escherichia coli using the autodisplay technology and whole cells of E. coli are used to coat the microplates for serum testing. After establishing the setup of the SD-ELISA with polyclonal rabbit antiserum against bovine CSN1S1, the SD-ELISA was validated with 20 human sera, of which 10 sera were proven to have an IgG-mediated reaction against bovine CSN1S1 and 10 sera were shown to be negative for this reaction. Receiver operating characteristics (ROC) analysis revealed sensitivity of 100% and a specificity of 100% at a cut-off value of 0.133. Furthermore, human serum of 48 patients with known reactivity against human CSN1S1 (31 positive and 17 negative) was examined by the newly developed SD-ELISA to exclude cross-reactivity. Twenty human sera showed an IgG-mediated reaction against bovine CSN1S1. Eleven of these sera were positive for the reactivity against human CSN1S1, and nine were negative. In conclusion it was demonstrated that the performance of SD-ELISA is comparable to established ELISA without loss in sensitivity or specificity. Based on the advantages of this method - in particular no need for time-consuming and expensive antigen production and purification - the SD-ELISA is a potent alternative to convenient methods for identification and especially high-throughput screening of new antigens in the field of food allergies.

Biological activity of camel milk casein following enzymatic digestion

The aim of this study was to investigate the effects of enzymatic hydrolysis with digestive enzymes of camel whole casein and beta-casein (β-CN) on their antioxidant and Angiotensin Converting Enzyme (ACE)-inhibitory properties. Peptides in each hydrolysate were fractionated with ultra-filtration membranes. The antioxidant activity was determined using a Trolox equivalent antioxidant capacity (TEAC) scale. After enzymatic hydrolysis, both antioxidant and ACE-inhibitory activities of camel whole casein and camel β-CN were enhanced. Camel whole casein and β-CN showed significant ACE-inhibitory activities after hydrolysis with pepsin alone and after pepsinolysis followed by trypsinolysis and chymotrypsinolysis. Camel β-CN showed high antioxidant activity after hydrolysis with chymotrypsin. The results of this study suggest that when camel milk is consumed and digested, the produced peptides start to act as natural antioxidants and ACE-inhibitors.

Dual behavior of sodium dodecyl sulfate as enhancer or suppressor of insulin aggregation and chaperone-like activity of camel alphaS(1)-casein

Sodium dodecyl sulfate (SDS) at low concentrations considerably enhanced insulin aggregation and reduced the chaperone-like activity of purified camel alphaS(1)-casein (alphaS(1)-CN). These observed changes were the result of repulsive electrostatic interactions between both negative charged head groups of SDS and alphaS(1)-CN, and the net negative charge of insulin molecules, resulting in the greater exposure of hydrophobic patches of insulin and its enhanced aggregation. In contrast, enhanced hydrophobic interactions were primarily responsible for the conformational changes observed in insulin and alphaS(1)-CN at high SDS concentrations, resulting in increased binding of SDS and alphaS(1)-CN to insulin and its reduced aggregation.

Application of capillary zone electrophoresis to the characterisation of the human milk protein profile and its evolution throughout lactation

This work describes the use of capillary zone electrophoresis for the characterisation of human milk proteins. The major proteins were identified following different strategies, such as the treatment with enzymes for selective protein modification. Using this method we studied the proteins in human milk from different donors throughout lactation. Qualitative and quantitative differences in the composition of the individual proteins were observed. The different beta-casein phosphoforms were separated and quantified. The average proportion of the 0P:1P:2P:3P:4P:5P was, approximately, 3:6:9:4:10:2. The evolution of the ratio of the different beta-casein phosphoforms during lactation is reported.

On studying protein phosphorylation patterns using bottom-up LC–MS/MS: the case of human α-casein

Most proteomics studies involving mapping post-translational modifications, such as the phosphorylation of serine and threonine, are performed today using the 'bottom-up' approach. This approach involves enzymatic cleavage of proteins, most often by trypsin, with subsequent nano-LC-MS/MS. The occupancy rates of phosphosites in proteins may differ by orders of magnitude, and thus the occupancy rate must be reported for each occupied phosphosite. To highlight potential pitfalls in quantifying the occupancy rates, alpha(s1)-casein from human milk was selected as a model molecule representing moderately phosphorylated proteins. For this purpose, human milk from one Caucasian woman in the eighth month of lactation was used. The phosphorylation level of caseins is believed to have major implications for the formation of micelles that are involved in delivering valuable calcium phosphate and other minerals to the new-born. Human alpha(s1)-casein has been reported to be much less phosphorylated than ruminant caseins, which may indicate a different function of caseins in humans. Revealing the phosphorylation pattern in human casein can thus shed light on its function. The current study found that the sequence region between the residues Ser70 and Ser76 in human alpha(s1)-casein is in fact phosphorylated, contrary to previous knowledge. The site of the most abundant phosphorylation is Ser75, in agreement with the known action of the mammary gland casein kinase. There is evidence for the second phosphorylation in that region, possibly at Ser73. Earlier reported positions of phosphorylations at Ser18 and Ser26 are also confirmed, but not the dominance of Ser18 phosphorylation. The occupancy rates at Ser18, Ser26 and Ser75 are estimated to be (7 +/- 2), (20 +/- 6) and (27 +/- 9)%, respectively. Owing to differences in the ionization efficiency between phosphorylated and unphosphorylated peptides a 30% error margin is added to the occupancy rates. The highlighted pitfalls of the bottom-up strategy include the sensitivity of enzymes to proximal acidic and phosphorylated residues and the presence of multiple isoforms, including unexpected ones, of the tryptic peptides. The utility of the earlier introduced PhosTS_hunter and ModifiComb approaches for evading the latter pitfall is demonstrated.

A folding pattern that is stable to thermal cycling is achieved by long term storage of recombinant human β-casein with four extra N-terminals amino-acid residues at −20°C

Studies have followed the turbidity (OD400 nm) of beta-casein (CN) as temperature (T) increased from 4 to 37 degrees C. Native non-phosphorylated beta-CN showed a turbidity increase above 25 degrees C and precipitated at about 22 degrees C in 5mM Ca+2. These patterns were reproducible upon T-cycling while those of recombinant beta-CN proteins are not. Here, a wild-type recombinant that was thermally stable after being frozen in solution and stored at -20 degrees C for a prolonged period of time was denatured with guanidine HCl and refolded by dialysis against buffer. This protein was again not stable to T-cycling. A recombinant mutant with four extra N-terminal amino acids was very stable to T-cycling, both with and without 5mM Ca+2. However, it was still much different than the native protein. These results indicate that there are probably many energy minima for this protein and emphasize the possibility of "chaperon-like" conditions for proper folding of human beta-CN.

Reconstituted Micelle Formation Using Reduced, Carboxymethylated Bovine κ-Casein and Human β-Casein

In milk, kappa-casein, a mixture of disulfide-bonded polymers, stabilizes and regulates the size of the unique colloidal complex of protein, Ca2+ and inorganic phosphate (Pi) termed the casein (CN) micelle. However, reduced, carboxymethylated bovine kappa-CN (RCM-kappa) forms fibrils at 37 degrees C and its micelle-forming ability is in question. Here, the doubly- and quadruply-phosphorylated human beta-CN forms and 1:1 (wt:wt) mixtures were combined with RCM-kappa at different beta/kappa weight ratios. Turbidity (OD(400 nm)) and a lack of precipitation up to 37 degrees C were used as an index of micelle formation. Studies were with 0, 5 and 10 mM Ca2+ and 4 and 8 mM Pi. The RCM-kappa does form concentration-dependent micelles. Also, beta-CN phosphorylation level influences micelle formation. Complexes were low-temperature reversible and RCM-kappa fibrils were seen. There appears to be equilibrium between fibrillar and soluble forms since the solution still stabilized after fibril removal. The RCM-kappa stabilized better than native bovine kappa-CN.

Disulphide bonds in casein micelle from milk

Mammary epithelial cells synthesised and secreted caseins, the major milk proteins in most mammals, as large aggregates called micelles into the alveolar lumen they surround. We investigated the implication of the highly conserved cysteine(s) of kappa-casein in disulphide bond formation in casein micelles from several species. Dimers were found in all milks studied, confirming previous observation in ruminants. More importantly, the study of interchain disulphide bridges in mouse and rat casein micelles revealed that any casein possessing a cysteine is engaged in disulphide bond interchange; these species express four or five cysteine-containing caseins, respectively. We found that the main rodent caseins form both homo- and heterodimers. Additionally, disulphide bond formation among milk proteins was specific since the interaction of the caseins with cysteine-containing whey proteins was not observed in native casein micelles.

Colloidal Calcium Phosphate in the Reconstituted Milk Micelle May Direct Wild-type Recombinant Human β-Casein to Fold Like the Native Protein

Native human beta-casein (CN) at all phosphorylation levels exhibits reproducible behavior and appears to have a unique, stable folding pattern. In contrast, the recombinant non-phosphorylated form of human beta-CN (beta-CN-0P) with the exact amino acid sequence (wild-type), expressed and purified from Escherichia coli, differs greatly in its behavior from the native protein and the complexes formed are unstable to thermal cycling. However, when it was incorporated into reconstituted milk micelles, using bovine kappa-CN at a kappa/beta molar ratio of 1/3 with added Ca2+ ions and inorganic phosphate (P(i)) at levels that would ordinarily precipitate, its association behavior vs. temperature as monitored by turbidity (OD(400 nm)) approximated that of native beta-CN-0P. This suggests that the milk micelle system, and particularly the colloidal calcium phosphate, may act as a 'molecular chaperon' to direct the folding of the molecule into the highly stable conformation found in the purified native human beta-CN molecule.

The Formation of Casein Micelles Reconstituted with Ca+2 and Added Inorganic Phosphate is Influenced by the Non-phosphorylated Form of Human β-Casein

The beta-casein (CN) human milk fraction is comprised of a single protein phosphorylated at levels from 0 to 5. Component interactions are dependent on the phosphorylation level. Here, 3 mg/ml of beta-CN-0P, beta-CN-2P, beta-CN-4P, a 2P/4P 1:1 (wt:wt) mixture, or a mixture of all six forms in the ratio in human milk, were mixed with bovine kappa-CN at a kappa/beta molar ratio of 0.33. Measurements were with 0, 5 and 10 mM Ca+2 and 4 and 8 mM added inorganic phosphate (Pi). The turbidity (OD400 nm) and a lack of precipitation as T increased from 4 to 37 degrees C was an index of micelle formation. The results indicate: (1) while micelles will form with Ca+2 alone, added Pi has a significant enhancing effect on micelle formation; (2) the patterns of micelle formation as a function of T are influenced by the beta-CN-0P and beta-CN-1P forms of beta-CN to an unexpected extent.

Colorimetric Analysis of Protein Sulfhydyl Groups in Milk: Applications and Processing Effects

Methods for protein sulfhydryl (SH) group analysis in food systems have been largely overlooked. Nevertheless, changes in SH group concentration affect both physical and nutritional characteristics of high protein foods and ingredients. Food scientists and technologists require improved understanding of protein SH chemistry in order to design processes that minimize loss of thiol groups. This article surveys colorimetric methods for food protein SH group analysis with applications to fluid milk and dried milk powder. Most colorimetric assays (chloromeribenzoate, pyridine disulfide, Nitrobenzo-2-oxa-1,3-diazole, papain reactivation assay, etc.) were found to be inferior to the Ellman method based on the use of 5,5'dithio (bis-2 nitro benzoic acid). Techniques for SH group analysis in fluid milk and dried milk powder are described, along with typical results, their interpretations, and current research related to processing effects and the role of milk SH content on a wider range of technological issues, such as development of cooked flavors, fouling and cleaning of plate heat exchanges, protein-protein interactions, and the storage stability. Finally, a number of areas requiring further research are presented.

The phosphorylation pattern of human alphas1-casein is markedly different from the ruminant species

Caseins are highly phosphorylated milk proteins assembled in large colloidal structures termed micelles. In the milk of ruminants, alphas1-casein has been shown to be extensively phosphorylated. In this report we have determined the phosphorylation pattern of human alphas1-casein by a combination of matrix-assisted laser desorption mass spectrometry and amino acid sequence analysis. Three phosphorylation variants were identified. A nonphosphorylated form, a variant phosphorylated at Ser18 and a variant phosphorylated at Ser18 and Ser26. Both phosphorylation sites are located in the amino acid recognition sequence of the mammary gland casein kinase. Notably, no phosphorylations were observed in the conserved region covering residues Ser70-Glu78, which is extensively phosphorylated in the ruminant alphas1-caseins.

Comparison of native and recombinant non-phosphorylated human beta-casein: further evidence for a unique beta-casein folding pattern

Recombinant wild-type non-phosphorylated human beta-casein was obtained from Escherichia coli. Turbidity vs. temperature (T) without Ca(2+) showed wild-type self-association like native except for irreversibility upon T-cycling with the original pattern re-established after concentrated urea/dialysis. With Ca(2+), wild-type was more native-like. Intrinsic Trp fluorescence spectra were similar but with lowered intensity for the wild-type protein. Changes in extrinsic ANS fluorescence from 4 to 37 degrees C showed less exposure of hydrophobic surface for wild-type than native. Trp to ANS fluorescence resonance energy transfer was higher for wild-type than native at 4 degrees C but 2- to 3-fold lower at 37 degrees C. The native protein must be directed by the environment and/or a chaperone to fold into a unique, somewhat flexible, conformation, unaltered by urea during purification. Wild-type protein, with many native properties, does not spontaneously fold to the native conformation, even after solubilization with urea. T-cycling gives a stable conformation that is different from the native.

Kappa-casein interactions in the suspension of the two major calcium-sensitive human beta-caseins

The possible effects of both the beta-casein (beta-CN) phosphorylation level and the kappa-CN glycosylation level on micelle formation were studied using the doubly-phosphorylated form (beta-CN-2P) and the quadruply-phosphorylated form (beta-CN-4P) of human beta-CN, along with bovine kappa-CN to compare with previous studies using the more highly glycosylated human kappa-CN. Addition of bovine kappa-CN to human beta-CN-2P, beta-CN-4P, or a 1/1 (wt/wt) mixture of the two was at kappa/beta molar ratios from 0.0 to approximately 0.6 and micelles were reconstituted by addition of Ca+2 either directly at 37 degrees C for determination of the fraction suspended or at an initial temperature of 4 degrees that was gradually increased to 37 degrees C with the change in particle size monitored by turbidity measurements. Analysis of the data indicates that the 4P form requires more kappa-CN for stabilization than the 2P form but that the mixture of the two is more like the 4P form in that lateral kappa-kappa interactions may enhance beta-kappa interactions and micelle formation. Above a kappa/beta molar ratio of about 0.2, the caseins were fully suspended into reconstituted micelles. However, micelle size decreased at a higher ratio, indicating that the kappa-CN probably occupies a surface position and may regulate micelle size by its relative abundance. A comparison with published results suggests that the higher glycosylation level of human kappa-CN may protect a larger surface area and result in smaller micelles. Changes in reconstituted micelle size with pH indicate that positively charged groups in the kappa-CN may interact with the negatively charged phosphate esters in the beta-CN moieties in addition to kappa-beta hydrophobic interactions.

The use of lithium chloride to study human milk micelles

Various methods have been used to study the dissociation of milk micelles in attempts to determine their structure and the interactions that stabilize them. These include the addition of urea, cooling to alter hydrophobic bonding, the addition of EDTA to sequester calcium, and changes in pH to alter molecular charge. For this study, the mild chaotropic agent LiCl was added to human milk micelles, and measurements were made on the relative percentages of the six different phosphorylation levels of beta-casein (CN) at various LiCl concentrations for different lengths of time and at different temperatures. Added LiCl had little effect at 37 degrees C but caused maximal dissociation, mainly of the beta-CN species with higher phosphorylation levels, at 23 degrees C and 4 degrees C between 1 and 2 M concentration. Comparison was made with 2-M additions of NaCl, MgCl2, and KCl at 4 degrees C, with LiCl showing the only appreciable change. The results suggest that Li+ may displace Ca2+ in protein-Ca2+-protein or protein-colloidal calcium phos+ phate-protein salt bridges and that the nonphosphorylated form of human beta-CN may change its conformation and mode of interaction upon phosphorylation. Lithium chloride may be useful to study the dissociation of the different CN in bovine milk micelles.

Suspension of the calcium-sensitive human beta-caseins by human kappa-casein

The beta-casein (CN) fraction of human milk exists as a single protein entity phosphorylated at various levels from zero to five (beta-CN-0P to beta-CN-5P). Since the beta-CN fraction is precipitated by the calcium ions in milk, a stabilizing protein is needed to form a suspension of casein micelles for ready ingestion by the infant. That stabilization is known to be carried out by kappa-CN but it is also thought possible that the 0P and 1P beta-CN moieties may play a role. To examine the effects of different phosphorylation levels, 10 mM CaCl2 was added to each of the purified proteins phosphorylated (P) at different levels. Without kappa-CN, precipitation of the different beta-CN forms varied from 78 to 99%. Human kappa-CN was then added to each to give kappa/beta molar ratios varying from 0.01 to 0.25. Some stabilization was observed even at the lowest ratio and more than 90% of the protein was suspended in all cases at the highest ratio. Interaction of low levels of kappa-CN with the different forms of beta-CN to create a suspension was somewhat dependent on the phosphorylation level and the possibility of forming intra- or intermolecular Ca2+ bridges or cross-links. Similar ratios of the beta-CN-4P to either the 0P or 1P form and of the 2P to the 1P form showed that neither the 0P nor 1P form had any stabilizing ability. In fact, coprecipitation occurred so that with either the 4P or 2P forms present, higher percentages of the 0P and 1P forms precipitated.

Selection, load and inbreeding depression in a large metapopulation

The subdivision of a species into local populations causes its response to selection to change, even if selection is uniform across space. Population structure increases the frequency of homozygotes and therefore makes selection on homozygous effects more effective. However, population subdivision can increase the probability of competition among relatives, which may reduce the efficacy of selection. As a result, the response to selection can be either increased or decreased in a subdivided population relative to an undivided one, depending on the dominance coefficient F(ST) and whether selection is hard or soft. Realistic levels of population structure tend to reduce the mean frequency of deleterious alleles. The mutation load tends to be decreased in a subdivided population for recessive alleles, as does the expected inbreeding depression. The magnitude of the effects of population subdivision tends to be greatest in species with hard selection rather than soft selection. Population structure can play an important role in determining the mean fitness of populations at equilibrium between mutation and selection.

Association of mixtures of the two major forms of beta-casein from human milk

Human milk beta-casein (CN) is unique in that it may be phosphorylated at any level from zero (beta-CN-0P) to five (beta-CN-5P) organic phosphates per molecule. The 2P and 4P forms are the major components, with about 30 to 35% each. Here, we present the association properties of mixtures of these two moieties of human beta-CN. The aggregation patterns, as functions of temperature and ionic strength of these mixtures, generally follow those for the individual components. However, the mixtures yielded polymers with slightly different properties, which indicates extensive interaction between the two. Some properties of the mixtures were more like those for the 2P form, such as association in low salt buffer to give a peak with a sedimentation coefficient, s20,w, of approximately 11 S, in contrast to the 2P form alone with a peak of approximately 13 S and 4P alone with only a small amount of material with s20,w greater than 2 S at 27 degrees C. The solubility and interactions in the presence of Ca2+ ions were intermediate but more like the 4P form. A protein-concentration dependence for s20,w was seen, and laser light scattering indicated that there was an increase in size and/or a change in shape as the protein concentration increased. From the results, it is apparent that submicellar oligomers are probably formed by rapidly established equilibrium association reactions. The presence of an equal amount of the 2P form along with the 4P form does not appear to be a disadvantage in casein micelle formation and function.

Association of the quadruply phosphorylated beta-casein from human milk with the nonphosphorylated form

Human beta-casein (beta-CN) is phosphorylated at levels from zero (beta-CN-0P) to five (beta-CN-5P). The major constituent is the 4P form (approximately 35%), whereas the 0P form (approximately 5%) has been implicated in the formation of a framework upon which the forms with higher levels of phosphorylation may aggregate. At 4 degrees C in 0.01 M imidazole and 0.02 M NaCl, pH 7, with a 1:1 (wt:wt) 0P:4P ratio and a total protein concentration of 3 mg/ml, the s20,w was 1.4 S (monomer). Laser light scattering gave a radius of approximately 4.5 nm. As the temperature, T, increased, s20,w increased to 2 S. At 25 degrees C, peaks of 9.5 S and 2 S were observed. This transition T was different from that of either form. At 37 degrees C, a single peak was again observed with s20,w of 17.5 S, compared with 42 S for the 0P and 14 S for the 4P form. Laser light scattering at 37 degrees C revealed a polymer of approximately 16 nm radius and D20,w of 1.55 cm2/s. A combination of D20,w and S20,w gave a relative molecular mass suggesting about 45 monomers per polymer. An incubation of 3 h or more at 37 degrees C caused further aggregation, characteristic of the 0P form, and supported the concept of framework formation. At pH 6.6, s20,w was 38 S compared with 1.4 S at pH 10.4. Hydrostatic pressure did not have a large effect but supported a soap micelle-like structure for the polymer. The turbidity of the mixture increased with the amount of CaCl2 and T until the protein precipitated. The properties of the 1:1 mixture of these human beta-CN are intermediate but probably more biased toward those for the 4P form.

The pH-dependent dissociation of beta-casein from human milk micelles: role of electrostatic interactions

Evidence has been reported that supports the role of hydrophobic interactions in the association of the monomers and in the dissociation of different phosphorylation levels of beta-casein (CN) from human casein micelles. Here, the role of electrostatic interactions in the formation and structure of human casein micelles was examined by determining the beta-CN composition of micelle pellets from milk samples adjusted in the range from pH 5 to 10. As the pH was decreased from normal (approximately pH 7.5) at 37 degrees C, the proteins remained associated with the micelle, and the relative amounts of all of the phosphorylated forms remained essentially constant. As the pH was increased from normal, protein was lost from the micelles to the supernate. When the relative micelle compositions were corrected for the loss, all of the phosphorylated forms decreased in total amount: the largest decrease for both the nonphosphorylated and singly phosphorylated forms (approximately 95%), a sizeable decrease for the doubly phosphorylated form (approximately 70%), and only about a 25% decrease for the triply, quadruply, and fully phosphorylated forms. Laser light scattering measurements on micelle pellets resuspended in simulated milk ultrafiltrate at 37 degrees C yielded mean particle radii of approximately 63 nm at pH 7.5 and approximately 35 nm near pH 6. These results suggest that micelle stabilization depends not only on the net charge on the negatively charged organic phosphate esters and the inorganic orthophosphate, which vary with pH and calcium binding, but also on the positively charged amino groups of basic amino acid residues in the proteins.

Monomer characterization and studies of self-association of the major beta-casein of human milk

The casein form that has four organic phosphoryl groups, beta-casein (CN)-4P, is the major constituent (approximately 35%) of the beta-CN fraction of human milk and should play an important role in micelle structure and formation. In 3.3 M urea, the monomer is present with a molecular mass of 24,500 Da and a sedimentation coefficient of 1.3 S (Svedberg units, 10(-13)s). In 0.02 M NaCl and 0.01 M imidazole (low salt buffer) at pH 7, the sedimentation coefficient was 1.5 S, which increased to 14 S at 37 degrees C. Laser light scattering in low salt buffer and 9 mg/ml of protein indicated monomers with a radius of about 4 nm at 4 degrees C. The size of the radius increased as temperature increased, and, at 37 degrees C, the radius was about 12 nm. The molecular mass suggested the presence of about 47 monomers per polymer. In 0.25 M NaCl and with 10 nM Ca2+ prior to precipitation, the polymer attained a maximum radius of about 15 nm, which perhaps is the size of the smallest human milk micelles. The low value for reduced viscosity of 8.2 ml/g for the calcium-induced polymer was independent of protein concentration, suggesting a spherical shape and fixed size. Calcium apparently binds strongly to the phosphates; the dissociation constant was 8.1 x 10(-4) M. Other constituents of milk, such as inorganic orthophosphate, may contribute to differences in the manner by which beta-CN, with various phosphorylation levels, participate in micelle formation.

Expression of the human milk protein beta-casein in transgenic potato plants

A 1177 bp cDNA fragment encoding the human milk protein beta-casein was introduced into Solanum tuberosum cells under control of the auxin-inducible, bidirectional mannopine synthase (mas1',2') promoters using Agrobacterium tumefaciens-mediated leaf disc transformation methods. Antibiotic-resistant plants were regenerated and transformants selected based on luciferase activity carried by the expression vector containing the human beta-casein cDNA. The presence of human beta-casein cDNA in the plant genome was detected by PCR and DNA hybridization experiments. Human beta-casein mRNA was identified in leaf tissues of transgenic plants by RT-PCR analysis. Human beta-casein was identified in auxin-induced leaf and tuber tissues of transformed potato plants by immunoprecipitation and immunoblot analysis. Human beta-casein produced in transgenic plants migrated in polyacrylamide gels as a single band with an approximate molecular mass of 30 kDa. Immunoblot experiments identified approximately 0.01% of the total soluble protein of transgenic potato leaf tissue as beta-casein. The above experiments demonstrate the expression of human milk beta-casein as part of an edible food plant. These findings open the way for reconstitution of human milk in edible plants for replacement of bovine milk in baby foods for general improvement of infant nutrition, and for prevention of gastric and intestinal diseases in children.

Structural studies on casein micelles of human milk: dissociation of beta-casein of different phosphorylation levels induced by cooling and ethylenediaminetetraacetate

Information on the structure of human casein micelles has been obtained from dissociation of beta-casein (CN). Two approaches were used: cooling at 4 degrees C and addition of EDTA. An initial loss of about 80% of the protein optical density occurred upon cooling to 4 degrees C. Dissociation was time dependent, and at > or = 24 h about 10% remained. However, mean size and voluminosity of micelles increased, as indicated by laser light scattering and viscosity measurements. This process was reversible, and 95% of the protein reentered the micelles upon incubation for 3 h at 37 degrees C. Upon cooling, amounts of nonphosphorylated beta-CN increased, and singly phosphorylated beta-CN levels were almost constant relative to the total beta-CN in micelles. Upon addition of EDTA (0 to 5 mM), the forms with three to five phosphates were the major dissociating constituents; EDTA that was added by dialysis produced similar results but at lower concentrations. These data suggest that, in the absence of significant amounts of alpha s1-CN, nonphosphorylated and singly phosphorylated human beta-CN may form a framework, as proposed for alpha s1-CN for bovine milk, along with the colloidal calcium phosphate for the development of the final micelle structure by addition of the more highly phosphorylated forms. The results also indicate that human casein micelles have a less rigid structure than those of other species.