Interactions in human casein systems: self-association of nonphosphorylated human beta-casein

Coacervates of lactotransferrin and β- or κ-casein: structure determined using SAXS

Lactotransferrin (LF) is a large globular protein in milk with immune-regulatory and bactericidal properties. At pH 6.5, LF (M = 78 kDa) carries a net (calculated) charge of +21. β-Casein (BCN) and κ-casein (KCN) are part of the casein micelle complex in milk. Both BCN and KCN are amphiphillic proteins with a molar mass of 24 and 19 kDa and carry net charges of -14 and -4, respectively. Both BCN and KCN form soap-like micelles, with 40 and 65 monomers, respectively. The net negative charges are located in the corona of the micelles. On mixing LF with the caseins, coacervates are formed. We analyzed the structure of these coarcervates using SAXS. It was found that LF binds to the corona of the micellar structures, at the charge neutrality point. BCN/LF and KCN/LF ratios at the charge neutrality point were found to be ~1.2 and ~5, respectively. We think that the findings are relevant for the protection mechanism of globular proteins in bodily fluids where unstructured proteins are abundant (saliva). The complexes will prevent docking of enzymes on specific charged groups on the globular protein.

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.

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.

The effect of conserved residue charge reversal on the folding of recombinant non-phosphorylated human β-casein

A short stretch of 13 amino acids in the central portion of human beta-casein contains four positively charged conserved residues, three Lys and one Arg. We changed these individually to Glu, reversing their charge, and compared the resulting recombinant proteins to the wild-type recombinant, monitoring thermal aggregation with turbidity as well as using the fluorescence of the intrinsic Trp, of hydrophobically bound ANS and fluorescence resonance energy transfer from Trp to ANS to detect differences in structure. The results demonstrate the need to maintain the actual or functional identity of these conserved charged amino acid residues in order to attain the protein folding and functional properties of the wild-type human beta-casein molecule. They emphasize the probability that native human beta-casein has a unique folding pattern that is important for its function of suspending minerals and delivering the protein and minerals to the neonate in a readily ingestible form.

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.

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.

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.

Thermal cycling aids folding of a recombinant human beta-casein with four extra N-terminal amino acid residues

Due to the limited secondary structure, it is believed that the caseins of milk, particularly the beta-caseins (beta-CN), may be in a mostly random-coil conformation or in various structures that result from random association of hydrophobic residues. However, the self-association of the human proteins with increasing temperature (T) and in the presence of Ca2+ is reproducible, implying that they normally fold into fixed tertiary structures. A nonphosphorylated recombinant human beta-CN with four extra amino acids at the N-terminus (GSHM-) was prepared and studied by laser light scattering, analytical ultracentrifugation, fluorescence spectroscopy, turbidity, and circular dichroism. In 3.3 M urea or at 4 degrees C, the protein was monomeric, as expected. Increasing T both without and with the addition of Ca2+ ions caused self-association as it does for the nonphosphorylated native beta-CN but with a somewhat different interaction pattern. However, returning the protein to its monomeric state by reequilibration at 4 degrees C followed again by increasing T caused a shift in the pattern. Such thermal cycling eventually caused the protein to equilibrate to a particular conformation where no more change could be observed. The resulting interaction pattern was similar to that of the native protein but differed particularly in that there was more extensive self-association for the recombinant mutant. The equilibration to a stable conformation was more rapid in the presence of Ca2+ ions. This suggests that the native protein normally folds into a particular conformation which may be aided by Ca2+ in the mammary gland. Further study of a recombinant form with the native amino acid sequence is needed.

Characterization of zinc-depleted alanyl-tRNA synthetase from Escherichia coli: role of zinc

To evaluate the role of zinc in Escherichia coli alanyl-tRNA synthetase, hydrodynamic measurements and circular dichroism spectra were obtained for the zinc-depleted protein and compared with those of the native enzyme. At a protein concentration of 5 mg ml(-1), pH 7.5, the sedimentation coefficient (s(20,w)) was 6.3 S and was virtually independent of temperature between 10 and 37 degrees C, similar to the results reported for the native form. However, the s(20,w) now decreased significantly as the concentration increased, indicative of a possible change in conformation. The s(20,w) value did not appear to change as the pH was increased to 9.5. In standard buffer with 3.3 M added urea, a single peak with a s(20,w) of 3.6 S was obtained and with 6.6 M added urea, a peak with a s(20,w) of 2.7 S was seen. Added Gd-HCl (6 M) gave a single peak with s(20,w) of 2. 0 S. Like the native form, laser light scattering studies indicated some heterogeneity and a radius of 6.4 nm which was virtually independent of concentration and temperature in the range of 10-37 degrees C. At 25 degrees C, a diffusion coefficient (D(20,w)) of 3.3 x 10(-7) cm(2) s(-1) was obtained. The combination of s(0)(20,w) and D(20,w) yielded a molecular mass of approximately 179 kDa, which is slightly less than that reported for the native dimeric form (186 kDa). The intrinsic viscosity at 25 degrees C was extrapolated to 5. 3 ml g(-1), a value significantly higher than that reported for the native form, which increased with temperature. These results indicate some conformational and flexibility changes from the native to the zinc-depleted form, which may explain differences in activity. Furthermore, urea denaturation experiments demonstrate the role of zinc in stabilization of AlaRS structure.

Concentrations and anti-Haemophilus influenzae activities of beta-casein phosphoforms in human milk

BACKGROUND

The distribution and concentrations of six phosphorylated forms of human beta-casein, a major source of nutrition among breast-fed infants, have not been examined in milk samples without prior fractionation. In this study, the levels of beta-casein phosphoforms in untreated human milk samples were analyzed and their antiadhesion activities determined against Haemophilus influenzae, a pathogen implicated in middle ear infection in infants.

METHODS

Human milk samples were analyzed using urea-polyacrylamide gel electrophoresis of whole-milk samples and scanning densitometry to determine the concentrations of beta-casein and its phosphoforms. A nontypable H. influenzae strain was radiolabeled to monitor its attachment to human pharyngeal cells in microtiter plates. Purified phosphoforms of beta-casein were preincubated for 15 minutes with radiolabeled bacteria to determine their antiadhesion activities.

RESULTS

The average beta-casein concentration in 151 human milk samples was 5.37+/-2.26 mg/ml. On average, the phosphoforms in untreated milk are present in the following order ranked by concentration: tetra- > di- > non- > mono- > tri- > pentaphosphorylated beta-casein. The tri-, tetra-, and pentaphosphorylated forms of human beta-casein exhibited more than 60% inhibition of H. influenzae in the antiadhesion assay when used at a concentration of 0.6 to 0.9 mg/ml.

CONCLUSION

The beta-casein level in untreated human milk is significantly higher than previously reported. The phosphoform distribution of beta-casein in individual donors varies widely. Anti-H. influenzae activity was detected in vitro among human beta-casein molecules with three or more phosphate groups.

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.

Interaction properties of singly phosphorylated human beta-casein: similarities with other phosphorylation levels

The singly and doubly phosphorylated forms of the human beta-caseins may function differently in casein micelle formation in human milk than the forms with higher phosphorylation (levels 3 to 5). This paper reports properties of the singly phosphorylated protein and compares them with the same previously measured properties for the forms with 0, 2, 3, and 5 phosphoryl groups, including the monomer characteristics, such as the extinction coefficient, partial specific volume, and molecular weight; the Ca2+ binding; the self-association and solubility patterns as monitored visually and with laser light scattering, sedimentation velocity, intrinsic viscosity, and ultraviolet spectroscopy; and the fluorescence polarization and intensity that are due to the binding of a fluorescent probe and to environmental changes in the vicinity of an intrinsic fluorophore. Changes could be attributed to relatively minor electrostatic differences between entities that vary in phosphoryl content. Major differences in interaction patterns between phosphorylation levels must therefore reside in the more complicated system in which inorganic o-phosphate is also present, thus permitting the proteins to interact with and be crosslinked by colloidal calcium phosphate.

Characterization of human kappa-casein purified by FPLC

Because previous purification procedures for human kappa-casein may have caused the loss of some carbohydrate, relatively gentle methods were used. The protein was isolated by a four-step procedure which included isoelectric precipitation of whole casein, gel chromatography on Sephadex G-200 in the presence of SDS, removal of the SDS with Extracti-Gel D, and FPLC chromatography on Mono Q with buffers containing 6 M urea. The purified protein was nearly identical in amino acid composition to that found earlier by amino acid analysis and peptide sequencing and a molar extinction coefficient of 11.2 +/- 0.1 was determined on the basis of amino acid analysis with a norleucine internal standard. Hydrolysis, acylation, and methylsilylation of the carbohydrate, followed by gas chromatographic analysis on a fused silica column, yielded approximately 5% fucose, 17% galactose, 18% N-acetylglucosamine, 8% N-acetylgalactosamine and 7% sialic acid, totaling almost 55% by weight. The percentages from two different donors were almost the same. About 1 mole phosphorus per mole of kappa-casein was also detected. Using low-speed sedimentation equilibrium methods, a molecular weight of only 33,400 was obtained for human kappa-casein, suggesting carbohydrate lability. Human beta-casein with four phosphoryls was stabilized against precipitation by 10 mM Ca+2 ions at a level greater than 95% when the molar ratio of kappa/beta exceeded 0.15.

Interaction properties of doubly phosphorylated beta-casein, a major component of the human milk caseins

Doubly phosphorylated beta-casein constitutes nearly 30% of the total human beta-caseins and is thus one of the major components of that fraction. The properties and mode of association of doubly phosphorylated beta-casein are therefore important determinants of the structure and function of the human casein micelle. Doubly phosphorylated beta-casein has an absorbency of 6.2 and a partial specific volume of .74. The protein precipitated at room temperature when 10 mM Ca2+ was added but produced a clear solution in 1 M NaCl. Equilibrium dialysis produced an average of 2.06 major Ca(2+)-binding sites at 37 degrees C with a dissociation constant of 12.1 x 10(-4) M. The monomer at 20 degrees C was calculated to have a solvation of 2.1 g of H2O/g of protein and an axial ratio of 6.8, suggesting a prolate ellipsoid of about 11 by 2 nm. At high ionic strength, evidence exists for a spherical structure with a molecular weight of 2.25 x 10(6). This structure would represent a polymer of about 90 monomers with a radius of 14.8 nm and a solvation of 1.93 g of H2O/g of protein. This association behavior is similar to that of other phosphorylated human beta-caseins but differs from the nonphosphorylated form. It changes when both Ca2+ and inorganic orthophosphate are present.

Interactions of triply phosphorylated human beta-casein: fluorescence spectroscopy and light-scattering studies of conformation and self-association

Structural changes of triply phosphorylated human beta-casein, caused by shifts in temperature between 5 and 40 degrees C, were studied using intrinsic and extrinsic fluorescence, fluorescence polarization, turbidity, and light scattering measurements. Intrinsic fluorescence declined between 5 and 20 degrees C then rose between 25 and 40 degrees C, indicative of a shift of the tryptophan fluor toward a more nonpolar environment. The fluorescence of the extrinsic probe, 8-anilino-1-naphthalene-sulfonic acid (ANS), increased only slightly between 5 and 25 degrees C, and then more sharply between 25 and 40 degrees C, suggesting a change in conformation leading to a change in either the dissociation constant, Kd, or the number of ANS binding sites, N. The presence of Ca+2 ions did not significantly alter the pattern of changes of intrinsic and extrinsic fluorescence with changing temperature. For ANS binding, values of Kd and N were calculated by two different procedures, each based upon different assumptions. The results point to increased exposure of hydrophobic surfaces with increased temperature, strongly supportive of conformational changes. Although more opportunity for hydrophobic interaction leads to increased protein-protein association, turbidity and light-scattering also suggest ion bridge formation between protein molecules. A comparison of the primary sequences of beta-caseins from six species reveal residues that are common in all species examined and thus are pivotal in protein folding and conformation, intermolecular hydrophobic interactions and ion bridge formation with Ca+2 and inorganic phosphate.

Interactions of triply phosphorylated human beta-casein: monomer characterization and hydrodynamic studies of self-association

The triply phosphorylated form of human beta-casein comprises about 15% of that fraction and is thus a significant component about midway between the two extremes of zero and five phosphoryls. Its partial specific volume, v, of 0.74 +/- 0.01 and absorbancy, E1% 1 cm, 280 nm, of 6.2 +/- 0.2 are almost identical to the other human beta-caseins. Equilibrium dialysis gave an average of 3.1 +/- 0.4 major Ca2+ binding sites at 37 degrees C with Kdiss = 8.6 x 10(-4) M. Sedimentation and viscosity at low temperatures or in 3.3 M urea suggested a prolate ellipsoidal monomer with 1.4 g H2O/g protein, 10 nm in length and 1.4 nm in width. The concentrated charge of the phosphoryls may be near one end of the ellipsoid, allowing the molecules to align with the flow in the viscometer at low concentration but, due to intermolecular electrostatic interactions, not when concentration is high. This would provide a reason for the heretofore unexplained curvature in the plots of reduced viscosity, eta red, vs beta-casein protein concentration. Self-association increased with temperature. At 37 degrees C in low salt buffer, s20,W was 16 S, which increased to about 33 S as ionic strength, I, was increased to 0.2 and above. At the same time, eta red in low salt buffer decreased from about 22 ml/g at 4 degrees C to a constant value of about 5 ml/g above 23 degrees C. A similar value for eta red at 37 degrees C, which was almost independent of protein concentration, was obtained at I greater than 0.25, giving an extrapolated intrinsic viscosity value of [eta] = 4.0 ml/g. Using this value and assuming a spherical aggregate, calculations suggest a radius of 9 nm with about 48 monomers and 0.86 g H2O/g protein.

Hydrophobic interactions in human casein micelle formation: beta-casein aggregation

The association of non-phosphorylated (0-P) and fully phosphorylated (5-P) human beta-caseins was studied by fluorescence spectroscopy and laser light scattering. The tryptophan fluorescence intensity (FI) level increased between 20 and 35 degrees C, indicating a change in the environment of that residue. A similar transition occurred when ANS was used as a probe. Transition temperatures were slightly lower in 10 mM-CaCl2 but were not affected by an equivalent increase in ionic strength caused by NaCl. The magnitude of the FI change was less for the 5-P than the 0-P protein but was increased for both by CaCl2 addition. These FI data were characteristic of a conformational change and this was supported by fluorescence polarization which indicated that with CaCl2, tryptophan and ANS mobility increased at the transition temperature even though the extent of protein association also increased. Light scattering suggested that protein association proceeded with the primary formation of submicellar aggregates containing 20-30 monomers which then associated further to form particles of minimum micelle size (12-15 submicelles), and eventually larger. The temperature of precipitation of the 5-P form in the presence of CaCl2 was lower than the conformational transition and suggested that both hydrophobic interactions and Ca bridges between phosphate esters on adjacent molecules are important in micelle formation.