Hydrogen exchange of the tryptophan residues in bovine, goat, guinea pig, and human alpha-lactalbumin

Hydrogen exchange of the individual tryptophan residues of bovine, goat, guinea pig, and human alpha-lactalbumin has been studied by both ultraviolet and NMR spectra. The assignment of the slowly exchanging imino proton resonances to the tryptophan residues (Trp26 and Trp60) was obtained by comparison of the nuclear Overhauser effect difference spectra of bovine, guinea pig, and human alpha-lactalbumin. Taking account of the thermal unfolding of each alpha-lactalbumin, the hydrogen exchange rates of the individual tryptophan residues are analyzed. The temperature dependence of the exchange rates classified their exchange mechanisms into two exchange processes: the "low activation energy process" and the "high activation energy process" which is associated directly with the global thermal unfolding of the protein. Trp26 of alpha-lactalbumin exchanges through the high activation energy process. The exchange behavior of Trp26 of guinea pig alpha-lactalbumin suggests a difference of the globally unfolded state of the protein from the other species. The exchange mechanism of Trp60 of human alpha-lactalbumin is the low activation energy process in contrast with those of the bovine and goat proteins, although their global thermodynamic properties are similar to each other. Trp104 and Trp118 of alpha-lactalbumin exchange through the low activation energy process, and the reaction rates are affected by the local structural differences around the tryptophan residues among these proteins. The results presented in this paper indicate that the hydrogen exchange rate through the low activation energy process provides the information only about the local nature of a protein while that through the high activation energy process provides the information about the global nature of a protein.     

Characterization of the unfolded state of bovine alpha-lactalbumin and comparison with unfolded states of homologous proteins

The unfolded states of three homologous proteins with a very similar fold have been investigated by heteronuclear NMR spectroscopy. Secondary structure propensities as derived from interpretation of chemical shifts and motional restrictions as evidenced by heteronuclear (15)N relaxation rates have been analyzed in the reduced unfolded states of hen lysozyme and the calcium-binding proteins bovine alpha-lactalbumin and human alpha-lactalbumin. For all three proteins, significant deviations from random-coil predictions can be identified; in addition, the unfolded states also differ from each other, despite the fact that they possess very similar structures in their native states. Deviations from random-coil motional properties are observed in the alpha- and the beta-domain in bovine alpha-lactalbumin and lysozyme, while only regions within the alpha-domain deviate in human alpha-lactalbumin. The motional restrictions and residual secondary structure are determined both by the amino acid sequence of the protein and by residual long-range interactions. Even a conservative single point mutation from I to L in a highly conserved region between the two alpha-lactalbumins results in considerable differences in the motional properties. Given the differences in oxidative folding between hen lysozyme and alpha-lactalbumin, the results obtained on the unfolded states suggest that residual long-range interactions, i.e., those between the alpha- and the beta-domain of lysozyme, may act as nucleation sites for protein folding, while this property of residual structure is replaced by the calcium-binding site between the domains in alpha-lactalbumin.     

Comparison of the denaturant-induced unfolding of the bovine and human alpha-lactalbumin molten globules.

Residue-specific information on the urea-induced unfolding of the molten globule state of bovine alpha-lactalbumin (BLA) has been obtained using NMR spectroscopy. In agreement with previous studies on human alpha-lactalbumin (HLA) the unfolding process for BLA has been found to be non-cooperative. Both the alpha and beta-domains of the protein are substantially collapsed in the absence of denaturant but in both proteins the majority of the structure in the beta-domain unfolds prior to that in the alpha-domain. However, in BLA the protein unfolds completely in 10 M urea at 50 degrees C, whilst in HLA a stable core region persists even under these extreme conditions. Previous studies on HLA have identified eight residues that are crucial for the stability of the molten globule. Of these residues, only three are conserved in the sequence of BLA. By taking into consideration the differences in inter-residue contacts between the four alpha-helices arising from these substitutions, and the relative hydrophobicity of the helices in the two proteins, we show that it is possible to rationalise the observed differences in the behaviour of the molten globule states of the two proteins. Taken together, these results suggest that there may be a number of ways of stabilising a given protein fold, and the specific manner that this is achieved for a particular protein is determined by details of its sequence.     

COMPARATIVE STUDIES OF GUINEA PIG AND BOVINE MYELIN BASIC PROTEINS. PARTIAL CHARACTERIZATION OF CHEMICALLY DERIVED FRAGMENTS AND THEIR ENCEPHALITOGENIC ACTIVITIES IN LEWIS RATS

Guinea pig and bovine myelin basic proteins were chemically cleaved at the carboxyl peptide bonds of methionyl and tryptophanyl residues to yield several fragments. Comparison of the bovine fragment consisting of the first 20 residues of the protein with the corresponding guinea pig fragment showed that the latter differs in containing histidine and glycine (one residue of each), an additional threonyl residue, and one fewer alanyl residues. Comparison of the bovine fragment consisting of the C-terminal 54 residues of the protein (residues 117-170) with the corresponding guinea pig fragment showed that the latter differs in containing one fewer histidyl and leucyl residues and an additional phenylalanyl residue. Tests of encephalitogenic activity in Lewis rats showed that these two fragments from both species were much less active, on a molar basis, than the uncleaved protein. On the other hand, examination of the bovine fragments consisting of residues 1-116 and 21-116 and the corresponding fragments obtained from the guinea pig protein revealed activity at least as high as that of the respective uncleaved proteins.     

Complete nucleotide sequence of ovine alpha-lactalbumin mRNA

The nucleotide sequence of ovine alpha-lactalbumin mRNA has been determined by chemical sequencing of two cDNA recombinant plasmids and a primer extension product. Ovine alpha-lactalbumin mRNA contains 723 nucleotides (excluding the poly(A) tail), with a 5' non-coding region of 26 nucleotides, followed by the 426 nucleotides of the coding region which determines a sequence signal of 19 amino acid residues and the 123 amino acid residues of mature alpha-lactalbumin. The coding region is followed by a 3' untranslated sequence of 271 nucleotides. The derived amino acid sequence of ovine pre-alpha-lactalbumin differs from that of its bovine counterpart by 8 amino acid substitutions, all but one originating from single mutations. Comparison of sequences of guinea pig, rat and human alpha-lactalbumin mRNAs with their ovine and bovine counterparts has revealed that these molecules have rapidly evolved. The highest degree of conservation was observed in the region coding for the mature protein and corresponds essentially to sequences which interact with UDP-galactosyltransferase and Ca2+ ions.     

Sequence of Guinea Pig Myelin Basic Protein

This paper proposes a tentative amino acid sequence of guinea pig myelin basic protein obtained by comparison of peptide fragments of the guinea pig and bovine proteins. Analyses of the tryptic peptides confirmed the known sequence differences in the NH2-terminal half of the molecule and showed that in the COOH-terminal half of the guinea pig protein Ser131 was missing, Ala136 - His137 was deleted, Leu140 was replaced by Phe, and an extra Ala was inserted somewhere within sequence 142-151 (tryptic peptide T23 ). Sequence determination of guinea pig tryptic peptides corresponding to residues 130-134 ( T20 ), 135-138 ( T21 ), and 142-151 ( T23 ) of the bovine protein confirmed the above sequence changes and placed the extra Ala between Gly142 and His143 . The sequence of the region corresponding to bovine residues 130-143 is thus Ala-Asp-Tyr-Lys-Ser-Lys-Gly-Phe-Lys-Gly-Ala-His. No species differences were observed in the amino acid compositions of the remaining tryptic peptides obtained from the COOH-terminal half of the molecule. Based upon these results, the guinea pig basic protein contains 167 amino acid residues and has a molecular weight of 18,256.     

Purification and characterization of mouse alpha-lactalbumin from lactating mammary glands

The whey protein, alpha-lactalbumin, was purified from lactating mammary glands of mice at high yields. It exists as two major charge forms (pI values of 6.2 and 5.8) with similar molecular weights (approx. 14600). Antibodies prepared against these peptides precipitate newly synthesized and secreted alpha-lactalbumin from organ cultures of mid-pregnancy mammary glands. The antibody is specific for mouse alpha-lactalbumin as it does not react with mouse casein, mouse serum or purified bovine alpha-lactalbumin or galactosyl transferase. In addition, it blocks enzymatic activity of alpha-lactalbumin in mouse milk but has no effect on guinea pig or human milk. A very sensitive radioimmunoassay has been developed with this antibody which can detect alpha-lactalbumin levels as low as 0.25 ng.     

The complete primary structure of alpha-lactalbumin isolated from pig (Sus scrofa) milk

The complete amino-acid sequence of pig alpha-lactalbumin has been determined. It was obtained by microsequencing of the native protein and the peptides derived after tryptic or cyanogen bromide cleavage. The tryptic peptides were separated by a rapid microbore HPLC method. Pig alpha-lactalbumin is 122 amino acids long and differs from the bovine homologue by 26 exchanged residues. Of the two prolines present in bovine alpha-lactalbumin, one has been deleted in the pig structure. All previously sequenced alpha-lactalbumins have shown glutamic acid at position 49, which is known to be the active site in the homologous lysozyme c structure. This residue is replaced by phenylalanine in pig alpha-lactalbumin indicating that the pig protein is the first alpha-lactalbumin with complete loss of all lysozyme functional residues.     

Structural studies of cytochrome b5: complete sequence-specific resonance assignments for the trypsin-solubilized microsomal ferrocytochrome b5 obtained from pig and calf

We report complete sequence-specific proton resonance assignments for the trypsin-solubilized microsomal ferrocytochrome b5 obtained from calf liver. In addition, sequence-specific resonance assignments for the main-chain amino acid protons (i.e., C alpha, C beta, and amide protons) are also reported for the porcine cytochrome b5. Assignment of the majority of the main-chain resonances was rapidly accomplished by automated procedures that used COSY and HOHAHA peak coordinates as input. Long side chain amino acid spin system identification was facilitated by long-range coherence-transfer experiments (HOHAHA). Problems with resonance overlap were resolved by examining differences between the two-dimensional 500-MHz NMR spectra of rabbit, pig, and calf proteins and by examining the temperature-dependent variation of amide proton resonances. Calculations of the aromatic ring-current shifts for protons that the X-ray crystal structure indicated were proximal to aromatic residues were found to be useful in corroborating assignments, especially those due to the large shifts induced by the heme. Assignment of NOESY cross peaks was greatly facilitated by a prediction of intensities using a complete relaxation matrix analysis based on the crystal structure. These results suggest that the single-crystal X-ray structure closely resembles that of the solution structure although there is evidence that the solution structure has a more dynamic character.     

Secondary structure of proteins from circular dichroism spectra. V. Secondary structure of proteins in a "molten globule" state

A method that accounts for the contribution made by aromatic amino acid residues in circular dichroism spectra of proteins has been used in order to analyze the structure of bovine carboanhydrase B, bovine and human alpha-lactalbumin in the native state and when denatured with acid and temperature. At acid- and temperature-induced transitions of the secondary structure of these proteins has been shown not to change. However the rigidity of their tertiary structure decreases (the environment of aromatic amino acid residues is made more symmetrical).     

Characterization of wild-type and mutant M13 gene V proteins by means of 1H-NMR

Recording of good quality NMR spectra of the single-stranded DNA binding protein gene V of the bacteriophage M13 is hindered by a specific protein aggregation effect. Conditions are described for which NMR spectra of the protein can best be recorded. The aromatic part of the spectrum has been reinvestigated by means of two-dimensional total correlation spectroscopy. Sequence-specific assignments were obtained for all of the aromatic amino acid residues with the help of a series of single-site mutant proteins. The solution properties of the mutants of the aromatic amino acid residues have been fully investigated. It has been shown that, for these proteins, either none or only local changes occur compared to the wild-type molecule. Spin-labeled oligonucleotide-binding studies of wild-type and mutant gene V proteins indicate that tyrosine 26 and phenylalanine 73 are the only aromatic residues involved in binding to short stretches of single-stranded DNA. The degree of aggregation of wild-type gene V protein is dependent on both the total protein and salt concentration. The data obtained suggest the occurrence of specific protein-protein interactions between dimeric gene V protein molecules in which the tyrosine residue at position 41 is involved. This hypothesis is further strengthened by the observation that the solubility of tyrosine 41 mutants of gene V protein is significantly higher than that of the wild-type protein. The discovery of the so-called 'solubility' mutants of M13 gene V protein has finally made it possible to study the solution structure of gene V protein and its interaction with single-stranded DNA by means of two-dimensional NMR.     

A 1H-NMR study of human interleukin-1 beta. Sequence-specific assignment of aromatic residues using site-directed mutant proteins

Complete identification of spin systems in the aromatic region of recombinant human interleukin-1 beta has been achieved using two-dimensional homonuclear Hartmann-Hahn spectroscopy. In addition, sequence-specific assignments for the four tyrosine residues have been carried out with the help of a series of mutant proteins, obtained by site-directed mutagenesis of the cloned gene. It is shown that, for the mutant proteins investigated, either none or only local structural changes occur. The use of NMR spectroscopy to determine the structural identity of site-directed mutant proteins with respect to the wild-type protein is discussed.     

The primary structure of alpha-lactalbumin from camel milk

The primary structure of camel alpha-lactalbumin was determined by analysis of the intact protein, and of CNBr fragments and enzymatic peptides from the carboxymethylated protein chain. Results show that camel alpha-lactalbumin has 123 residues and a molecular mass of 14.6 kDa. The amino acid sequence is strictly homologous to alpha-lactalbumins characterized, but also exhibits extensive differences: 39 residues differ in relation to the bovine protein and only 35 residues are conserved among hitherto known alpha-lactalbumins with characterized structures. All residues ascribed critical structural or functional roles are strictly invariant in the camel protein.     

Apolipoprotein CIII from guinea pig (Cavia porcellus) is shorter and less homologous than apolipoprotein CIII from other mammals

Apolipoprotein (apo) CIII plays an important role in metabolism of triglyceride-rich lipoproteins as a regulator of lipolysis and/or lipoprotein-receptor interaction. With the method of RT-PCR, the cDNA of guinea pig apo CIII was cloned and sequenced. The deduced amino acid sequence of 91 amino acids residues consists of a highly conserved signal peptide of 20 residues and a mature protein of 71 residues. Compared to mouse, rat, dog, bovine and human apo CIII, guinea pig apo CIII has a deletion of eight or nine amino acids at its C-terminus and it shows the lowest degree of homology to the presently known apo CIII sequences. Interestingly, the most conserved areas of guinea pig apo CIII are found in two regions, residues 16-33 and residues 50-69. Corresponding regions in human and dog apo CIII were previously predicted to form amphipathic helices, which are assumed to play important roles in the inhibition of lipoprotein lipase (LPL) and binding to lipid. Our present study could be helpful for the future elucidation of the structure-function relationships and evolution of apo CIII.     

Structural elucidation of a hydrophobic box in bovine alpha-lactalbumin by NMR: nuclear Overhauser effects

The proton nuclear Overhauser effects of bovine alpha-lactalbumin were studied at 200 MHz by irradiation of an upfield ring current shifted methylene at -2.45 ppm (assigned to Ile-95) and two aromatic protons, Tyr-103 (8.36 ppm) and Trp-60 (5.85 ppm). The experimental results were consistent with a putative three-dimensional alpha-lactalbumin model [Warne, P. K., Momany, F. A., Rumball, S. V., Tuttle, R. W., & Scheraga, H. A. (1974) Biochemistry 13, 768-782], which predicted the close proximity of Ile-95, Tyr-103, Trp-60, and Trp-104. Several of the assignments correlated with those previously made from chemically induced dynamic nuclear polarization experiments [Berliner, L. J., & Kaptein, R. (1981) Biochemistry 20, 799-807]. Subtle differences in the structure of this hydrophobic box region in alpha-lactalbumin were found between the Ca(II) and apo forms of the protein. The existence of this "hydrophobic box" in alpha-lactalbumin was strikingly similar to that in lysozyme, as verified in solution.     

Local and long-range interactions in the molten globule state: A study of chimeric proteins of bovine and human alpha-lactalbumin

The molten globule state of alpha-lactalbumin has ordered secondary structure in the alpha-domain, which comprises residues 1 to 34 and 86 to 123. In order to investigate which part of a polypeptide is important for stabilizing the molten globule state of alpha-lactalbumin, we have produced and studied three chimeric proteins of bovine and human alpha-lactalbumin. The stability of the molten globule state formed by domain-exchanged alpha-lactalbumin, in which the amino acid sequence in the alpha-domain comes from human alpha-lactalbumin and that in the beta-domain comes from bovine alpha-lactalbumin, is the same as that of human alpha-lactalbumin and is substantially greater than that of bovine alpha-lactalbumin. Therefore, our results show that the stability of the molten globule state of alpha-lactalbumin is determined by the alpha-domain and the beta-domain is not important for stabilizing the molten globule state. The substitution of residues 1 to 34 of bovine alpha-lactalbumin with those of human alpha-lactalbumin substantially increases the stability of the molten globule state, while the substitution of residues 86 to 123 of bovine alpha-lactalbumin with those of human alpha-lactalbumin decreases the stability of the molten globule state. Therefore, residues 1 to 34 in human alpha-lactalbumin is more important for the stability of the human alpha-lactalbumin molten globule state than residues 86 to 123. The stabilization of the molten globule state due to substitution of both residues 1 to 34 and 86 to 123 is not identical with the sum of the two individual substitutions, demonstrating the non-additivity of the stabilization of the molten globule state. This result indicates that there is a long-range interaction between residues 1 to 34 and 86 to 123 in the molten globule state of human alpha-lactalbumin. The differences in the stabilities of the molten globule states are well correlated with the averaged helical propensity values in the alpha-domain when the long-range interactions are negligible, suggesting that the local interaction is the dominant term for determining the stability of the molten globule state. Our results also indicate that the apparent cooperativity is closely linked to the stability of the molten globule state, even if the molten globule state is weakly cooperative.     

Molecular cloning and nucleotide sequence of a bovine alpha-lactalbumin cDNA

A cDNA clone for the bovine milk protein, alpha-lactalbumin (alpha LA), has been identified using a rat cDNA probe. The bovine cDNA clone is 703 nucleotides (nt) long, contains 8 nt of 5'-untranslated sequence and 269 nt of 3'-untranslated sequence. When compared with previously reported sequences, the bovine alpha LA mRNA sequence has 74% similarity with rat alpha LA mRNA, 79% similarity with human mRNA and 74% similarity with guinea pig mRNA.     

Assignment of aromatic resonances in the 1H nuclear magnetic resonance spectra of cardioactive polypeptides from sea anemones

High-resolution 1H NMR spectroscopy at 300 MHz has been used to investigate the aromatic residues of a series of homologous polypeptides from sea anemones: anthopleurin-A from Anthopleura xanthogrammica and toxins I and II from Anemonia sulcata. Using two-dimensional NMR techniques, specific assignments to individual protons have been made for all aromatic resonances in the spectra of these molecules. In all three polypeptides the resonances from the two conserved Trp residues, 23 and 33, are shifted significantly from their random coil values, and the indole NH resonance of Trp-23 is not observed. These shift perturbations are due in part to a mutual interaction of the two indole rings, which is also indicated by the observation of nuclear Overhauser enhancements between protons of the two rings. Several other nonpolar side chains also interact with these two Trp residues, forming a hydrophobic region, the overall structure of which is conserved throughout the series. The other aromatic residues in these polypeptides appear not to participate in this structural region.     

Effects of the stabilization of the molten globule state on the folding mechanism of alpha-lactalbumin: a study of a chimera of bovine and human alpha-lactalbumin

The N-terminal half of the alpha-domain (residues 1 to 34) is more important for the stability of the acid-induced molten globule state of alpha-lactalbumin than the C-terminal half (residues 86 to 123). The refolding and unfolding kinetics of a chimera, in which the amino acid sequence of residues 1 to 34 was from human alpha-lactalbumin and the remainder of the sequence from bovine alpha-lactalbumin, were studied by stopped-flow tryptophan fluorescence spectroscopy. The chimeric protein refolded and unfolded substantially faster than bovine alpha-lactalbumin. The stability of the molten globule state formed by the chimera was greater than that of bovine alpha-lactalbumin, and the hydrophobic surface area buried inside of the molecule in the molten globule state was increased by the substitution of residues 1 to 34. Peptide fragments corresponding to the A- and B-helix of the chimera showed higher helix propensity than those of the bovine protein, indicating the contribution of local interactions to the high stability of the molten globule state of the chimera. Moreover, the substitution of residues 1-34 decreased the free energy level of the transition state and increased hydrophobic surface area buried inside of the molecule in the transition state. Our results indicate that local interactions as well as hydrophobic interactions formed in the molten globule state are important in guiding the subsequent structural formation of alpha-lactalbumin.