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

Detection of Local Polarity of α-Lactalbumin by N-Terminal Specific Labeling with a New Tailor-Made Fluorescent Probe

To detect the local polarity such as the N-terminal domain of a protein molecule, 3-(4-chloro-6-hydrazino-1,3,5-triazinylamino)-7-(dimethylamino)-2-methylphenazine has been designed and synthesized as a polarity-sensitive fluorescent probe by using an s-triazine ring as a backbone, neutral red and hydrazine as a polarity-sensitive fluorophore, and a labeling group, respectively. The fluorescence properties of the probe have been characterized. The probe has the following features: (1) stable in various solvents; (2) the long-wavelength emission of >550 nm that can avoid the interferences of the background fluorescence shorter than 500 nm from common biomacromolecules; and (3) the maximum emission wavelength (lambda(em)) sensitive to solvent polarity only but not to pH and temperature. The hydrazino group in such a probe reacts readily with an active carbonyl produced by transamination of a protein molecule, leading to N-terminal specific attachment of the fluorophore and thereby allowing the monitoring of local polarity. With this probe, the polarity of the N-terminal domain in both native and heat-denatured alpha-lactalbumin has been first determined, which corresponds to that with a dielectric constant of about 16, and the hydrophobic core near the N-terminus is found to be conservative for heating. The present strategy may provide a general method to study the local environmental changes of a protein molecule under different denaturation conditions.

Nomenclature of the Proteins of Cows’ Milk—Sixth Revision

This report of the American Dairy Science Association Committee on the Nomenclature, Classification, and Methodology of Milk Proteins reviews changes in the nomenclature of milk proteins necessitated by recent advances of our knowledge of milk proteins. Identification of major caseins and whey proteins continues to be based upon their primary structures. Nomenclature of the immunoglobulins consistent with new international standards has been developed, and all bovine immunoglobulins have been characterized at the molecular level. Other significant findings related to nomenclature and protein methodology are elucidation of several new genetic variants of the major milk proteins, establishment by sequencing techniques and sequence alignment of the bovine caseins and whey proteins as the reference point for the nomenclature of all homologous milk proteins, completion of crystallographic studies for major whey proteins, and advances in the study of lactoferrin, allowing it to be added to the list of fully characterized milk proteins.

Partly folded states of members of the lysozyme/lactalbumin superfamily: a comparative study by circular dichroism spectroscopy and limited proteolysis

The partly folded states of protein members of the lysozyme (LYS)/alpha-lactalbumin (LA) superfamily have been analyzed by circular dichroism (CD) measurements and limited proteolysis experiments. Hen, horse, dog, and pigeon LYSs and bovine LA were used in the present study. These are related proteins of 123- to 129-amino-acid residues with similar three-dimensional structures but low similarity in amino acid sequences. Moreover, notable differences among them reside in their calcium-binding properties and capability to adopt partly folded states or molten globules in acid solution (A-state) or on depletion of calcium at neutral pH (apo-state). Far- and near-UV CD measurements revealed that although the structures of hen and dog LYS are rather stable in acid at pH 2.0 or at neutral pH in the absence of calcium, conformational transitions to various extents occur with all other LYS/LA proteins herewith investigated. The most significant perturbation of tertiary structure in acid was observed with bovine LA and LYS from horse milk and pigeon egg-white. Pepsin and proteinase K were used as proteolytic probes, because these proteases show broad substrate specificity, and therefore, their sites of proteolysis are dictated not by the specific amino acid sequence of the protein substrate but by its overall structure and dynamics. Although hen LYS at pH 2.0 was fully resistant to proteolysis by pepsin, the other members of the LYS/LA superfamily were cleaved at different rates at few sites of the polypeptide chain and thus producing rather large protein fragments. The apo-form of bovine LA, horse LYS, and pigeon LYS were attacked by proteinase K at pH 8.3, whereas dog and hen LYSs were resistant to proteolysis when reacted under identical experimental conditions. Briefly, it has been found that the proteolysis data correlate well with the extent of conformational transitions inferred from CD spectra and with existing structural informations regarding the proteins herewith investigated, mainly derived from NMR and hydrogen exchange measurements. The sites of initial proteolytic cleavages in the LYS variants occur at the level of the beta-subdomain (approximately chain region 34-57), in analogy to those observed with bovine LA. Proteolysis data are in agreement with the current view that the molten globule of the LYS/LA proteins is characterized by a structured alpha-domain and a largely disrupted beta-subdomain. Our results underscore the utility of the limited proteolysis approach for analyzing structure and dynamics of proteins, even if adopting an ensemble of dynamic states as in the molten globule.

Molten globule of bovine alpha-lactalbumin at neutral pH induced by heat, trifluoroethanol, and oleic acid: a comparative analysis by circular dichroism spectroscopy and limited proteolysis

The calcium-depleted form of alpha-lactalbumin (alpha-LA) at neutral pH can be induced to adopt a partly folded state or molten globule upon moderate heating, by dissolving the protein in aqueous TFE or by adding oleic acid. This last folding variant of the protein, named HAMLET, can induce apoptosis in tumor cells. The aim of the present work was to unravel from circular dichroism (CD) measurements and proteolysis experiments structural features of the molten globule of apo-alpha-LA at neutral pH. CD spectra revealed that the molten globule of apo-alpha-LA can be obtained upon mild heating at 45 degrees C, as well as at room temperature in the presence of 15% TFE or by adding to the protein solution 7.5 equivalents of oleic acid. Under these various conditions the far- and near-UV CD spectra of apo-alpha-LA are essentially identical to those of the most studied molten globule of alpha-LA at pH 2.0 (A-state). Proteolysis of the 123-residue chain of apo-alpha-LA by proteinase K at 4 degrees C occurs slowly as an all-or-none process leading to small peptides only. At 37 degrees C, proteinase K preferentially cleaves apo-alpha-LA at peptide bonds Ser34-Gly35, Gln39-Ala40, Gln43-Asn44, Phe53-Gln54, and Asn56-Asn57. All these peptide bonds are located at level of the beta-subdomain of the protein (chain region 34-57). Similar sites of preferential cleavage have been observed with the TFE- and oleic acid-induced molten globule of apo-alpha-LA. A protein species given by the N-terminal fragment 1-34 linked via the four disulfide bridges to the C-terminal fragment 54-123 or 57-123 can be isolated from the proteolytic mixture. The results of this study indicate that the same molten globule state of apo-alpha-LA can be obtained at neutral pH under mildly denaturing conditions, as indicated by using a classical spectroscopic technique such as CD and a simple biochemical approach as limited proteolysis. We conclude that the molten globule of alpha-LA maintains a native-like tertiary fold characterized by a rather well-structured alpha-domain and a disordered chain region encompassing the beta-subdomain 34-57 of the protein.

Stepwise proteolytic removal of the beta subdomain in alpha-lactalbumin. The protein remains folded and can form the molten globule in acid solution

Bovine alpha-lactalbumin (alpha-LA) is an alpha/beta protein which adopts partly folded states when dissolved at low pH (A-state), by removal of the protein-bound calcium at neutral pH and low salt concentration (apo-state), as well as in aqueous trifluoroethanol. Previous spectroscopic studies have indicated that the A-state of alpha-LA at pH 2.0, considered a prototype molten globule, has a native-like fold in which the helical core is mostly retained, while the beta subdomain is less structured. Here, we investigate the conformational features of three derivatives of alpha-LA characterized by a single peptide bond fission or a deletion of 12 or 19/22 amino-acid residues of the beta subdomain of the native protein (approximately from residue 34 to 57). These alpha-LA derivatives were obtained by limited proteolysis of the protein in its partly folded state(s). A nicked alpha-LA species consisting of fragments 1-,3-40 and 41-123 (nicked-LA) was prepared by thermolytic digestion of the 123-residue chain of alpha-LA in 50% (v/v) aqueous trifluoroethanol. Two truncated or gapped protein species given by fragments 1-40 and 53-123 (desbeta1-LA) or fragments 1-34 and 54-,57-123 (desbeta2-LA) were obtained by digestion of alpha-LA with pepsin in acid or with proteinase K at neutral pH in its apo-state, respectively. The two protein fragments of nicked or gapped alpha-LA are covalently linked by the four disulfide bridges of the native protein. CD measurements revealed that, in aqueous solution at neutral pH and in the presence of calcium, the three protein species maintain the helical secondary structure of intact alpha-LA, while the tertiary structure is strongly affected by the proteolytic cleavages of the chain. Temperature effects of CD signals in the far- and near-UV region reveal a much more labile tertiary structure in the alpha-LA derivatives, while the secondary structure is mostly retained even upon heating. In acid solution at pH 2.0, the three alpha-LA variants adopt a conformational state essentially identical to the molten globule displayed by intact alpha-LA, as demonstrated by CD measurements. Moreover, they bind strongly the fluorescent dye 8-anilinonaphthalene-1-sulfonate, which is considered a diagnostic feature of the molten globule of proteins. Therefore, the beta subdomain can be removed from the alpha-LA molecule without impairing the capability of the rest of the chain to adopt a molten globule state. The results of this protein dissection study provide direct experimental evidence that in the alpha-LA molten globule only the alpha domain is structured.

Cryoprotective activities of group 3 late embryogenesis abundant proteins from Chlorella vulgaris C-27

The nucleotide sequence of hiC12, isolated as a cDNA clone of hardening-induced Chlorella (hiC) genes, was identified. The clone encodes a late embryogenesis abundant (LEA) protein having six repeats of a 11-mer amino acid motif, although in a slightly imperfect form. To overexpress the hiC61) and hiC12 genes, their coding regions were PCR amplified and subcloned into a pGEX-1lambdaT vector. The HIC6 and HIC12 proteins were expressed as GST fusion proteins in E. coli, then purified. The two HIC proteins were found to be effective in protecting a freeze-labile enzyme, LDH, against freeze-inactivation. On a molar concentration basis, they were about 3.1 x 10(6) times more effective in protecting LDH than sucrose and as effective as BSA. Cryoprotection tests with five kinds of chain-shortened polypeptides, synthesized based on the 11-mer amino acid motif of the HIC6 protein showed that the cryoprotective activity decreased with a decrease in the repeating units of the 11-mer motif. In fact, cryoprotective activities of three kinds of single 11-mer amino acids were very low even at high concentrations. All the results suggested that the sufficiently repeated 11-mer motif is required for the cryoprotective activities of Chlorella LEA proteins.

Limited proteolysis of bovine alpha-lactalbumin: isolation and characterization of protein domains

The partly folded states of alpha-lactalbumin (alpha-LA) exposed to acid solution at pH 2.0 (A-state) or at neutral pH upon EDTA-mediated removal of the single protein-bound calcium ion (apo form) have been probed by limited proteolysis experiments. These states are nowadays commonly considered to be molten globules and thus protein-folding intermediates. Pepsin was used for proteolysis at acid pH, while proteinase K and chymotrypsin at neutral pH. The expectations were that these proteolytic probes would detect sites and/or chain regions in the partly folded states of alpha-LA sufficiently dynamic, or even unfolded, capable of binding and adaptation to the specific stereochemistry of the protease's active site. A time-course analysis of the proteolytic events revealed that the fast, initial proteolytic cuts of the 123-residue chain of alpha-LA in its A-state or apo form by the three proteases occur at the same chain region 39-54, the actual site(s) of cleavage depending upon the protease employed. This region in native alpha-LA encompasses the beta-sheets of the protein. Subsequent cleavages occur mostly at chain regions 31-35 and 95-105. Four fragment species of alpha-LA have been isolated by reverse-phase high-performance liquid chromatography, and their conformational properties examined by circular dichroism and fluorescence emission spectroscopy. The single chain fragment 53-103, containing all the binding sites for calcium in native alpha-LA and cross-linked by two disulfide bridges, maintains in aqueous buffer and in the presence of calcium ions a folded structure characterized by the same content of alpha-helix of the corresponding chain segment in native alpha-LA. Evidence for some structure was also obtained for the two-chain species 1-40 and 104-123, as well as 1-31 and 105-123, both systems being covalently linked by two disulfide bonds. In contrast, the protein species given by fragment 1-34 connected to fragment 54-123 or 57-123 via four disulfide bridges adopts in solution a folded structure with the helical content expected for a native-like conformation. Of interest, the proteolytic fragment species herewith isolated correspond to the structural domains and subdomains of alpha-LA that can be identified by computational analysis of the three-dimensional structure of native alpha-LA (Siddiqui AS, Barton GI, 1995, Protein Sci 4:872-884). The fast, initial cleavages at the level of the beta-sheet region of native alpha-LA indicate that this region is highly mobile or even unfolded in the alpha-LA molten globule(s), while the rest of the protein chain maintains sufficient structure and rigidity to prevent extensive proteolysis. The subsequent cleavages at chain segment 95-105 indicate that also this region is somewhat mobile in the A-state or apo form of the protein. It is concluded that the overall domain topology of native alpha-LA is maintained in acid or at neutral pH upon calcium depletion. Moreover, the molecular properties of the partly folded states of alpha-LA deduced here from proteolysis experiments do correlate with those derived from previous NMR and other physicochemical measurements.

Effects of secretion removal on bovine mammary gland function following an extended milk stasis

The objective of this study was to determine whether lactation function could be reinitiated after a period of extended milk stasis. Involution was induced by milk stasis in lactating Holstein cows for a period of 11 d. On d 11, one side of the mammary gland was milked twice daily for 3 d. The contralateral side remained unmilked for the 14-d experimental period. Cows were slaughtered, and mammary tissue was collected from both udder halves for further analysis. Mammary secretion volume was partially restored in the milked udder half, but reestablished milk yields were variable among cows. A partial recovery of lactation function was further indicated by elevated levels of lactose and protein profiles resembling milk in mammary secretions from the milked glands. Lactose and protein profiles from the unmilked glands were similar to those of glands undergoing involution. Lactoferrin levels were elevated in secretions from the milked and unmilked udder halves. Casein and lactoferrin synthesis by mammary explants and beta-casein and lactoferrin mRNA abundance in mammary tissues corresponded to protein profiles from milked and unmilked mammary secretions. alpha-Lactalbumin mRNA was variable but was more abundant in the milked glands compared with the unmilked glands. Lectin fluorescence microscopy for soybean agglutinin preferentially stained the apical surface of the mammary epithelial cells from the milked glands. Staining was absent in the unmilked glands and suggested resumption of lactation function in all such milked glands. These results suggest that mammary involution can be partially reversible after 11 d of milk stasis.

Correlation of the alpha-lactalbumin (+15) polymorphism to milk production and milk composition of Holsteins

The alpha-lactalbumin (+15) polymorphism (a single base variation 15 basepairs 3' of the alpha-lactalbumin transcription start point) was examined for its usefulness as a genetic marker for Holsteins. The +15 polymorphism is located in a region of the gene that is potentially involved in the regulation of alpha-lactalbumin gene expression. Animals from two dairy herds and young sires from progeny-testing programs of four AI organizations were used in the analysis. A group of sons from a heterozygous sire were also evaluated. Each individual animal was genotyped at the alpha-lactalbumin (+15) locus, and differences of genotypes were investigated. Estimated differences among alleles were calculated for PTA for milk, kilograms of protein, protein percentage, protein dollars, kilograms of fat, fat percentage, and fat dollars. Animals having the alpha-lactalbumin (+15) AA (an adenine on both alleles at position +15) genotype had statistically higher PTA for milk, kilograms of protein, protein dollars, kilograms of fat, and fat dollars than did the alpha-lactalbumin (+15) BB (a cytosine, guanine, or thymine on both alleles at position +15) animals. The alpha-lactalbumin (+15) BB animals had higher protein and fat percentages than the alpha-lactalbumin (+15) AA animals. Animals that were heterozygous at this locus, alpha-lactalbumin (+15) AB, had intermediate values for all traits analyzed. These results indicate a potential marker or actual locus effect of the alpha-lactalbumin (+15) polymorphism in Holstein cattle.

Physical mapping of the lysozyme gene family in cattle

Amplification of an ancestral lysozyme gene in artiodactyls is associated with the evolution of foregut fermentation in the ruminant lineage and has resulted in about ten lysozyme genes in true ruminants. Hybridization of a cow stomach lysozyme 2 cDNA clone to restricted DNAs of a panel of cow x hamster hybrid cell lines revealed that all but one of the multiple bovine-specific bands segregate concordantly with the marker for bovine syntenic group U3 [Chromosome (Chr) 5]. The anomalous band was subsequently mapped to bovine syntenic group U22 (Chr 7) with a second panel of hybrids representing all 31 bovine syntenic groups. By two-dimensional pulsed-field gel electrophoresis the lysozyme genes on cattle Chr 5 were shown to be clustered on a 2- to 3-Mb DNA fragment, while the lactalbumin gene and pseudogenes that are paralogous and syntenic with the lysozymes were outside the lysozyme gene cluster. Chromosomal fluorescence in situ hybridization of a cocktail of lysozyme genomic clones localized the lysozyme gene cluster to cattle Chr 5 band 23, corroborating the somatic cell assignment.

Sequence and single-base polymorphisms of the bovine alpha-lactalbumin 5'-flanking region

The alpha-lactalbumin (alpha LA)-encoding gene is a potential quantitative trait locus in dairy animals. In cattle, the production of alpha LA is tightly coupled to the onset of lactation and it serves as a regulatory subunit of the enzyme responsible for lactose synthesis. Lactose is the major osmole controlling water movement in the mammary gland. To better understand the control of bovine alpha LA expression, the 5'-flanking region of a Holstein alpha LA gene was cloned and sequenced. The sequenced clone contains 1952 bp of 5'-flanking region and 66-bp of the protein-coding region. Three single-bp polymorphisms were identified within this region. These polymorphisms occur at positions +15, +21 and +54 relative to the mRNA transcription start point (tsp). The +15 and +21 variations occur in the region encoding the 5'-untranslated region of the mRNA-coding sequence. The +54 polymorphism is a silent mutation in the SP-coding region of the gene. A polymerase chain reaction (PCR, Cetus)-based screening method has been employed to analyze the genotype of cattle at the +15 position. A total of 501 randomly selected cattle from seven breeds were screened for this allele. Of these animals, only the Holstein breed of cattle was found to contain the +15 variation and it occurs at a gene frequency of 32%. Sequence comparisons were conducted between the 5'-flanking regions of the bovine-milk-protein encoding genes, alpha LA, beta-casein and alpha S1-casein, which are coordinately expressed. Regions of similarity extending to 350 bp in length were observed between these sequences.

Molecular sequence accuracy: analysing imperfect data

Molecular sequences are experimentally derived data that can be expected to contain errors as a result of diverse phenomena such as biological variation, molecular cloning artifacts, imperfect sequence determination, and data handling during contig assembly. Errors will affect the reliability of database searches and sequence alignments, but their impact may be minimized by the use of analytical techniques that anticipate that the data will be imperfect.

The sequence of a porcine cDNA encoding alpha-lactalbumin

A cDNA clone encoding porcine alpha-lactalbumin (alpha LA) was isolated and sequenced. The longest clone was 688 nucleotides (nt) long and encoded a preprotein of 141 amino acids (aa) including a leader peptide of 19 aa. The porcine cDNA exhibited a nt similarity of between 72.2%-83.5% to other alpha LA cDNAs and an aa similarity of between 50.8%-85.2% with other alpha LA aa sequences. The derived aa sequence varied at three positions from a previously reported sequence for porcine alpha LA obtained by direct aa sequencing.

Genomic analysis of the major bovine milk protein genes

The genomic arrangement of the major bovine milk protein genes has been determined using a combination of physical mapping techniques. The major milk proteins consist of the four caseins, alpha s1 (CASAS1), alpha s2 (CASAS2), beta (CASB), and kappa (CASK), as well as the two major whey proteins, alpha-lactalbumin (LALBA) and beta-lactoglobulin (LGB). A panel of bovine X hamster hybrid somatic cells analyzed for the presence or absence of bovine specific restriction fragments revealed the genes coding for the major milk proteins to reside on three chromosomes. The four caseins were assigned to syntenic group U15 and localized to bovine chromosome 6 at q31-33 by in situ hybridization. LALBA segregated with syntenic group U3, while LGB segregated with U16. Pulsed-field gel electrophoresis confirmed genetic mapping results indicating tight linkage of the casein genes. The four genes reside on less than 200 kb of DNA in the order CASAS1-CASB-CASAS2-CASK. Multiple restriction fragment length polymorphisms were also found at the six loci in three breeds of cattle.

The evolution of lysozyme and alpha-lactalbumin

From the analysis of phylogenetic trees constructed from the amino acid sequences and metal-binding properties of various lysozymes c and alpha-lactalbumins, it was found that before the divergence of the lineages of birds and mammals, calcium-binding lysozyme diverged from non-calcium-binding lysozyme. alpha-Lactalbumin evolved from the calcium-binding lysozyme along the mammalian lineage after the divergence of birds and mammals. Rapid evolution took place, not in the process of acquisition of the activity of alpha-lactalbumin, but after the loss of lysozyme activity, due to the change in the distribution of selective pressure on each amino acid site. A general process for the change in function of a protein during evolution is suggested to be as follows: after duplication of the gene, one of their protein products acquires a new function, besides that already present; the old function is eventually lost.