Rat beta casein cDNA: sequence analysis and evolutionary comparisons

MiR-486 regulates lactation and targets the PTEN gene in cow mammary glands

Mammary gland development is controlled by several genes. Although miRNAs have been implicated in mammary gland function, the mechanism by which miR-486 regulates mammary gland development and lactation remains unclear. We investigated miR-486 expression in cow mammary gland using qRT-PCR and ISH and show that miR-486 expression was higher during the high-quality lactation period. We found that miR-486 targets phosphoinositide signaling in the cow mammary gland by directly downregulating PTEN gene expression and by altering the expression of downstream genes that are important for the function of the mammary gland, such as AKT, mTOR. We analyzed the effect of β-casein, lactose and triglyceride secretion in bovine mammary gland epithelial cells (BMECs) transfected by an inhibitor and by mimics of miR-486. Our results identify miR-486 as a downstream regulator of PTEN that is required for the development of the cow mammary gland.

The sequence of porcine beta-casein cDNA

Porcine cDNAs clones encoding beta-casein were isolated and sequenced. The porcine beta-casein cDNA is 1100bp in length, excluding the poly(A) tail, and encodes a preprotein of 232 amino acids.

Multispecies comparison of the casein gene loci and evolution of casein gene family

Caseins, the major milk proteins, are present in a genomic cluster spanning 250-350 kb. The divergence at the coding level between human, rodent, and cattle sequences is rather extensive for most of the genes in this region. Nevertheless, comparative analysis of genomic sequences harboring the casein gene cluster region of these species (with equal evolutionary distances 79-88 Myr) shows that the organization and orientation of the genes is highly conserved. The conserved gene structure indicates that the molecular diversity of the casein genes is achieved through variable use of exons in different species and high evolutionary divergence. Comparative analysis also revealed the presence within two species of uncharacterized casein family members and ruled out the previously held notion that another gene family, located in this region, is primate-specific. Several other new genes as well as conserved noncoding sequences with potential regulatory functions were identified. All genes identified in this region are, or are predicted to be, secreted proteins involved in mineral homeostasis, nutrition, and/or host defense, and are mostly expressed in the mammary and/or salivary glands. These observations suggest a possible common ancestry for the genes in this region.

Stat5 phosphorylation status and DNA-binding activity in the bovine and murine mammary glands

The transcription factors Stat5a and Stat5b are mediators of prolactin signalling in mammary epithelial cells, and are thought to play a role in lactogenesis. In cultured cells, activation of Stat5 activity through phosphorylation results in Stat5 binding to the promoters of at least some of the milk protein genes, thereby stimulating their transcription. However, the mammary biology of Stat5 differs between species, and the role of Stat5 in the bovine mammary gland is not fully understood. We have generated an antibody that specifically recognises the phosphorylated forms of Stat5a and Stat5b and used it to compare the levels of phosphorylated Stat5 with Stat5 DNA-binding activity in bovine and murine mammary tissue. Both Stat5 DNA-binding activity and phosphorylation status in the bovine mammary gland were at near-maximal levels at late pregnancy (27-35 days prior to calving), when at least three of the major milk proteins are not highly expressed. In addition, these studies revealed significant animal-to-animal variation in the level of Stat5 activity in both species. The results are consistent with a role in terminal differentiation of mammary epithelial cells. They also suggest that the stimulation of high-level expression of milk protein genes in the bovine mammary gland is not through activation of the prolactin receptor-Jak2-Stat5 pathway.

Comparative aspects of milk caseins

The caseins comprise the major protein component of milk of most mammals and are secreted as micelles that also carry high concentrations of calcium. They are phosphoproteins that represent the products of four genes, equivalent to those that encode the bovine alpha s1, alpha s2, beta, and kappa-caseins. There is considerable variation in the relative proportions of the particular caseins across species. The primary sequences of the alpha s1, alpha s2, and beta-caseins also show considerable species variation consistent with rapidly evolving genes that are proposed to have a common precursor. In contrast, the kappa-caseins exhibit features that demonstrate a separate origin and function where they are proposed to stabilise the micelle structure. This review focuses on comparative aspects of the caseins across a number of species for which information is now available.

Mammary Stat5 abundance and activity are not altered with lactation state in cows

Stat5 is a key intracellular mediator of prolactin signalling and can activate transcription of milk proteins in response to prolactin. Therefore, in animals such as mice where lactation is dependent on prolactin, Stat5 is likely to play an important role in establishing or maintaining lactation in the mammary gland. However, little is known about its role in lactation in the dairy cow. In order to address this, the levels of Stat5a and Stat5b protein, mRNA and Stat5 DNA-binding activity were measured in mammary tissue from mice and cows at different lactational states. In the cow, Stat5a and Stat5b protein and mRNA levels, as well as Stat5 DNA-binding activity were unaltered between pregnancy and established lactation. In contrast, in the mouse Stat5a and Stat5b protein, as well as Stat5 DNA-binding activity were clearly increased during lactation whereas Stat5a and Stat5b mRNA levels were highest during pregnancy as has been previously described. In both species only a minority of the epithelial cell nuclei were Stat5 positive during established lactation. These results suggest that there are significant differences in the biological role of Stat5 in controlling lactation between ruminants and rodents.

Structures and functionalities of milk proteins

During the last decade, marked progress has been made in the study of the fine details of the structures of milk proteins such as caseins, beta-lactoglobulin, alpha-lactalbumin, and lactotransferrin. Many of the functional properties of the individual milk proteins, as well as the milk protein products, may be described at the molecular level. This article is an attempt to thoroughly review the three-dimensional structures of major milk proteins, and to correlate them with the functional aspects of these proteins as food ingredients.

beta-Casein mRNA sequesters a single-stranded nucleic acid-binding protein which negatively regulates the beta-casein gene promoter

beta-Casein gene expression in mammary epithelial cells is under the control of the lactogenic hormones, glucocorticoids, insulin, and prolactin. The hormonal control affects gene transcription, and several regulatory elements in the beta-casein gene promoter between positions -80 and -221 have previously been identified. A region located in the promoter between positions -170 and -221 contains overlapping sequences for negative and positive regulatory elements. A sequence-specific single-stranded DNA-binding factor (STR), composed of two proteins with molecular masses of 35 and 54 kDa, recognizes the upper strand of this region and has a repressing role in transcription. High-level STR binding activity was observed in nuclear extracts from mammary glands of pregnant and postlactating mice and from noninduced HC11 mammary epithelial cells, cells with a low level of transcriptional activity of the beta-casein gene. STR activity is downregulated in mammary epithelial cells during lactation of the animals and after lactogenic hormone induction of HC11 cells in culture. These cells strongly transcribe the beta-casein gene. We investigated the mechanism of downregulation and found that a lactogenic-hormone-induced molecule (I-STR) inhibits STR from binding to its DNA target. I-STR is composed of RNA. STR is sequestered into the cytoplasm by I-STR after lactogenic hormone induction of mammary epithelial cells and remains present in an RNA-bound form. A high-affinity STR binding site was found in the 5' untranslated region of beta-casein mRNA. We propose that beta-casein mRNA can function as I-STR. beta-Casein mRNA may positively regulate its own transcription by translocating STR from the nucleus to the cytoplasm. The beta-casein STR binding sequence increases expression of a transfected beta-galactosidase gene when it is placed into the 5' untranslated region sequence of the mRNA. STR may have a positive role in posttranscriptional regulation.

beta-Casein mRNA sequesters a single-stranded nucleic acid-binding protein which negatively regulates the beta-casein gene promoter

beta-Casein gene expression in mammary epithelial cells is under the control of the lactogenic hormones, glucocorticoids, insulin, and prolactin. The hormonal control affects gene transcription, and several regulatory elements in the beta-casein gene promoter between positions -80 and -221 have previously been identified. A region located in the promoter between positions -170 and -221 contains overlapping sequences for negative and positive regulatory elements. A sequence-specific single-stranded DNA-binding factor (STR), composed of two proteins with molecular masses of 35 and 54 kDa, recognizes the upper strand of this region and has a repressing role in transcription. High-level STR binding activity was observed in nuclear extracts from mammary glands of pregnant and postlactating mice and from noninduced HC11 mammary epithelial cells, cells with a low level of transcriptional activity of the beta-casein gene. STR activity is downregulated in mammary epithelial cells during lactation of the animals and after lactogenic hormone induction of HC11 cells in culture. These cells strongly transcribe the beta-casein gene. We investigated the mechanism of downregulation and found that a lactogenic-hormone-induced molecule (I-STR) inhibits STR from binding to its DNA target. I-STR is composed of RNA. STR is sequestered into the cytoplasm by I-STR after lactogenic hormone induction of mammary epithelial cells and remains present in an RNA-bound form. A high-affinity STR binding site was found in the 5' untranslated region of beta-casein mRNA. We propose that beta-casein mRNA can function as I-STR. beta-Casein mRNA may positively regulate its own transcription by translocating STR from the nucleus to the cytoplasm. The beta-casein STR binding sequence increases expression of a transfected beta-galactosidase gene when it is placed into the 5' untranslated region sequence of the mRNA. STR may have a positive role in posttranscriptional regulation.

Structure of the human beta-casein encoding gene

The entire human beta-casein-encoding gene, Bca, was cloned and sequenced. The gene consists of eight exons ranging from 21 to 531 nucleotides (nt) in length and extending over 10,466 nt. Exon-2 contains the translational start, the entire signal sequence and the codons for the two first amino acids of the mature protein. This corresponds to the organization found in other species. The translational stop is localized to exon-7. Exon/intron boundaries are in accordance with the AG/GT rule and conform to suggested consensus sequences. Splice junctions are located between coding triplets. In all other species analyzed, Bca has been found to consist of nine exons; however, within intron-2 of the human gene, a sequence omitted from human mRNA, but corresponding to exon-3 of other known Bca genes, was revealed.

Characterization of two novel casein transcripts in rabbit mammary gland

Two clones were isolated from a cDNA library corresponding to mRNAs which accumulate in mid-lactating (14 day) rabbit mammary gland and characterized by DNA sequencing. The two clones sequenced corresponded to two novel casein transcripts (pBRM5 and pBRM42). Relative mRNA abundances for the two clones were assessed by dot-blot analysis. Phylogenetic analysis and comparison of both pBRM5 and pBRM42 with other members of the casein family revealed that the rabbit may be unique among mammals in expressing two alpha s2-casein genes. The presence of two alpha s2-casein genes in the rabbit may be the result of a relatively recent intergenic duplication event.

Arguments against the prostatic origin of the R-3327 Dunning H tumor

The Dunning tumor, originally described as a carcinoma of the rat dorsal prostate, has for long been used as an experimental model of prostatic cancer. We have recently presented a number of morphological findings that are incompatible with the prostatic origin of the H-subline of the Dunning tumor. In this paper, biochemical and immunohistochemical markers of rat prostate and mammary gland are studied in the R-3327 Dunning H tumor. Pieces of the H tumor were inoculated in male or lactating female rats. The electrophoretic protein pattern of Dunning tumor extracts was more similar to that of the mammary gland than the dorsolateral prostate. Proteins selectively appearing after metabolic labeling in Dunning tumors grown in lactating rats corresponded to labeled proteins in mammary glands from the same animals. Secretory proteins typical of the lateral prostate (SVS II) and dorsal prostate (transglutaminase) could not be detected immunohistochemically in the Dunning tumor. Western blot studies of tumor extracts and slot blot analysis of RNA preparations from the tumor confirmed the absence of SVS II and prostate specific transglutaminase from the Dunning tumor. On the other hand, the presence of mammary gland proteins such as milk fat globule membrane proteins, lactoperoxidase and lactalbumin were detected in the Dunning tumor by immunohistochemistry and Western blotting, but were absent from the dorsolateral prostate. Transferrin-mRNA, expressed in the male urogenital tract and also in the liver and other tissues, was detected in the mammary gland and Dunning tumor, but not in the dorsolateral prostate. The absence of mammary gland secretory beta-casein in the Dunning tumor was related to the elevated Ha-ras oncogene expression in the tumor, previously reported to suppress casein expression. The findings clearly demonstrate that the prostate cannot be the origin of the Dunning tumor, presently being used in prostatic cancer research. The designation prostatic adenocarcinoma for this tumor is therefore invalid. Furthermore, the data support our view that mammary gland might be the origin of the Dunning tumor, although the derivation from the bulbourethral or the parotid glands cannot strictly be excluded.

Cloning of the goat beta-casein-encoding gene and expression in transgenic mice

The goat beta-casein-encoding gene (CSN2), which encodes the most abundant protein of goat milk, has been cloned and sequenced. The intron/exon organization of the 9.0-kb goat CSN2 gene is similar to that of other CSN2 genes. Expression of the goat gene was principally restricted to the mammary gland of lactating transgenic animals. A low level of expression was also observed in skeletal muscle and skin. In contrast to a rat CSN2 transgene [Lee et al., Nucleic Acids Res. 16 (1988) 1027-1041], the goat gene was expressed to a high degree in the lactating mammary gland. Differences in the content or context of regulatory elements may account for the enhanced performance of the goat relative to the rat CSN2 gene in transgenic mice.

Developmental and environmental regulation of a mammary gland-specific nuclear factor essential for transcription of the gene encoding beta-casein

During the lactation period, mammary epithelial cells secrete large amounts of milk proteins. The coordinate regulation of milk protein expression is effected by the lactogenic hormones. We have investigated the activity of a mammary gland-specific transcription factor (MGF), which mediates hormonal influences at the level of a milk protein gene promoter. MGF-binding sites are present in the promoters of the most abundantly expressed milk protein genes. Mutation of the MGF-binding site in the beta-casein gene promoter completely abolishes responsiveness of the promoter to lactogenic hormones in cultured mammary epithelial cells. MGF activity is closely controlled in vivo. High MGF levels were found in mouse mammary gland nuclear extracts toward the end of pregnancy and during lactation. Withdrawal of suckling pups from their mothers during the lactation period caused a strong and rapid decrease of MGF activity. Readdition of pups to their mothers restored maximal MGF levels within 4 hr. We investigated MGF phosphorylation as a possible posttranslational modification responsible for regulation of the DNA-binding activity of MGF. Treatment of nuclear extracts from lactating mammary glands with potato acid phosphatase abolished MGF-binding activity. Casein kinase II phosphorylation of nuclear extracts from animals withdrawn from their pups for 24 hr enhanced MGF-binding activity. These results suggest that the reversible activation of MGF by suckling and withdrawal might be mediated by the action of kinases and phosphatases.

Exon-skipping is responsible for the 9 amino acid residue deletion occurring near the N-terminal of human beta-casein

Interspecies comparison and alignment of the beta-casein N-terminal sequence, taking into account its exon modular splitting derived from the known structural organization of the relevant genes, has revealed that a 9 amino acid residue sequence, corresponding to that encoded by the third exon of the other species genes, is lacking in human beta-casein. Using the polymerase chain reaction technique, we have amplified a human genomic 1-kb fragment, spanning from exon 2 to exon 4, which was subsequently cloned and sequenced. One hundred base pairs (bp) upstream from exon 4 and 737 bp downstream of exon 2, a 27-bp virtual exon 3 sequence, probably skipped during the course of pre-mRNA splicing, was identified. We discuss the possibility that this out-splicing event might be due to the weak strength of the 3' acceptor site and/or to the secondary structure sequestering of the branch site sequence.

Exon skipping in human beta-casein

Earlier amino acid alignments of mature beta-caseins showed that the human protein was shifted in alignment relative to other species, with amino acid deletions in the N-terminal region and others inserted in the C-terminal region. Our alignment, based on cDNA sequences and their translation products, has shown that the amino acid deletions correspond exactly to exon 3 in the other species. Cloning and sequencing of a segment of the human beta-casein gene between exons 2 and 4 revealed the presence of an intact exon 3 sequence in the gene. An interruption of the polypyrimidine tract adjacent to the 5' end of exon 3 sequence may account for the omission of the exon from human beta-casein mRNA.

Genomic organization of the bovine alpha-S1 casein gene

We report the sequence of the complete bovine alpha-s1 casein gene eludicating for the first time the genomic organization of an alpha-s type casein gene. Extending over 17508 bp the gene is split into 19 exons, ranging in size from 24 bp to 385 bp. Except for the translational stop codon not a single coding triplet of the alpha-s1 reading frame is disrupted by any of the splice junctions, which all confirm to known splice consensus sequences. Nine out of 16 coding exons begin with a 'GAX' codon, specific for glutamate. Splicing of this codon from exon 10 to the preceding exon creates a major phosphorylation site. An intron-exon-intron stretch of 154 bp comprising exons 10 and 13 is found precisely duplicated. Associated with the gene, copies of 8 atriodactyla retroposons are found, 6 of which are interspersed into the sequences of the three longest introns. We discuss the possibility that three functional parts of the gene have been recruited and evolutionary conserved at a time before gene diversification gave rise to the separate evolution of alpha- and beta-type casein-genes.

Cloning and sequencing of a cDNA encoding human milk beta-casein

A cDNA of 1065 bp encoding the human milk beta-casein was cloned and sequenced using a synthetic oligodeoxyribonucleotide probe and a human mammary gland library. The nucleotide (nt) sequence contained an open reading frame sufficient to encode the entire amino-acid (aa) sequence of a beta-casein precursor protein consisting of 210 aa and a signal peptide of 15 aa. The nt sequence shows 45-62% homology to those of bovine, ovine, rat, and mouse beta-caseins. The highly phosphorylated site, which is responsible for the calcium-binding capacity of beta-casein, the signal peptide, and a sequence encoding for an inhibitor to the angiotensin-converting enzyme seem highly conserved among the beta-caseins with known sequences.

Primary culture of normal rat mammary epithelial cells within a basement membrane matrix. II. Functional differentiation under serum-free conditions

A serum-free primary culture system is described which allows normal rat mammary epithelial cells (RMECs) embedded within a reconstituted basement membrane to undergo extensive growth and functional differentiation as detected by synthesis and secretion of the milk products casein and lipid. RMECs isolated from mammary glands of immature virgin rats were seeded within an extracellular matrix preparation derived from the Engelbreth-Holm-Swarm sarcoma and cultured in a serum-free medium consisting of Dulbecco's modified Eagle's medium-F12 containing insulin, prolactin, progesterone, hydrocortisone, epidermal growth factor, bovine serum albumin, transferrin, and ascorbic acid. Casein synthesis and secretion were documented at the electron microscopic level as well as by an enzyme-linked immunosorbent assay (ELISA) assay using a polyclonal antibody against total rat caseins. Numerous secretory vesicles with casein micelles were noted near the apical surface of the RMECs, and secreted casein was observed in the lumen. These ultrastructural data were confirmed by the ELISA assay which showed that microgram amounts of casein per well were synthesized by the RMECs and that the amount of casein increased with time in culture. Using immunoblot analysis it was demonstrated that the full complement of casein proteins was synthesized. In addition to casein protein, beta-casein mRNA levels were shown to increase with time. Synthesized lipid was detected at both the light and electron microscopic levels. Phase contrast photomicrographs demonstrated extensive intracellular lipid accumulation within the ductal and lobuloalveolarlike colonies, and at the electron micrograph level, lipid droplets were predominantly localized near the apical surface of the RMECs. The lipid nature of these droplets was verified by oil red O staining. Results from this study demonstrate that RMECs from immature virgin rats proliferate extensively and rapidly develop the capacity to synthesize and secrete casein and lipid when grown within a reconstituted basement membrane under defined serum-free conditions. This unique system should thus serve as an excellent model in which the regulation of mammary development and gene expression can be investigated.

cDNA of the 24 kDa subunit of the bovine respiratory chain NADH dehydrogenase: high sequence conservation in mammals and tissue-specific and growth-dependent expression

We have isolated and sequenced several overlapping cDNA clones from a bovine lambda gt10 library which encode all but the first five amino acids of the entire mature 24 kDa subunit of NADH:ubiquinone oxidoreductase (EC 1.6.99.3), the first enzyme of the respiratory chain. The derived amino acid sequence agrees with that determined by direct sequencing of the purified protein, filling in a gap in the published sequence. A comparison of the nucleotide and amino acid sequences of the bovine 24 kDa subunit with those recently determined for the rat homologue has shown that this nuclear-encoded subunit of an OX-PHOS complex has diverged in these two species much less than the mitochondrial DNA-encoded subunits of the same enzyme complex, and also less than a set of available non-mitochondrial nuclear DNA-coded proteins. The sequence analysis of the clones has revealed the expression in the brain of two mRNAs differing in the length of the 3'-untranslated region. Furthermore, two polyadenylated RNA species, 930 and 1080 nucleotides in length, probably corresponding to the above mRNAs, have been detected in bovine brain and other tissues by RNA gel blot hybridization. The level of expression of the 24kDa subunit gene varies by more than an order of magnitude among different tissues. A cross-hybridizing mRNA species of 930 nucleotides has also been observed in HeLa cells and found to be strongly growth regulated.

Isolation and structural analysis of the mouse beta-casein gene

Three overlapping clones containing the entire beta-casein gene were isolated from a mouse genomic library constructed in bacteriophage lambda EMBL3. Within the three clones the 6.8-kb casein gene was flanked by about 6-kb (5') and about 10-kb (3') sequences. The complete nucleotide sequence of the gene and its immediate flanking regions was determined. The gene consisted of nine exons ranging from 21 bp to 525 bp separated by introns ranging from 81 bp to 1288 bp. The length of the first exon was 43 bp, as determined by primer extension. The comparison of this sequence with that of the rat beta-casein gene showed that the overall structure of the gene was highly conserved. However, the first intron of the mouse gene was 400 bp shorter than that of the rat gene. Further analysis of homology between this gene and four other casein genes from rat and bovine revealed the presence of four highly conserved sequences. A search for the consensus sequences of transcription-regulatory elements showed that several potential regulatory sequences were present in the gene and its 5' flanking region.

Caseins of various origins and biologically active casein peptides and oligosaccharides: structural and physiological aspects

The first part of the present review is focused on structural aspects concerning the so far studied casein fractions of various origins: they are compared to the four classical major bovine caseins (alpha s1-, alpha s2-, beta- and kappa). The calcium-sensitive casein fractions are always phosphorylated whereas kappa-caseins are glycosylated. The study of the casein genes showed that the calcium-sensitive caseins diverged from a common ancestral gene and during the evolution, intergenic and intragenic duplications occurred. The considerable conservation of the phosphorylation sites emphasizes the importance of phosphorylated residues for the function of caseins, i.e. the formation of micelles and the binding of Ca2+. In kappa-caseins all the prosthetic sugar groups are linked by O-glycosidic linkages: their number varies from 0 to 5 in bovine kappa-casein and up to 10 in human kappa-casein. The structures of the known kappa-casein carbohydrate moieties are described. Finally the milk clotting process (interaction kappa-casein/chymosin) is compared to the blood clotting process (interaction fibrinogen/thrombin): a large number of similarities could be noted between both clotting phenomena. The second part of the review is devoted to the study of short casein peptides endowed with various biological activities. Some of them behaved as immunomodulators or casomorphins or angiotensin I converting enzyme inhibitors; others demonstrated an effect on platelet functions. A 'strategic zone' containing immunostimulating and opioid peptides could be located in cow and human beta-caseins. Furthermore bitter peptides, emulsifying peptides, calcium absorption enhancing peptides, chymosin-inhibiting peptides, have also been described and several further properties have been attributed to the kappa-caseinoglycopeptide; two tetrasaccharides isolated from the latter possess blood group activities. In conclusion caseins, the main milk proteins, should not only be considered as a nutriment but as a possible source of biologically active components. If, in the future, some of the discussed active peptides cannot be characterized in vivo, they can all, nevertheless, be synthesized and used either as food additives or in pharmacology.

Isolation and characterization of the Bos taurus beta-casein gene

The expression of casein genes in the mammary cells is regulated by peptide and steroid hormones. To investigate the controlling mechanisms we have isolated and characterized the bovine beta-casein gene. The gene has the size of 8.6 kb, which is 7.8 times longer than the corresponding mRNA composed of nine exons. The genomic clones include additional 8.5-kb and 4.5-kb sequences of the 5'- and 3'-flanking regions. We have determined the sequences of the 5' and 3' ends of the gene and compared them with the respective sequences of the rat beta-casein gene. Conserved sequences are identical or homologous to the potential binding sites for nuclear factors and for glucocorticoid and progesterone receptors. The regulatory region contains two different TATA signals and a repeat sequence between them.

Digestion by pancreatic juice of a beta-casomorphin-containing fragment of buffalo beta-casein

Degradation by pig pancreatic juice of a beta-casomorphin-containing fragment (tryptic peptide corresponding to residues 49-68 of buffalo beta-casein) was investigated. The FAB/MS (fast atom bombardment mass spectrometry) technique was used to identify the fragments produced by the concerted action of pancreatic proteases. Pancreatic juice, under our experimental conditions, is not able to release beta-casomorphins or morphiceptin from the tryptic peptide sequence. Furthermore, the present report shows that the rapid hydrolysis of a peptide bond by a single protease can prevent the cleavage of peptide bonds by a different protease. Therefore the formation of some peptides in the gastrointestinal tract can depend on the protease ratio.

Cloning and sequence analysis of bovine beta-casein cDNA

A bovine beta-casein cDNA clone was isolated from a cDNA library prepared from mammary gland mRNA. Sequence analysis revealed 25 nucleotides (nt) of the 5' noncoding region, 672 nt of the complete sequence coding and a 3' region of approximately 500 nt. When the nucleotide sequence of bovine beta-casein cDNA is compared to rat beta-casein cDNA (5), a high degree of homology is observed in the first 100 nt corresponding to the signal peptide of the pre-beta-caseins.

Evolution of the casein multigene family: conserved sequences in the 5' flanking and exon regions

The rat alpha- and bovine alpha s1-casein genes have been isolated and their 5' sequences determined. The rat alpha-, beta-, gamma- and bovine alpha s1-casein genes contain similar 5' exon arrangements in which the 5' noncoding, signal peptide and casein kinase phosphorylation sequences are each encoded by separate exons. These findings support the hypothesis that during evolution, the family of casein genes arose by a process involving exon recruitment followed by intragenic and intergenic duplication of a primordial gene. Several highly conserved regions in the first 200 base pairs of the 5' flanking DNA have been identified. Additional sequence homology extending up to 550 base pairs upstream of the CAP site has been found between the rat alpha- and bovine alpha s1-casein sequences. Unexpectedly, the 5' flanking promoter regions are conserved to a greater extent than both the entire mature coding and intron regions of these genes. These conserved 5' flanking sequences may contain potential cis regulatory elements which are responsible for the coordinate expression of the functionally-related casein genes during mammary gland development.

Complete sequence of ovine alpha s2-casein messenger RNA

The primary structure of mRNA coding for ovine alpha s2 casein has been determined by chemical sequencing of three cDNA clones and the primer extension products of the longest one. The mRNA was 1,024 nucleotides long, excluding the poly(A) tail. The length of the 5' noncoding, coding and 3' noncoding regions was 53, 669 and 302 nucleotides, respectively. A comparison of the nucleotide sequence of ovine alpha s2-casein and guinea-pig casein A mRNAs revealed an extensive homology in the 5' and 3' noncoding regions. The deduced amino acid sequence of ovine alpha s2-casein was compared with its bovine and guinea-pig counterparts. Moreover, an heterogeneity was evidenced in the mRNA population of the alpha s2-casein.

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.

Molecular cloning of mouse mammary gland kappa-casein: comparison with rat kappa-casein and rat and human gamma-fibrinogen

A cDNA clone for kappa-casein mRNA from the lactating mouse mammary gland was isolated and its nucleotide sequence determined. Analysis of the deduced amino acid sequence revealed a precursor protein with a 21-amino-acid signal sequence and a mature protein of 160 amino acids, the mature mouse protein being 3 amino acids longer than the rat kappa-casein. Northern blot analysis of the lactating rat and mouse mammary gland showed a specific mRNA for rat kappa-casein and two distinct mRNAs for mouse kappa-casein. This result is explained by the presence of two putative polyadenylation sites in mouse kappa-casein cDNA, whereas rat kappa-casein cDNA has only one polyadenylation site. Comparison of the nucleotide sequence and of the deduced amino acid sequence of kappa-casein from mouse with that of the rat showed 85% homology between the two sequences. However, when amino acid sequences of kappa-casein from rat and mouse were compared with ovine kappa-casein, only a 45% homology was observed. Amino acid sequences of kappa-casein from rat, mouse, and sheep were 36.53% homologous with rat and human gamma-fibrinogen. The extent of homology was similar (32%) when nucleotide sequences of corresponding cDNAs were compared. The stretches of homology existing at different regions between the two proteins were more confined toward the amino-terminal half of gamma-fibrinogen. However, when nucleotide sequences were compared, mouse kappa-casein cDNA showed homology only with the second half of the rat gamma-fibrinogen cDNA, i.e., between nucleotides 661-1135. The homology with the human gamma-fibrinogen cDNA spanned over two regions, one between nucleotides 1-328 and the second between nucleotides 591-726.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleotide sequence determination of guinea-pig casein B mRNA reveals homology with bovine and rat alpha s1 caseins and conservation of the non-coding regions of the mRNA

Nucleotide sequence analysis of cloned guinea-pig casein B cDNA sequences has identified two casein B variants related to the bovine and rat alpha s1 caseins. Amino acid homology was largely confined to the known bovine or predicted rat phosphorylation sites and within the 'signal' precursor sequence. Comparison of the deduced nucleotide sequence of the guinea-pig and rat alpha s1 casein mRNA species showed greater sequence conservation in the non-coding than in the coding regions, suggesting a functional and possibly regulatory role for the non-coding regions of casein mRNA. The results provide insight into the evolution of the casein genes, and raise questions as to the role of conserved nucleotide sequences within the non-coding regions of mRNA species.

Nucleotide sequences of bovine alpha S1- and kappa-casein cDNAs

The nucleotide sequences corresponding to bovine alpha S1- and kappa-casein mRNAs are presented. An unusual alpha S1-casein cDNA has been characterised whose 5' end commences upstream from its putative TATA box. The alpha S1-casein mRNA is compared to rat alpha-casein mRNA and two components of divergence are identified. Firstly, the two sequences have diverged at a high point mutation rate and the rate of amino acid replacement by this mechanism is at least as great as the rate of divergence of any other part of the mRNAs. Secondly, the protein coding sequence has been subjected to several insertion/deletion events, one of which may be an example of exon shuffling . The kappa-casein mRNA sequence verifies the proposition that it has arisen from a different ancestral gene to the other caseins.

Compilation and analysis of sequences upstream from the translational start site in eukaryotic mRNAs

5-Noncoding sequences have been tabulated for 211 messenger RNAs from higher eukaryotic cells. The 5'-proximal AUG triplet serves as the initiator codon in 95% of the mRNAs examined. The most conspicuous conserved feature is the presence of a purine (most often A) three nucleotides upstream from the AUG initiator codon; only 6 of the mRNAs in the survey have a pyrimidine in that position. There is a predominance of C in positions -1, -2, -4 and -5, just upstream from the initiator codon. The sequence CCAGCCAUG (G) thus emerges as a consensus sequence for eukaryotic initiation sites. The extent to which the ribosome binding site in a given mRNA matches the -1 to -5 consensus sequence varies: more than half of the mRNAs in the tabulation have 3 or 4 nucleotides in common with the CCACC consensus, but only ten mRNAs conform perfectly.

Immunochemical characterization with monoclonal antibodies of three major caseins and alpha-lactalbumin from rat milk

Rat milk contains at least three major caseins with apparent molecular weights of 41,000 (alpha-casein), 25,000 (beta-casein), and 22,000 (gamma-casein) (estimated in 10% sodium dodecyl sulfate-polyacrylamide gels). These three caseins and alpha-lactalbumin, a major whey protein, were purified from rat milk. The purified caseins and alpha-lactalbumin were used to immunize BALB/c mice, and spleen cells from these mice were hybridized with cells of the mouse myeloma SP-2/0 cell-line. We have isolated a small library of hybridoma cell-lines secreting monoclonal antibodies specific for each of the major caseins and alpha-lactalbumin from rat milk. Antibodies were tested for immunoreactivity with each of the purified milk proteins and with total rat milk proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Some heterogeneity in apparent molecular weight was observed for purified alpha-casein, gamma-casein, and alpha-lactalbumin. Monoclonal antibodies against alpha-casein, gamma-casein, and alpha-lactalbumin recognized all of the molecular weight forms of the antigen for which they were specific. Each monoclonal antibody was specific for one of the caseins or alpha-lactalbumin and did not react with the other caseins or alpha-lactalbumin, suggesting that there is limited structural homology among these proteins. All of the monoclonal antibodies against the rat caseins reacted with components of mouse milk, and the monoclonal antibodies against rat gamma-casein reacted with a component of human milk of apparent molecular weight 27,000. No interspecies reactivity was observed with the antibodies against rat alpha-lactalbumin. These monoclonal antibodies are being used to develop sensitive assays for each of these major rat milk proteins.

Characterization of a cDNA clone for a rare mRNA modulated by ovariectomy in mammary carcinomas

A complementary DNA (cDNA) clone (p13) for a rare mRNA was isolated from a cDNA library generated from total polyA+ RNA of 14-day lactating rat mammary gland. In vitro translation of the positively selected mRNA from p13 cDNA revealed on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) a polypeptide of 24 kDa. The p13 cDNA clone hybridized on northern blots predominantly to approximately 1100 base size RNA and weakly to approximately 3800 base size RNA from lactating mammary gland. It hybridized only to approximately 3800 base size RNA from rat liver. Southern blot analysis of genomic DNA showed differences in gene organization in mammary gland and liver. The mRNA level for the 24 kDa polypeptide was higher in 7-12 DMBA-induced tumor and lower in the MTW9 carcinoma as compared to lactating mammary gland. After ovariectomy, the mRNA level in mid pregnant gland increased but was reduced in the 7-12 DMBA tumors.