Primary structure of ovine alpha s1-caseins: localization of phosphorylation sites and characterization of genetic variants A, C and D

Combining different proteomic approaches to resolve complexity of the milk protein fraction of dromedary, Bactrian camels and hybrids, from different regions of Kazakhstan

Nutritional suitability of milk is not only related to gross composition, but is also strongly affected by the microheterogeniety of the protein fraction. Hence, to go further into the evaluation of the potential suitability of non-bovine milks in human/infant nutrition it is necessary to have a detailed characterization of their protein components. Combining proven proteomic approaches (SDS-PAGE, LC-MS/MS and LC-ESI-MS) and cDNA sequencing, we provide here in depth characterization of the milk protein fraction of dromedary and Bactrian camels, and their hybrids, from different regions of Kazakhstan. A total 391 functional groups of proteins were identified from 8 camel milk samples. A detailed characterization of 50 protein molecules, relating to genetic variants and isoforms arising from post-translational modifications and alternative splicing events, belonging to nine protein families (κ-, α_s1-, α_s2-, β-; and γ-CN, WAP, α-LAC, PGRP, CSA/LPO) was achieved by LC-ESI-MS. The presence of two unknown proteins UP1 (22,939 Da) and UP2 (23,046 Da) was also reported as well as the existence of a β-CN short isoform (946 Da lighter than the full-length β-CN), arising very likely in both genetic variants (A and B) from proteolysis by plasmin. In addition, we report, for the first time to our knowledge, the occurrence of a α_s2-CN phosphorylation isoform with 12P groups within two recognition motifs, suggesting thereby the existence of two kinase systems involved in the phosphorylation of caseins in the mammary gland. Finally, we demonstrate that genetic variants, which hitherto seemed to be species- specific (e.g. β-CN A for Bactrian and β-CN B for dromedary), are in fact present both in Camel dromedarius and C. bactrianus.

Ser71 of αS1-Casein is Phosphorylated in Breast Milk-Evidence from Targeted Mass Analysis

SCOPE

The casein phosphoproteins in mother's milk supply calcium and phosphate ions and make them biologically available to the newborn. Human αS1-casein is of particular interest being also an autoantigen and proinflammatory cytokine. Phosphorylation of αS1-casein by casein kinase 2 completely abolishes binding to toll-like receptor 4 and proinflammatory effects. It is, however, not known, which amino acids are affected. Therefore, breast milk samples were analyzed in an effort to detect the phosphorylation sites of αS1-casein.

METHODS AND RESULTS

Breast milk samples were tryptically digested. Target tandem MS analysis confirmed the known phosphorylation sites S33 and S41; evidence for pS89 was found in some samples. Experimental support for the presence of pS31 and pS34 was weak. Phosphorylation of a new site in αS1-casein, S71, was reproducibly measured in all samples, albeit at much lower intensity than pS33 and pS41.

CONCLUSION

Phospho-occupancy rates varied greatly and could not be confidently correlated to other parameters within the cohort of 20 donors. The new phosphosite S71 is located in the neighborhood of the serine-rich region and may contribute to the cluster of high charge density at normal milk pH, likely exerting an influence on protein tertiary structure and thus function.

Proteins and bioactive peptides from donkey milk: The molecular basis for its reduced allergenic properties

The legendary therapeutics properties of donkey milk have recently been supported by many clinical trials who have clearly demonstrated that, even if with adequate lipid integration, it may represent a valid natural substitute of cow milk for feeding allergic children. During the last decade many investigations by MS-based methods have been performed in order to obtain a better knowledge of donkey milk proteins. The knowledge about the primary structure of donkey milk proteins now may provide the basis for a more accurate comprehension of its potential benefits for human nutrition. In this aspect, experimental data today available clearly demonstrate that donkey milk proteins (especially casein components) are more closely related with the human homologues rather than cow counterparts. Moreover, the low allergenic properties of donkey milk with respect to cow one seem to be related to the low total protein content, the low ratio of caseins to whey fraction, and finally to the presence in almost all bovine IgE-binding linear epitopes of multiple amino acid differences with respect to the corresponding regions of donkey milk counterparts.

Molecular Characterization of the Llamas (Lama glama) Casein Cluster Genes Transcripts (CSN1S1, CSN2, CSN1S2, CSN3) and Regulatory Regions

In the present paper, we report for the first time the characterization of llama (Lama glama) caseins at transcriptomic and genetic level. A total of 288 casein clones transcripts were analysed from two lactating llamas. The most represented mRNA populations were those correctly assembled (85.07%) and they encoded for mature proteins of 215, 217, 187 and 162 amino acids respectively for the CSN1S1, CSN2, CSN1S2 and CSN3 genes. The exonic subdivision evidenced a structure made of 21, 9, 17 and 6 exons for the αs1-, β-, αs2- and κ-casein genes respectively. Exon skipping and duplication events were evidenced. Two variants A and B were identified in the αs1-casein gene as result of the alternative out-splicing of the exon 18. An additional exon coding for a novel esapeptide was found to be cryptic in the κ-casein gene, whereas one extra exon was found in the αs2-casein gene by the comparison with the Camelus dromedaries sequence. A total of 28 putative phosphorylated motifs highlighted a complex heterogeneity and a potential variable degree of post-translational modifications. Ninety-six polymorphic sites were found through the comparison of the lama casein cDNAs with the homologous camel sequences, whereas the first description and characterization of the 5'- and 3'-regulatory regions allowed to identify the main putative consensus sequences involved in the casein genes expression, thus opening the way to new investigations -so far- never achieved in this species.

Investigating the genetic polymorphism of sheep milk proteins: a useful tool for dairy production

Sheep is the second most important dairy species after cow worldwide, and especially in the Mediterranean and Middle East regions. In some countries, the difficult environmental conditions require a peculiar adaptation and, in these contexts, sheep are able to provide higher quality protein than cattle. In the least-developed countries, the amount of dairy sheep and ovine milk production is progressively increasing. In order to improve dairy productions, in particular those with local connotations, it is necessary to obtain in-depth information regarding milk quality and rheological properties. The genetic polymorphisms of milk proteins are often associated with quantitative and qualitative parameters in milk and are potential candidate markers that should be included in breeding strategies similar to those already available for cattle. Due to the current and growing interest in this topic and considering the large amount of new information, the aim of this study was to review the literature on sheep milk protein polymorphisms with a particular emphasis on recent findings in order to give scientists useful support. Moreover, the effects of different protein variants on milk yield and composition are discussed.

Influence of autochthonous adjunct cultures on ripening parameters of Argentinean goat's milk cheeses

BACKGROUND

Argentinean semi-hard goat's cheeses manufactured with and without the addition of autochthonous adjunct cultures of Lactobacillus plantarum ETC17, Lactobacillus rhamnosus ETC14 and Enterococcus faecium ETC3 were analysed to evaluate the effect of these strains on ripening parameters.

RESULTS

Gross composition was similar among cheeses. Microbiological analysis indicated that lactic acid bacteria added to cheeses reached high levels. None of the strains assayed affected the primary proteolysis. Overall, E. faecium had a clearer effect on the peptide and lipolysis profiles of cheeses. Analysis of the volatile fraction of cheeses indicated that the levels of several compounds involved in the overall flavour of goat's cheeses were affected by the presence of E. faecium. This could explain the differences detected in the global perception of cheeses made with this strain compared with control cheeses.

CONCLUSION

The present work represents a first contribution to knowledge of the ripening process of Argentinean goat's cheeses made with the addition of autochthonous adjunct cultures. The results suggest that E. faecium ETC3 showed a significant effect during ripening, which was reflected both in the profiles of proteolysis, lipolysis and volatile compounds and in the global sensory perception of cheeses.

Extracellular phosphorylation and phosphorylated proteins: not just curiosities but physiologically important

Mining of the literature and high-throughput mass spectrometry data from both healthy and diseased tissues and from body fluids reveals evidence that various extracellular proteins can exist in phosphorylated states. Extracellular kinases and phosphatases (ectokinases and ectophosphatases) are active in extracellular spaces during times of sufficiently high concentrations of adenosine triphosphate. There is evidence for a role of extracellular phosphorylation in various physiological functions, including blood coagulation, immune cell activation, and the formation of neuronal networks. Ectokinase activity is increased in some diseases, including cancer, Alzheimer's disease, and some microbial infections. We summarize the literature supporting the physiological and pathological roles of extracellularly localized protein kinases, protein phosphatases, and phosphorylated proteins and provide an analysis of the available mass spectrometry data to annotate potential extracellular phosphorylated proteins.

Evolution of major milk proteins in Mus musculus and Mus spretus mouse species: a genoproteomic analysis

Background

Due to their high level of genotypic and phenotypic variability, Mus spretus strains were introduced in laboratories to investigate the genetic determinism of complex phenotypes including quantitative trait loci. Mus spretus diverged from Mus musculus around 2.5 million years ago and exhibits on average a single nucleotide polymorphism (SNP) in every 100 base pairs when compared with any of the classical laboratory strains. A genoproteomic approach was used to assess polymorphism of the major milk proteins between SEG/Pas and C57BL/6J, two inbred strains of mice representative of Mus spretus and Mus musculus species, respectively.

Results

The milk protein concentration was dramatically reduced in the SEG/Pas strain by comparison with the C57BL/6J strain (34 ± 9 g/L vs. 125 ± 12 g/L, respectively). Nine major proteins were identified in both milks using RP-HPLC, bi-dimensional electrophoresis and MALDI-Tof mass spectrometry. Two caseins (β and α_s1) and the whey acidic protein (WAP), showed distinct chromatographic and electrophoresis behaviours. These differences were partly explained by the occurrence of amino acid substitutions and splicing variants revealed by cDNA sequencing. A total of 34 SNPs were identified in the coding and 3'untranslated regions of the SEG/Pas Csn1s1 (11), Csn2 (7) and Wap (8) genes. In addition, a 3 nucleotide deletion leading to the loss of a serine residue at position 93 was found in the SEG/Pas Wap gene.

Conclusion

SNP frequencies found in three milk protein-encoding genes between Mus spretus and Mus musculus is twice the values previously reported at the whole genome level. However, the protein structure and post-translational modifications seem not to be affected by SNPs characterized in our study. Splicing mechanisms (cryptic splice site usage, exon skipping, error-prone junction sequence), already identified in casein genes from other species, likely explain the existence of multiple α_s1-casein isoforms both in SEG/Pas and C57BL/6J strains. Finally, we propose a possible mechanism by which the hallmark tandem duplication of a 18-nt exon (14 copies) may have occurred in the mouse genome.

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

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

Toward milk speciation through the monitoring of casein proteotypic peptides

The possibility of detecting extraneous milk in singles species cheese-milk has been explored. A mass spectrometry (MS)-based procedure has been developed to detect 'signature peptides', corresponding to the predefined subset of 'proteotypic peptides', as matchless analytical surrogates of the parent caseins. Tryptic digests of skimmed milk samples from four species were analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS. Amongst the candidate signature peptides that are able to differentiate milks from the four species, the alpha(s1)-casein (CN) f8-22 peptide was selected as a convenient marker for bovine, ovine and water buffalo milk while the f4-22 peptide was selected as a marker for the two caprine alpha(s1)-CN A and B variants, which differ by a Pro(16) (B)->Leu(16) (A) substitution. MALDI analysis of the digest allowed the detection of alpha(s1)-CN f8-22 and caprine alpha(s1)-CN f4-22. The accurate evaluation of caprine milk in a quaternary mixture required the development of a liquid chromatography/electrospray ionization (LC/ESI)-MS procedure. Five synthetic signature peptide analogues, which differed from their natural counterparts by a single amino acid substitution, were used as internal standards to quantify the alpha(s1)-CN, which was chosen as a reference milk protein, from the different species. The limits of detection were 0.5% (1% for caprine) for either the MALDI or the LC/ESI-MS method. The isotopic-label-free quantification of isoform- or variant-specific signature peptides has disclosed a convenient approach for targeting proteins in complex mixtures.

Short communication: molecular genetic characterization of ovine alpha(S1)-casein allele H caused by alternative splicing

Sequencing of ovine CSN1S1*H cDNA showed an absence of exon 8 in comparison with GenBank sequences; the absence was confirmed by protein sequencing. We demonstrated that this allelic aberration is the result of a deletion of 4 nucleotides, the last 3 of exon 8 and the first 1 of intron 8, which are replaced by an insertion of 13 nucleotides in the DNA sequence. The insertion is a precise duplication of a part of the adjacent intronic sequence of CSN1S1*C''. These sequence differences result in an inactivation of the splice donor sequence distal to exon 8, leading to upstream exon skipping during the serial splice reactions of the ovine CSN1S1*H pre-mRNA, and may affect the specific casein expression as well as protein characteristics.

Characterisation of proteolysis profile of Argentinean sheep cheeses made by two different production methods

BACKGROUND

In this work the proteolysis profiles of Argentinean sheep cheeses made by two different production methods were studied in order to develop products with typical and defined features. Cheeses with a starter of Streptococcus thermophilus, curd cut to corn grain size, washed and heated to 43 degrees C (S cheeses) and cheeses with a mixed starter of Streptococcus thermophilus, Lactobacillus helveticus and Lactobacillus bulgaricus, curd cut to rice grain size, unwashed and heated to 47 degrees C (L cheeses) were manufactured. The cheeses were ripened at 12 degrees C and 80% relative humidity for 180 days and samples were taken throughout this period.

RESULTS

Gross composition and primary proteolysis were similar for both types of cheeses. Streptococci counts diminished from 10(9) to 10(7) colony-forming units g(-1) during ripening in both S and L cheeses. Lactobacilli counts in L cheeses decreased during ripening and disappeared at 180 days. L cheeses had significantly lower pH values and showed higher peptidolysis than S cheeses. Triangle sensory evaluation indicated important differences between the two types of cheeses.

CONCLUSION

S cheeses had a low proteolysis level and a soft flavour, making them appropriate for consumption after a short ripening time. L cheeses had a higher proteolysis level and more intense sensory characteristics, making them appropriate for consumption after a longer ripening time.

Sequence determination of alphas1-casein isoforms from donkey by mass spectrometric methods

Four co-eluting components, with experimentally measured M(r) of 23 658, 23 786, 24 278 and 24 406 Da, were detected by reversed-phase high-performance liquid chromatography (RP-HPLC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis in the dephosphorylated casein fraction of a milk sample collected at middle lactation stage from an individual donkey belonging to the Ragusano breed. By coupling RP-HPLC, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), enzymatic digestions, MALDI-TOF MS and capillary RP-HPLC/nano-electrospray ionization tandem mass spectrometry (nESI-MS/MS) analyses, the four components were identified as donkey's alpha(s1)-CNs and their sequences completely characterized, using the known mare's alpha(s1)-CN (GenBank Acc. No. AAK83668; M(r) 23750.7 Da) as reference. The proteins with M(r) of 23 786 and 23 658 Da differ in the presence of a glutamine residue at position 83 in the full-length component and present the amino acid substitutions Q(8)-->H and H(115)-->Y with respect to the mare's alpha(s1)-CN. The other two components with M(r) 24 406 and 24 278 Da, which also differ in the presence of a glutamine residue at position 88 in the full-length component, show the insertion of the pentapeptide HTPRE between Leu(33) and the Glu(34). The two alpha(s1)-CNs bearing the pentapeptide insertion were named variants A (202 amino acids; M(r) 24 406) and A(1) (201 amino acids; M(r) 24 278), whereas the two alpha(s1)-CNs without the pentapeptide were named variants B (197 amino acids; M(r) 23 786) and B(1) (196 amino acids; M(r) 23 658).

A critical analysis of production-associated DNA polymorphisms in the genes of cattle, goat, sheep, and pig

Increasing productivity is one of the main objectives in animal production. Traditional breeding methods have led to increased gains in some traits but gains are not easily attainable in traits with low heritabilities. Exploiting the genetic variations underlying desired phenotypes is the goal of today's animal producers. Such positive genetic variants must, however, be known before possible application. Consequently, candidate genes of traits of interest have been searched for possible relationships with such traits or to explain reported quantitative trait loci (QTL) for such traits. DNA variants or polymorphisms have been identified in many such genes and their relationships with production traits determined. However, only a few genes have been evaluated, given the wealth of information on reported QTL for production traits, and in most cases genes are only partially investigated. This review presents available information on DNA variants for production traits and discusses steps that are required for effective utilization of this information for successful marker-assisted selection programs.

The primary structure of a low-Mr multiphosphorylated variant ofβ-casein in equine milk

Highly phosphorylated casein with a low molecular mass was isolated from Haflinger mare's milk by RP-HPLC. It accounts for 4.0% of the casein content. Its mass was determined by LC-ESI-MS before and after treatment by alkaline phosphatase. The molecular mass found for the apo-form (10,591 +/- 2 Da) is in agreement with its primary structure, which was established by ESI-MS/MS from tryptic peptides. It appeared that this short protein (94 amino acid residues) is an internally truncated form of the full-length equine beta-casein (226 residues). This low-Mr variant of equine beta-casein displays a large deletion (residues 50-181), due to a cryptic splice site usage occurring within exon 7 during the course of primary transcripts processing. The phosphorylation pattern of this equine beta-casein variant was investigated by LC-ESI-MS and 2-DE. Seven phosphorylation forms were identified with one to seven phosphate groups with pIs ranging between 4.67 and 4.01. The major isoforms carry five and six phosphate groups.

A novel strategy for quantitative proteomics using isotope-coded protein labels

Stable isotope labelling in combination with mass spectrometry has emerged as a powerful tool to identify and relatively quantify thousands of proteins within complex protein mixtures. Here we describe a novel method, termed isotope-coded protein label (ICPL), which is capable of high-throughput quantitative proteome profiling on a global scale. Since ICPL is based on stable isotope tagging at the frequent free amino groups of isolated intact proteins, it is applicable to any protein sample, including extracts from tissues or body fluids, and compatible to all separation methods currently employed in proteome studies. The method showed highly accurate and reproducible quantification of proteins and yielded high sequence coverage, indispensable for the detection of post-translational modifications and protein isoforms. The efficiency (e.g. accuracy, dynamic range, sensitivity, speed) of the approach is demonstrated by comparative analysis of two differentially spiked proteomes.

Single nucleotide polymorphisms in the ovine casein genes detected by polymerase chain reaction-single strand conformation polymorphism

Casein genetic polymorphisms are important and well known due to their effects on quantitative traits and technological properties of milk. At the DNA level, polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) allows for the simultaneous typing of several alleles at casein loci, as well as the detection of unknown polymorphisms. Here we describe the usefulness of the PCR-SSCP technique for casein typing in sheep. In particular, three single-nucleotide polymorphisms (SNP) are described at CSN1S1, CSN2, and CSN3, all resulting in amino acid exchanges. At CSN1S1, a transition T-->C was found, resulting in the deduced amino acid exchange Ile186-->Thr186. A transition A-->G resulting in the deduced amino acid exchange Met183-->Val183 was identified at CSN2. The 2 SNP showed a rather high frequency (ranging from 0.12 to 0.26) in 3 Italian breeds (Sarda, Comisana, Sopravissana). Another transition C-->T (Ser104-->Leu104) was found at CSN3 in one heterozygous animal.

Mass spectrometric approach for the analysis of food proteins

In the study of food proteins, the need for accurate protein structural analysis has been acknowledged because of the fact that nucleotide sequencing alone is of limited analytical value if not combined with relevant information regarding the specific protein expressed and the occurrence of phosphorylation, glycosylation and disulphide bridges, and with the modification induced by the technological treatment. Mass spectrometry, whether used alone or to complement the traditional molecular-based techniques has become fundamental to the structural analysis of proteins. It is, moreover, virtually irreplaceable in determining post-translational modifications as conventional methods cannot deliver reliable data. What lies at the root of this methodological breakthrough is the combination of high-resolution separation techniques such as two-dimensional electrophoresis or capillary reverse- phase high-performance liquid chromatography with mass spectrometric analysis, what is termed "proteomic" analysis. Thus, it appears appropriate to state that the new mass spectrometric techniques have been established as a valuable and efficient tool for protein and peptide analysis in complex mixtures, like those from food matrices, enabling us therefore to provide accurate information on molecular weight and also to put forth a structural assessment at a low-picomole level of material. Thus, a series of alternative approaches have been developed based on advanced mass spectrometric analysis in conjunction with classic protein chemistry in order to provide an in-depth view of food protein structure. This review outlines several of these novel methodologies as they apply to structural characterization of food products.

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.

Proteolysis of bovine beta-lactoglobulin during thermal treatment in subdenaturing conditions highlights some structural features of the temperature-modified protein and yields fragments with low immunoreactivity

Bovine beta-lactoglobulin was hydrolyzed with trypsin or chymotrypsin in the course of heat treatment at 55, 60 and 65 degrees C at neutral pH. At these temperatures beta-lactoglobulin undergoes significant but reversible structural changes. In the conditions used in the present study, beta-lactoglobulin was virtually insensitive to proteolysis by either enzyme at room temperature, but underwent extensive proteolysis when either protease was present during the heat treatment. High-temperature proteolysis occurs in a progressive manner. Mass spectrometry analysis of some large-sized breakdown intermediates formed in the early steps of hydrolysis indicated that both enzymes effectively hydrolyzed some regions of beta-lactoglobulin that were transiently exposed during the physical treatments and that were not accessible in the native protein. The immunochemical properties of the products of beta-lactoglobulin hydrolysis were assessed by using various beta-lactoglobulin-specific antibodies, and most epitopic sites were no longer present after attack of the partially unfolded protein by the two proteases.

Interallelic recombination is probably responsible for the occurrence of a new alpha(s1)-casein variant found in the goat species

The alphas1-casein (alphas1-Cas) locus in the goat is characterized by a polymorphism, the main feature of which is to be qualitative as well as quantitative. A systematic analysis performed in an autochthon southern Italy breed identified a new rare allele (M), which was characterized at both the protein and genomic level. The M protein displays the slowest electrophoretic mobility of the alphas1-Cas variants described so far. MS and automated Edman degradation experiments showed that this behavior was due to the loss of two phosphate residues in the multiple phosphorylation site (64SP-SP-SP-SP-SP-E-70E) consecutively to a Ser-->Leu substitution at position 66 of the peptide chain (64S-SP-L-SP-SP-E-70E). This was confirmed by sequencing a genomic DNA fragment encompassing exon 9 where the 8th codon (TCG) was shown to be mutated to TTG. Sequencing of amplified genomic DNA segments spanning the 5' and 3' flanking regions of each exon allowed us to identify 23 single nucleotide polymorphisms and two insertion/deletion events in the coding as well as the noncoding regions. A comparison of specific haplotypes defined for each of the alphas1-CasF, A and M alleles indicates that the M allele probably arises from interallelic recombination between alleles A and B2, followed by a C-->T transition at nucleotide 23 of the ninth exon. The region encompassing the recombination break point was putatively located between nucleotide 86 upstream and nucleotide 40 downstream of exon 8. Interallelic recombination therefore appears to be a possible means of generating allelic diversity at the alphas1-Cas locus, at least in the goat. The previously proposed molecular phylogeny must now be revised, possibly starting from two ancestral allelic lineages.

Reliable sequence determination of ribosome- inactivating proteins by combining electrospray mass spectrometry and Edman degradation

The primary structure of saporin-S9 and MAP-S, two type-1 ribosome-inactivating proteins isolated from the seeds of Saponaria officinalis L. and Mirabilis jalapa, respectively, was determined using a combined approach based on Edman degradation and electrospray ionization mass spectrometry (ESMS). Saporin-S9 has 253 amino acids with a calculated molecular mass of 28,492.99, which is in good agreement with that determined by ESMS (28 495 +/- 2 Da). Unlike other saporins with known primary structure, saporin-S9 contains four histidinyl residues (positions 111, 121, 216 and 248). By comparing the amino acid sequence of saporin-S9 with that of saporin-S6, we found 22 amino acid substitutions (8.7%), 13 of which are conservative and nine non-conservative. The residues known to be involved in the definition of the active site and with RNA base recognition are conserved. The four histidinyl residues and especially Lys for Gln203 contribute to the higher calculated pI value (10.17) of saporin-S9 compared with saporin-S6 (9.98). MAP-S contains 250 amino acid residues with a calculated molecular mass of 27,789.49, in good agreement with that determined by ESMS (27,789 +/- 2). Cys36 and Cys220 form a disulphide bridge and only four amino acid residues are different from the amino acid sequence of MAP, isolated from the roots of the same plant, i.e. Leu34 (Glu), Ile161 (Leu), Asp185 (Glu) and Asp191 (Glu) (in parentheses, the residues present in MAP). The reported approach can provide rapid and reliable sequence screening in the analysis of homologous proteins, including the presence of disulphide bridges.

Two-dimensional gel electrophoresis/matrix-assisted laser desorption/ionisation mass spectrometry of commercial bovine milk

Proteins in commercial bovine milk have been separated by two-dimensional gel electrophoresis and examined by matrix-assisted laser desorption/ionisation mass spectrometry. Gel separation was conducted in two different pH gradients, 3-10 and 6-11; the latter range resulted in a higher spot resolution and favoured the basic proteins. We have limited the time-of-flight mass spectrometry analysis to the linear mode to examine the capability of reliable relative molecular masses of the intact proteins in their characterisation. The present study draws attention to the difficulty of identifying basic proteins with low molecular masses (below 12000 Da) that are commonly encountered in milk samples.

Applications of mass spectrometry to food proteins and peptides

The application of mass spectrometry (MS) to large biomolecules has been revolutionized in the past decade with the development of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) techniques. ESI and MALDI permit solvent evaporation and sublimation of large biomolecules into the gaseous phase, respectively. The coupling of ESI or MALDI to an appropriate mass spectrometer has allowed the determination of accurate molecular mass and the detection of chemical modification at high sensitivity (picomole to femtomole). The interface of mass spectrometry hardware with computers and new extended mass spectrometric methods has resulted in the use of MS for protein sequencing, post-translational modifications, protein conformations (native, denatured, folding intermediates), protein folding/unfolding, and protein-protein or protein-ligand interactions. In this review, applications of MS, particularly ESI-MS and MALDI time-of-flight MS, to food proteins and peptides are described.

Application of chromatography and mass spectrometry to the characterization of food proteins and derived peptides

The following review describes the development of mass spectrometry off-line and on-line coupled with liquid chromatography to the analysis of food proteins. It includes the significant results recently obtained in the field of milk, egg and cereal proteins. This paper also outlines the research carried out in the area of food protein hydrolysates, which are important components in foodstuffs due to their functional properties. Liquid chromatography and mass spectrometry have been particularly used for the characterization of food peptides and especially in dairy products.

Analysis of major ovine milk proteins by reversed-phase high-performance liquid chromatography and flow injection analysis with electrospray ionization mass spectrometry

Ovine milk proteins were analyzed both by coupling HPLC and electrospray ionization mass spectrometry (ESI-MS) and by flow injection analysis and ESI-MS detection after separation and collection of fractions from gel permeation chromatography. These methods resolved the four ovine caseins and whey proteins and made it possible to study the complexity of these proteins associated with genetic polymorphism, post-translational changes (phosphorylation and glycosylation) and the presence of multiple forms of proteins. The experimental molecular masses of ewe milk proteins were: 19,373 for kappa-casein 3P; 25,616 for alpha(s2)-casein 10P; 23,411 for alpha(s1)-casein C-8P; 23,750 for beta-casein 5P; 18,170 and 18,148 for beta-lactoglobulins A and B; 14,152 for alpha-lactalbumin A and 66,322 for serum albumin.

Analysis of major caprine milk proteins by reverse-phase high-performance liquid chromatography and electrospray ionization-mass spectrometry

Major proteins from caprine milk were separated by preparative gel permeation and cation-exchange fast protein liquid chromatography and were characterized by flow injection analysis by electrospray ionization mass spectrometry. In addition, proteins from whole skim milk and whole casein were analyzed by coupling reverse-phase HPLC and electrospray ionization mass spectrometry by two different chromatographic methods. These methods successfully resolved the major caprine milk proteins and main casein variants. The experimental molecular masses of major milk proteins and variants were: 19,302 for kappa-CN 2P; 25,599 for alphas2-CN A-11P; 25,514 for alphas2-CN B-10P; 23,370 for alphas1-CN A-8P; 23,345 for alphas1-CN B-8P; 23,264 for alphas1-CN E-8P; 18,817 for alphas1-CN F-3P; 23,835 for beta-CN 6P; 18,181 for beta-LG; 14,180 for alpha-LA and 66,318 for serum albumin.

Alternative nonallelic deletion is constitutive of ruminant alpha(s1)-casein

Multiple forms of alpha(s1)-casein were identified in the four major ruminant species by structural characterization of the protein fraction. While alpha(s1)-casein phenotypes were constituted by a mixture of at least seven molecular forms in ovine and caprine species, there were only two forms in bovine and water buffalo species. In ovine and caprine forms the main component corresponded to the 199-residue-long form, and the deleted proteins differed from the complete one by the absence of peptides 141-148, 110-117, or Gln78, or a combination of such deletions. The deleted segments corresponded to the sequence regions encoded by exons 13 and 16, and by the first triplet of exon 11 (CAG), suggesting that the occurrence of the short protein forms is due to alternative skipping, as previously demonstrated for some caprine and ovine phenotypes. The alternative deletion of Gln78 in alpha(s1)-casein, the only form common to the milk of all the species examined and located in a sequence region joining the polar phosphorylation cluster and the hydrophobic C-terminal domain of the protein, may play a functional role in the stabilization of the milk micelle structure.

Isolation and identification of some major water-soluble peptides in Feta cheese

Peptides were isolated from the water-soluble fraction of Feta cheese by reversed-phase HPLC in three successive steps. Peptide sequencing was performed by automatic Edman degradation. Most of the peptides originated from alpha s1-casein (CN), especially from the N-terminal half of the molecule. Two peptides originated from the C-terminal domain of beta-CN. Only one peptide, which was rich in histidine, originated from kappa-CN. beta-Lactoglobulin and alpha-lactalbumin were also identified in the water extract of Feta cheese. The origin of most of these peptides could be explained on the basis of known specificities of chymosin and lactococcal cell-wall proteinases.

The primary structure of water buffalo alpha(s1)- and beta-casein identification of phosphorylation sites and characterization of a novel beta-casein variant

The primary structure of water buffalo alpha(s1)-casein and of beta-casein A and B variants has been determined using a combination of mass spectrometry and Edman degradation procedures. The phosphorylated residues were localized on the tryptic phosphopeptides after performing a beta-elimination/thiol derivatization. Water buffalo alpha(s1)-casein, resolved in three discrete bands by isoelectric focusing, was found to consist of a single protein containing eight, seven, or six phosphate groups. Compared to bovine alpha(s1)-casein C variant, the water buffalo alpha(s1)-casein presented ten amino acid substitutions, seven of which involved charged amino acid residues. With respect to bovine betaA2-casein variant, the two water buffalo beta-casein variants A and B presented four and five amino acid substitutions, respectively. In addition to the phosphoserines, a phosphothreonine residue was identified in variant A. From the phylogenetic point of view, both water buffalo beta-casein variants seem to be homologous to bovine betaA2-casein.