Amino terminal sequences of the precursors of ovine caseins

Complete sequence of the ovine beta-casein-encoding gene and interspecies comparison

The 9149-bp transcription unit encoding ovine beta-casein (Cas) and 4636 bp of 5' flanking region were completely sequenced. The gene is composed of nine exons and its overall organization is similar to that of its counterparts from other species. Intron 4, the largest, shares three similar stretches (sizes ranging from 0.1 to 0.3 kb) with the region upstream from the transcription unit. These common sequences are part of short interspersed nuclear elements (SINE) specific to Bovidae (Bov). Intron 4 contains two 274-bp Bov-A2 SINE in opposite orientation, as well as a full-length 569-bp Bov-B SINE. This latter SINE, also present in caprine intron 4, is missing in cattle. This suggests that the amplification of Bov-SINE has continued after the divergence of cattle from sheep and goats, assuming that the presently known sequences are representative of these species.

Structure and function of milk protein genes

Interspecies comparisons of cDNA and mosaic milk protein genes have confirmed their high rate of evolution, but the overall gene organization has been conserved. The three Ca-sensitive casein genes, which share common motifs in the promoter region and contain similar sequences that encode signal peptide and multiple phosphorylation sites, probably derived from a common ancestor. alpha s1- and alpha s2-casein genes, divided into many small exons, undergo complex splicing, and the deleted caseins arise from exon skipping. The four bovine casein genes are clustered on 200 kb of chromosome 6. alpha-Lactalbumin and beta-lactoglobulin pseudogenes occur in ruminants. Study of the expression of native and modified milk protein genes in mammary cell lines and transgenic animals and DNA footprinting have shown the occurrence of important regulatory motifs in the proximal 5' flanking region, including one recognized by a specific mammary nuclear factor. Good stage- and tissue-specific expression has been obtained in transgenic animals with milk protein genes having less than a 3-kb 5' flanking region. Better knowledge of both the structure and function of milk protein genes, which has already allowed the use of powerful techniques for the rapid identification of alleles, offers the potential for the genetic modification of milk composition.

The complete sequence of the gene encoding bovine alpha s2-casein

From a bovine genomic library, five overlapping clones, spanning some 50 kb, have been isolated. These clones contain the complete alpha s2-casein-encoding gene (alpha s2ca) and its 5' and 3' flanking regions. The nucleotide (nt) sequence of the complete gene including 2510 bp of the 5' flanking region and 276 bp of the 3' region has been determined. The total length of alpha s2ca appears to be 18483 bp and, therefore, it is the longest of the four bovine casein-encoding genes. The alpha s2ca gene is comprised of 18 exons ranging in size from 21 to 266 nt. There are 16 Alu-like artiodactyla retroposons inserted at ten different locations within the gene. About 14% of the gene is composed of these repetitive sequences. Although the organization of alpha s2ca appears to be similar to that of the alpha s1-casein-encoding gene (alpha s1ca), sequence comparisons and the length of the exons indicate that it is more closely related to the beta-casein-encoding gene. Furthermore, it is shown that both genes could have evolved from a common ancestor by means of internal duplications.

Construction and identification of recombinant plasmids carrying cDNAs coding for ovine alpha S1-, alpha S2-, beta-, kappa-casein and beta-lactoglobulin. Nucleotide sequence of alpha S1-casein cDNA

An ovine mammary cDNA library has been constructed from total poly(A)+ RNA isolated from the mammary gland of a lactating ewe, using a classical procedure. Blunt-ended double-stranded cDNAs prepared with reverse transcriptase and nuclease S1 were tailed with dCTP, inserted into the dGMP-tailed PstI site of plasmid pBR322 and cloned in E. coli. Five series of homologous clones representing abundant messenger RNAs (strong hybridization with a single-stranded cDNA probe generated from total poly(A)+ RNA) were selected using each time a different predominant cloned ds-cDNA as probe, then identified by positive hybridization-translation of the cognate mRNA and subsequent immunoprecipitation and electrophoresis of the protein. The lengths of alpha s1-, alpha s2-, beta-, kappa-casein and beta-lactoglobulin mRNAs are in the range of 1.2, 1.1, 1.25, 1.0 and 0.85 kb, respectively, as determined by Northern blotting analysis. Five homologous mRNAs of similar sizes were identified in the porcine species by dot blot hybridization and Northern analyses. The nucleotide sequence of alpha s1-casein mRNA was determined by sequencing, according to Maxam and Gilbert, both a 1080 bp long cloned ds-cDNA and a ss-cDNA (268 nucleotides) generated by 5' extension of a 5' terminal truncated radiolabeled fragment (83 bp) of the relevant ds-cDNA, used as primer for reverse transcription. The 3' non coding region (431 nucleotides, excluding the poly(A) tail) represents 70% of the length of the coding region (618 nucleotides) flanked by a 61 nucleotide 5' region. Comparison of sequences of ovine and bovine, rat and guinea-pig alpha s1-casein mRNAs has revealed a greater homology in the 3' and especially 5' non coding regions. In the reading frame, the conserved regions are essentially those corresponding to the signal peptide and phosphopeptide domains. The derived 206 amino acid sequence of ovine pre-alpha s1-casein differs from that of its bovine counterpart (genetic variant B) by 24 amino acid substitutions and a deletion of 8 amino acid residues occurring in the polypeptide chain of the mature protein. Such a variation (84% homology only) in two phylogenetically closely related species indicates a high rate of evolution of alpha s1-casein.

Nucleotide sequence of cloned cDNA coding for mouse epsilon casein

We isolated cDNA clones, which correspond to the mRNA coding for the smallest of the seven mouse caseins. From the nucleotide sequence of the cDNA we deduced the amino acid sequence of the protein, which we named epsilon casein. Mouse epsilon casein and cow alpha s2 casein show amino acid sequence homologies in the N-terminal region of the mature protein. The signal peptide of mouse epsilon casein shows in length and sequence remarkable homology to the signal peptides of the calcium-precipitable caseins of other species. In accordance with this group of caseins mouse epsilon casein contains in the sequence -Ser-Ser-Glu-Glu- a site for potential multiple phosphorylation.

Comparative sequence analysis of the mRNAs coding for mouse and rat whey protein

Whey acidic protein (WAP) is a major milk protein found in mouse and rat. Cloned WAP cDNAs from both species have been sequenced and the respective protein sequences have been deduced. Mouse and rat WAP (134 and 137 amino acids respectively) are acidic, cysteine rich proteins which contain a N-terminal signal peptide of 19 amino acids. Most of the cysteines are located in two clusters containing six cysteine residues each, arranged in an identical pattern. Comparison of the mouse and rat WAPs show that the signal peptide and the first cysteine domain are conserved to a greater extent than the rest of the protein. This result is reflected in the nucleotide sequence homology, where the regions coding for the signal peptide and cysteine domain I are the only regions where the rate of replacement substitution is lower than the rate of silent substitution. The 3' non-coding regions show a 91% conservation which is half the substitution rate for the coding region. This low rate of sequence divergence in the 3' non-translated region of the mRNA may indicate a functional importance for this region.

Mouse whey acidic protein is a novel member of the family of 'four-disulfide core' proteins

Unlike in other mammalian species, the major whey protein in mouse is not alpha-lactalbumin, but a cysteine rich, acidic protein with a molecular weight of 14.0 kDa. We have deduced the amino acid sequence of this mouse acidic of whey protein from the nucleotide sequence of cloned cDNA. The positions of the half cysteines suggest that mouse whey acidic protein (WAP) is a two domain protein, very similar in structure to the plant lectin wheat germ agglutinin and the hypothalamic carrier protein neurophysin.

Localization of the casein gene family to a single mouse chromosome

A series of mouse-hamster somatic cell hybrids containing a variable number of mouse chromosomes and a constant set of hamster chromosomes have been used to determine the chromosomal location of a family of hormone-inducible genes, the murine caseins. Recombinant mouse cDNA clones encoding the alpha-, beta-, and gamma-caseins were constructed and used in DNA restriction mapping experiments. All three casein cDNAs hybridized to the same set of somatic cell hybrid DNAs isolated from cells containing mouse chromosome 5, while negative hybridization was observed to ten other hybrid DNAs isolated from cells lacking chromosome 5. A fourth cDNA clone, designated pCM delta 40, which hybridized to an abundant 790 nucleotide poly(A)RNA isolated from 6-d lactating mouse mammary tissue, was also mapped to chromosome 5. The chromosomal assignment of the casein gene family was confirmed using a mouse albumin clone. The albumin gene had been previously localized to mouse chromosome 5 by both breeding studies and analogous molecular hybridization experiments. An additional control experiment demonstrated that another hormone-inducible gene, specifying a 620 nucleotide abundant mammary gland mRNA, hybridized to DNA isolated from a different somatic cell hybrid line. These studies represent the first localization of a peptide and steroid hormone-responsive gene family to a single mouse chromosome.

Cell-free synthesis, proteolytic processing, core glycosylation, and amino terminal sequence of rabbit pre-alpha-lactalbumin

Two different forms of alpha-lactalbumin were isolated from rabbit milk and partially characterized. The major and the minor species had apparent molecular weights of 18000 and 14000, respectively, according to their electrophoretic mobilities on SDS polyacrylamide gels. Analyses of their amino acid compositions and amino-and carboxy-terminal sequences did not reveal any difference, but sugar analysis showed the occurrence of carbohydrates in the major species. Rabbit alpha-lactalbumin was synthesized in a cell-free translation system as a precursor with an amino terminal extension of 19 amino acid residues whose primary structure is rather different from those of its ovine and porcine counterparts, in contrast with the extensive similarity so far observed between the known signals of homologous milk proteins. When mammary microsomal membranes were added during translation, the preprotein was converted to authentic alpha-lactalbumin, as demonstrated by amino terminal sequence analyses. However, one of the two processed forms migrated more slowly than pre-alpha-lactalbumin on SDS polyacrylamide gels and this was related to the occurrence of carbohydrates: only the "slower moving" polypeptide was specifically adsorbed on concanavalin A Sepharose and its electrophoretic mobility was enhanced after treatment with endoglycosidase H, an enzyme known to remove clustered mannosyl residues linked to di-N-acetylchitobiose. It was also observed that the rate of translocation of alpha-lactalbumin across the microsomal membrane was lower than that of beta-casein.

Characterization of the translation products of the major mRNA species from rabbit lactating mammary glands and construction of bacterial recombinants containing casein and alpha-lactalbumin complementary DNA

Total cytoplasmic polyadenylated RNA from lactating rabbit mammary glands was analysed on methylmercury hydroxide-agarose gels. The size of the most abundant mRNA species ranged between 0.5 and 5.0 kb (kilobases), with major bands at 0.55, 0.84, 0.92, 1.18 and 2.4 kb and discrete minor bands of 1.5, 1.7, 3.0 and 3.9 kb. Translation in vitro of total mRNA with [3H]leucine or [35S]methionine as precursor yielded four major bands with apparent Mr values of 16 000, 25 000, 26 000 and 29 000. The four protein bands were identified by immunoprecipitation by using specific antisera as alpha-lactalbumin and x-, kappa- and alpha-caseins, respectively. Labelling with (35S]cysteine followed by immunoprecipitation with anti-transferrin or anti-alpha-lactalbumin sera allowed the identification of two whey proteins. Translated transferrin was resolved as an 80 000-dalton band and alpha-lactalbumin appeared as a 16 000-dalton protein. A library of recombinant plasmids containing cDNA (complementary DNA) sequences representing cytoplasmic polyadenylated RNA was used to isolate clones for the major rabbit caseins and alpha-lactalbumin. A preliminary characterization of these cDNA clones was achieved by colony hybridization with enriched RNA fractions as probes. Positive clones were identified by use of hybrid-promoted translation in vitro and immunoprecipitation of the translation products. The corresponding mRNA species were further identified by hybridizing RNA blots with radioactively labelled cDNA clones. We present the restriction map of alpha-casein and kappa-casein cDNA clones.

Heterogeneity of guinea-pig caseins synthesized and sequestered by cell-free protein-synthesizing systems

1. Individual mRNA species encoding guinea-pigs caseins A, B and C, and alpha-lactalbumin, were purified by hydridization to recombinant milk-protein plasmid DNA immobilized on diazobenzyloxymethyl-paper or diazobenzyloxymethyl-cellulose. Addition of the purified mRNA species to a reticulocyte-lysate cell-free system, in the presence or absence of a dog pancreas microsomal membrane fraction, established a precursor-product relationship between the primary translation products and those sequestered within microsomal vesicles, as determined by polyacrylamide-gel analysis in one and two dimensions. 2. Three sequestered variants of sequestered casein A were identified, but only single forms of sequestered casein B and alpha-lactalbumin. Sequestered variants of casein C proved to be unexpectedly basic, and did not focus on the pH gradient utilized. 3. Comparative analysis of milk proteins synthesized in the reticulocyte-lysate and wheat-germ cell-free systems by two-dimensional gel electrophoresis demonstrated both quantitative and qualitative differences. In particular, marked but variable heterogeneity was apparent within the primary translation products of casein A and casein B. Pre-casein C did not focus. Limited N-terminal processing of the primary translation products was also evident. These observations are discussed in relation to (i) unscheduled post-translational modifications by cell-free protein-synthesizing systems and (ii) multiplicity of signal sequences. 4. Overall we demonstrate that complex precursor-product relationships between primary translation products and their sequestered variants, programmed in vitro by a mixed mRNA population, may be readily analysed by using individual mRNA sequences purified by hybridization to immobilized cloned complementary-DNA sequences.

Purification and properties of a major casein component of rat milk

A casein component (C2-casein) was purified by ion-exchange and gel filtration chromatography from rat milk, and the properties of this protein were examined. The molecular weight of C2-casein, as determined by Sepharose 4B gel filtration in 6 M guanidine hydrochloride, was 34 000 +/- 1000. The average hydrophobicity calculated from the amino acid composition showed that C2-casein is a rather hydrophilic protein. The alpha-helix content obtained from optical rotatory dispersion experiments was about 12%. In ultracentrifugation analyses, monomer and polymer peaks of C2-casein were both seen, and the monomer-to-polymer ratio was not affected by changing temperature conditions. C2-casein was precipitated by the presence of 2.5 mM CaCl2, and the precipitability was greatly decreased by the dephosphorylation of the protein. C2-casein was stabilized from Ca2+-dependent precipitation by the addition of another rat casein component (C3-casein) or of bovine kappa-casein.

Phosphorylation of caseins, present evidence for an amino acid triplet code posttranslationally recognized by specific kinases

The fifty of so phosphorylated hydroxyamino acid residues hitherto investigated in caseins from different species have been found to occur in tripeptide sequences -Ser/Thr-X-A- where X represents any amino acid residue and A is an acidic residue. This strongly suggests that phosphorylation of caseins involves basically the stepwise enzymatic recognition of primary and secondary anionic amino acid triplets where the determinants are dicarboxylic residues and phosphoseryl residues, respectively. Studies of genetic variants of bovine caseins have provided clear-cut evidence for the actual occurrence of the former recognition sites. The occurrence of the above tripeptide sequences is a necessary but not a sufficient condition for phosphorylation of caseins to occur. Possible factors of constraint such as different intrinsic properties of both phosphate acceptor residues and acidic determinants, the characteristics of the local environment in terms of overall charge and hydrophilicity, secondary structure and steric hindrance, an insufficient available pool of casein kinase(s)... are discussed. All evidence now available supports the concept that phosphorylation of caseins is a posttranslational event and it is suggested that the process may occur during the transfer of completed polypeptide chains from the smooth endoplasmic reticulum to the Golgi apparatus where most of phosphate incorporation is presumably carried out. This organelle is rich in membrane-bound specific cyclic AMP-independent kinase(s) able in vitro to rephosphorylate specifically although not completely phosphatase-treated caseins and caseinophosphopeptides.

Processing mechanisms in the biosynthesis of proteins

Limited proteolysis is a widely occurring mechanism in protein biosynthesis. Protein precursors can be classified according to their functions, localization within cell compartments, and enzymic cleavage mechanisms. The presecretory proteins represent an important class of very rapidly turning over precursors which play an early role in the sequestration of secretory products and whose cleavage appears to be intimately associated with structures formed at the ribosome-membrane junction during protein synthesis. A model is proposed which predicts that the prepeptide forms a beta-pleated sheet structure with other components of the membrane which results in the transfer of a loop of peptide across the microsomal membrane. Proinsulin is representative of the general class of proproteins that are processed post-translationally within their secretory cells either during the formation and maturation of secretory granules (peptides hormones and neurotransmitters, serum albumins) or during the assembly of macromolecular structures (virus capsules, membrane-associated enzyme complexes). The former group are cleaved by Golgi-associated proteases having tryptic and carboxypeptidase B-like specificity. Some precursors are secreted as such and processed extracellularly either in the circulation or at special sites (procollagens, zymogens, provenoms, vitellogenins).

The biosynthesis of bovine fibrinogen, prothrombin, and albumin in a cell-free system

Preparative amounts of poly A-containing mRNA were isolated from bovine liver. In the presence of a cell-free system from rabbit reticulocytes, this mRNA stimulated the synthesis of a number of plasma proteins including albumin, fibrinogen, prothrombin, and antithrombin III. Automatic Edman degradation of immunoprecipitated albumin showed that this protein was synthesized as preproalbumin containing an NH2-terminal extension of 18 amino acid residues. Prothrombin is also synthesized as a precursor (preprothrombin), and the signal sequence for this protein is also rich in hydrophobic amino acids. The three chains of fibrinogen are synthesized from individual mRNAs, which are then linked by disulfide bonds to form the mature protein.

Sequestration and turnover of guinea-pig milk proteins and chicken ovalbumin in Xenopus oocytes

The stability and distribution of proteins within the living cell can be studied using Xenopus laevis oocytes. Microinjection of messenger RNAs and secretory proteins, followed by cell fractionation, shows that transfer of ovalbumin and milk proteins across intracellular membranes of the oocyte only occurs during their synthesis. Thus milk protein primary translation products, made in the wheat germ cell-free system, when injected into oocytes remain in the cytosol and are not recovered within membrane vesicles. Such miscompartmentalized primary milk proteins are rapidly degraded (t 1/2 0.6 +/- 0.1 h). In contrast, processed milk proteins, extracted from oocytes injected with mammary gland RNA, are relatively stable when introduced into the cytosolic compartment (t 1/2 alpha-lactalbumin 20 +/- 8 h, casein A 6 h, casein B 4 h, casein C 8.3 h). The primary ovalbumin product is also stable (t 1/2 22 +/- 9 h). Indirect evidence that rapid degradation of miscompartmentalized milk protein primary translation products may occur in vivo was obtained by the injection of massive amounts of ovalbumin and milk protein mRNA. Under these conditions there is no accumulation of primary milk protein translation products, but a polypeptide resembling the unglycosylated ovalbium wheat germ primary product can be detected in the cytosol. Only the glyclosylated forms of ovalbumin are found in the oocyte membrane vesicle fraction. We discuss the roles played by the presence of detachable signal sequences and the absence of secondary modifications in determining the rate of degradation of primary translation products within the cytosol.

Role of glucocorticoids and progesterone in the development of rough endoplasmic reticulum involved in casein biosynthesis

Hydrocortisone acetate injected into pseudopregnant rabbits induced casein synthesis and a parallel accumulation of casein mRNA. These effects were not accompanied by any enrichment of total RNA in the mammary cell. Hydrocortisone acetate did not favour the attachment of polysomes to endoplasmic reticulum. Casein mRNA concentration was enhanced in free and membrane-bound polysomes. After long treatments, the concentration of casein mRNA reached a plateau in membrane bound polysomes whereas it continued to be accumulated in free polysomes, suggesting that a substantial part of casein synthesis is then carried out by free polysomes. Progesterone injected with high doses of prolactin was unable to prevent the stimulatory action of prolactin on the synthesis of casein, the accumulation of casein mRNA and mammary gland growth, as judged by DNA content. By contrast, the increase in the total RNA content of mammary gland was still significantly reduced by progesterone. In addition, progesterone inhibited almost completely the formation of membrane-bound polysomes and the anchorage of casein mRNA to endoplasmic reticulum. From these data, it was concluded that the formation of the endoplasmic reticulum is not a prerequisite for the initiation of casein synthesis. Glucocorticoids do not play a major role in the formation of the endoplasmic reticulum and the Golai apparatus and in the binding of casein synthesizing polysomes to membranes. Progesteronne is capable of inhibiting preferentially and gradually the stimulation of cellular functions requiring the most potent prolactin stimulation.

Study of the precursors of ovine lactoproteins: primary structures of the 'signals' and enzymic processing of prelactoproteins by mammary microsomal membranes

The radiolabelled primary translation products of ovine mammary mRNAs synthesized in a wheat germ cell-free system were isolated by immunoprecipitation and analysed by automated Edman degradation. The 3 'Ca-sensitive' caseins (alpha S1, alpha S2 and beta), kappa-casein, beta-lactoglobulin and alpha-lactalbumin were found to be synthesized as precursors with amino terminal extensions of 15, 21, 18 and 19 amino acid residues respectively. The extra pieces of these various lactoproteins are similar to 'signal' peptides of other secretory proteins in their length and hydrophobicity. The occurrence of an alanyl residue at the C-termini of the extra pieces of the 6 ovine prelactoproteins suggests that the mammary proteinase responsible for the cleavage of the signal peptides may have an elastase-like specificity. When mammary mRNAs were translated in a wheat germ cell-free system in the presence of mammary microsomal membranes, pre-beta-casein was converted into authentic beta-casein as demonstrated by amino terminal sequence analyses. Additionally, pre-beta-casein was post-translationally converted into authentic beta-casein by a specific proteinase(s) extracted from rough microsomes with Na deoxycholate.