Sequence of rat alpha- and gamma-casein mRNAs: evolutionary comparison of the calcium-dependent rat casein multigene family

Piriform spider silk sequences reveal unique repetitive elements

Orb-weaving spider silk fibers are assembled from very large, highly repetitive proteins. The repeated segments contain, in turn, short, simple, and repetitive amino acid motifs that account for the physical and mechanical properties of the assembled fiber. Of the six orb-weaver silk fibroins, the piriform silk that makes the attachment discs, which lashes the joints of the web and attaches dragline silk to surfaces, has not been previously characterized. Piriform silk protein cDNAs were isolated from phage libraries of three species: A. trifasciata , N. clavipes , and N. cruentata . The deduced amino acid sequences from these genes revealed two new repetitive motifs: an alternating proline motif, where every other amino acid is proline, and a glutamine-rich motif of 6-8 amino acids. Similar to other spider silk proteins, the repeated segments are large (>200 amino acids) and highly homogenized within a species. There is also substantial sequence similarity across the genes from the three species, with particular conservation of the repetitive motifs. Northern blot analysis revealed that the mRNA is larger than 11 kb and is expressed exclusively in the piriform glands of the spider. Phylogenetic analysis of the C-terminal regions of the new proteins with published spidroins robustly shows that the piriform sequences form an ortholog group.

The sequence of porcine alpha s1-casein cDNA: evidence for protein variants generated by altered RNA splicing

A cDNA library was constructed from mRNA isolated from lactating porcine mammary gland and screened with a bovine alpha s1-casein cDNA clone. Three classes of cDNA isolated varied in the number of bases within the coding region. The full length porcine alpha s1-casein cDNA is 1124bp and codes a preprotein of 206 amino acids. The other two classes of alpha s1-casein cDNA lacked 18bp and 60bp respectively when compared to the 1124-bp cDNA sequence. PCR amplification confirmed the presence of these sequences in total RNA. These differences appear to be due to altered RNA splicing.

Characterisation of monotreme caseins reveals lineage-specific expansion of an ancestral casein locus in mammals

Using a milk-cell cDNA sequencing approach we characterised milk-protein sequences from two monotreme species, platypus (Ornithorhynchus anatinus) and echidna (Tachyglossus aculeatus) and found a full set of caseins and casein variants. The genomic organisation of the platypus casein locus is compared with other mammalian genomes, including the marsupial opossum and several eutherians. Physical linkage of casein genes has been seen in the casein loci of all mammalian genomes examined and we confirm that this is also observed in platypus. However, we show that a recent duplication of β-casein occurred in the monotreme lineage, as opposed to more ancient duplications of α-casein in the eutherian lineage, while marsupials possess only single copies of α- and β-caseins. Despite this variability, the close proximity of the main α- and β-casein genes in an inverted tail–tail orientation and the relative orientation of the more distant kappa-casein genes are similar in all mammalian genome sequences so far available. Overall, the conservation of the genomic organisation of the caseins indicates the early, pre-monotreme development of the fundamental role of caseins during lactation. In contrast, the lineage-specific gene duplications that have occurred within the casein locus of monotremes and eutherians but not marsupials, which may have lost part of the ancestral casein locus, emphasises the independent selection on milk provision strategies to the young, most likely linked to different developmental strategies. The monotremes therefore provide insight into the ancestral drivers for lactation and how these have adapted in different lineages.

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.

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.

Structure and expression of the mouse casein gene locus

The analysis of yeast artificial chromosomes (YACs) containing the complete mouse casein gene locus revealed the presence of five casein genes, alpha-, beta-, gamma-, delta-, and kappa-casein, in this order, in the locus. The alpha- and beta-casein genes are only 10 kb apart and have convergent transcriptional orientations. The distance between the beta-casein gene and the alpha s2-like gamma-casein gene is about 70 kb, and these genes have divergent transcriptional orientations. The gamma- and delta-casein genes, both encoding a alpha s2-like casein, are linked within 60 kb and convergently transcribed. The kappa-casein gene is located about 100 kb from the delta-gene. Except for the presence of the delta-casein gene, the organization of the mouse casein locus resembles that of the bovine locus, including the transcriptional orientation of the genes. In contrast to the other casein genes, which are strongly induced at mid-lactation, expression of the delta-casein gene is abruptly induced upon parturition. Comparative analysis of alpha s2-like sequences from various species suggests that the ancestral alpha s2-like gene duplicated around the time of radiation of the rodent and artiodactylid ancestors.

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.

Characterization of three types of human alpha s1-casein mRNA transcripts

Here we report the molecular cloning and sequencing of three types of human alpha s1-casein transcripts and present evidence indicating that exon skipping is responsible for deleted mRNA transcripts. The largest transcript comprised 981 bp encoding a signal peptide of 15 amino acids followed by the mature alpha s1-casein sequence of 170 amino acids. Human alpha s1-casein has been reported to exist naturally as a multimer in complex with kappa-casein in mature human milk, thereby being unique among alpha s1-caseins [Rasmussen, Due and Petersen (1995) Comp. Biochem. Physiol., in the press]. The present demonstration of three cysteines in the mature protein provides a molecular explanation of the interactions in this complex. Tissue-specific expression of human alpha s1-casein was indicated by Northern-blot analysis. In addition, two cryptic exons were localized in the bovine alpha s1-casein gene.

A herpesvirus saimiri protein required for interleukin-2 independence is associated with membranes of transformed T cells

A region of the herpesvirus saimiri genome encoding an mRNA with two open reading frames (ORFs) has been identified to be essential for transformation of T cells. Deletion of either ORF resulted in the loss of transforming ability. ORF-1 has been shown to code for a collagen-like oncoprotein. This study shows for the first time that the bicistronic mRNA can translate a 32-kDa protein from ORF-2. Polyclonal serum to ORF-2 was generated by using a glutathione fusion protein. Using this antiserum, ORF-2 was localized in cell membranes and is expressed on the outer cell membrane. The half-life of this membrane protein was found to be about 5.5 h. Limited sequence similarity was found between ORF-2 and interleukin-11; however, no secretion of ORF-2 protein was detected in supernatants from transformed cells. Further studies are required to investigate the potential interaction with the interleukin-11 receptor.

Human alpha s1-casein: purification and characterization

The human counterpart of alpha s1-casein has been purified by a combination of gel-filtration and ion-exchange chromatography under denaturing conditions. SDS-PAGE analysis revealed the presence of a diffuse ladder with a high molecular mass which upon reduction was replaced by several closely spaced bands of lower molecular masses and a broad diffuse band corresponding to kappa-casein. Amino acid sequence analysis of the closely spaced bands all resulted in the same N-terminal sequence which was found to be homologous with alpha s1-casein from other species. Sequence analysis of a major radiolabelled tryptic peptide from purified 14C-carboxymethylated alpha s1-casein demonstrated that the protein contains at least two cysteine residues. As judged by SDS-PAGE in the presence or absence of a reducing agent, the molecular structure of the polymers constituting the ladder is composed of heteropolymers of alpha s1- and kappa-casein cross-linked by disulfide bonds.

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.

The secondary structure of milk proteins and their biological function

A brief overview is given of the methods of determining and predicting secondary structure in proteins. The secondary structures of the milk serum proteins, lactoferrin, alpha-lactalbumin, lysozyme, and beta-lactoglobulin, as determined by x-ray crystallography, are compared with the results of a joint prediction method. This comparison evaluates critically the degree of success achieved and helps define what can reasonably be expected from a prediction in the absence of a known structure. The value of supplementary information from spectroscopic methods and the use of templates and sequence information from related proteins in improving the confidence of predictions are illustrated. One point that emerges is the general overprediction of helix content by the joint prediction method such that, for an all-beta protein such as beta-lactoglobulin, the method of Garnier, Osguthorpe, and Robson, applied with the correctly selected decision constants, provides a somewhat better approach. Secondary structure of the caseins can be predicted with less confidence than for globular proteins, and the results should be interpreted as evidence of a propensity to form transient secondary structures of the indicated type in view of the generally open and flexible conformation of caseins in solution.

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.

Localization of two interchain disulfide bridges in dimers of bovine alpha s2-casein. Parallel and antiparallel alignments of the polypeptide chains

Carboxymethylation of bovine skimmed milk with 14C-labelled iodoacetic acid followed by purification of the alpha s2-casein dimer showed that all four cysteine residues in the protein are engaged in disulfide linkages. Mass spectrometry and sequence analysis of cystine-containing tryptic peptides revealed the presence of two interchain disulfide bridges in the protein. Sequence analysis of disulfide-linked peptides resulting from an enzymatic cleavage between the bridges demonstrated that the individual chains in the dimers are either aligned in an antiparallel or a parallel orientation. The identity of some of the disulfide-linked peptides was further verified by performic acid oxidation followed by sequence analysis of the resulting peptides.

Multiple mRNA species code for two non-allelic forms of ovine alpha s2-casein

The two non-allelic forms of alpha s2-casein, occurring in ovine milk, differ by an internal deletion of nine amino acid residues, including both cysteine residues at positions 34 and 42 in the mature chain. Sequencing of several alpha s2-casein cDNA, isolated from the mammary cDNA library of a single lactating ewe, showed three new types which differed from that previously studied. In addition to the expected deletion of codons +34 to +42 affecting 30-40% of mRNA, another structural difference involving an internal stretch of 44 nucleotides in the 5' untranslated region, was found. S1-nuclease protection assays confirmed the existence of several types of the relevant mRNA and sequencing of in-vitro-amplified genomic DNA demonstrated the presence of the 44-nucleotide stretch in the alpha s2-casein transcriptional unit, thus ruling out the possibility of a cloning artefact. The different alpha s2-casein mRNA, four in terms of deletion and two in terms of nucleotide substitutions for a given ewe, can be readily explained by partial exon skipping and allelic differences, respectively. This assumption is well supported by the following observations: 5' and 3' ends of both deleted DNA fragments are similar to those of exons; sequences neighbouring the 44-nucleotide stretch of the genomic DNA perfectly match consensus sequences described for 3' and 5' ends of introns; the rather simple patterns observed on Southern blots of different enzymatic digests of genomic DNA strongly suggest the occurrence of only 1 copy alpha s2-casein gene/haploid genome. During the course of evolution, the alpha s2-casein-encoding gene has undergone many mutations and some of them might have occurred in regions corresponding to consensus splicing regions of the pre-mRNA. Thus, complete skipping of some exons might be responsible for the shorter sizes of rat and mouse alpha s2-casein mRNA. If so, the overall organization of the alpha s2-casein gene in the different species might be more similar than expected from structural comparisons of the cognate mRNA or caseins.

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.

Casein expression in cytotoxic T lymphocytes

A cDNA that expresses a mRNA restricted to cytotoxic T lymphocytes (CTL) and mammary tissue has been isolated and characterized. The deduced amino acid sequence from this cDNA shows extensive homology with the previously reported amino acid sequence for rat alpha-casein. Indeed, the presence of a six-residue-repeated motif that is specific for rodent alpha-caseins strongly supports the identification of this cDNA as mouse alpha-casein. Northern (RNA) blot analysis of many hematopoietic cell types revealed that this gene is restricted to CTL, being expressed in four of six CTL lines examined. Furthermore, CTL that express this gene were also found to express other members of the casein gene family, such as beta- and kappa-casein. These results suggest that caseins may be important in CTL function, and their potential role in CTL-mediated lysis is discussed.

Cloning and sequencing of a complementary deoxyribonucleic acid coding for a bovine alpha s1-casein A from mammary tissue of a homozygous B variant cow

A cDNA clone for bovine alpha s1-casein variant A was isolated from a mammary gland cDNA library using a synthetic degenerate oligonucleotide probe. The largest Pst I insert containing an EcoR I site was sequenced. It contained 1090 base pairs, 47 in the 5' noncoding region, 603 in the coding region and 440 in the 3' noncoding region. The nucleotide sequence was compared with three published cDNA sequences for alpha s1-casein variant B. The most obvious difference was the absence of the 39 bases encoding the 13 amino acids that are present in the B variant but absent from the A variant. In addition, five other single base positions differed within individual codons among the four sequences at the third base for each codon, but this did not change the amino acids encoded. There were, however, a number of differences found in the 3' noncoding region. The isolated cDNA was subjected to site-directed mutagenesis to replace a Val-Ile dipeptide with Phe-Phe to increase the chymosin sensitivity of the protein. When the milk proteins from mammary gland tissue extracts were typed, the alpha s1-casein A gene product was not detected.

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.

Complete nucleotide sequence of ovine alpha-lactalbumin mRNA

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

Ovine beta-lactoglobulin messenger RNA: nucleotide sequence and mRNA levels during functional differentiation of the mammary gland

The nucleotide sequence of ovine beta-lactoglobulin mRNA has been determined by chemical sequencing of two cDNA recombinant plasmids and primer extension products. Ovine beta-lactoglobulin mRNA consists of a 540 nucleotide coding region, flanked by 39 nucleotide 5' and 206 nucleotide 3' non-coding regions including a 20 nucleotide poly A tail. The deduced 180 amino acid sequence of pre-beta-lactoglobulin is in agreement with the previously published amino acid sequence of signal peptide and mature protein. Northern blot analysis of poly A+ RNAs from the lactating mammary glands of porcine, rabbit and rat species, allowed us to identify a homologous RNA to beta-lactoglobulin mRNA solely in the porcine species. We also detected a mRNA transcript of a size similar to that of beta-lactoglobulin mRNA in hepatic poly A+ RNA from female rat liver treated by estrogens. Furthermore, we have examined the levels of beta-lactoglobulin mRNA during the functional differentiation of the mammary gland and after hormonal stimulation. During the last third of pregnancy, the expression of beta-lactoglobulin gene is significantly more elevated than that of alpha s1- or beta-casein whose mRNA levels were found to change very slightly during this period. Both beta-lactoglobulin and casein mRNAs showed a rapid response and a wide range of change in response to cortisol treatment. However, there was a significant difference in the rate at which these processes occurred, suggesting that beta-lactoglobulin gene expression is regulated independently of the casein genes.

Isolation and characterization of milk protein nuclear RNAs in rat mammary gland

Methods have been developed to isolate high-molecular-weight pre-mRNAs from lactating mammary gland, a tissue high in RNase levels. These methods involved isolation of nuclei at -20 degrees C in 50% glycerol, and nucleic acid extraction using a guanidine thiocyanate-CsCl protocol. Specific RNAs were detected using alpha-, beta-, and gamma-casein and whey acidic protein nick-translated cDNA and genomic DNA probes by hybridization in situ to pre-mRNAs fractionated on agarose gels containing 10 mM methylmercuric hydroxide. Using these techniques it was possible to isolate poly(A)-containing gene-sized primary transcripts in the case of the two smaller genes, beta-casein and whey acidic protein. A very complex pattern of pre-mRNAs was observed for the beta-casein transcripts, including detection of a species which may represent an excised intron. Probes for the alpha- and gamma-casein genes revealed much lower abundance and complexity of RNA precursors. These methods have proven useful in the initial analysis of RNA processing of these hormonally regulated milk protein gene transcripts.

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.

Spontaneous reiterations of DNA sequences near the ends of adenovirus type 3 genomes

Repeated passage of adenovirus type 3 in HeLa cells has led to a novel stock of variant genomes. Most of the DNA molecules in this stock are characterized by deletions and substitutions of DNA sequences near the left end of the adenovirus type 3 genome map, as reported earlier (C.C. Robinson and C. Tibbetts (1984) Virology 137, 276-286). In this report the characterization of the variant genomes is extended and reveals elongated DNA molecules bearing tandem repetitions of viral DNA sequences near the left and right ends of the viral DNA. Evidence is also presented supporting the cellular DNA origin of short insert sequences found in substitution variants. The elongated variants are of interest because of their novel repeated DNA structures. The locations of these aberrant sequences raise questions about their potential impact on viral gene expression.

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)

Variation in the lack of polyadenylation of the rat milk protein mRNAs during the lactation cycle

Translationally active milk protein mRNAs were found as nonpolyadenylated mRNAs in the rat mammary gland during pregnancy, lactation and involution. Analyses of whey protein mRNA and casein mRNA with the corresponding cDNAs showed that the lack of polyadenylation of these mRNAs at different time points of the lactation cycle is not consistent with the hypothesis that polyadenylation may be incomplete in the mammary gland when large amounts of mRNA are synthesized. The fraction of whey protein mRNA and casein mRNA that lacked polyadenylation was inversely proportional to the concentration of each sequence in the tissue during pregnancy, lactation and involution. A model is proposed to explain the finding that in each animal the ratio of casein mRNA to whey protein mRNA was similar in polyadenylated RNA and in nonpolyadenylated RNA at all stages of the lactation cycle.

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.

Effect of 7-methylguanosine-5'-phosphate on the translation of rat milk-protein mRNAs in the wheat-germ cell-free system

The translation of polyadenylated and of non-polyadenylated RNA obtained from lactating rat mammary gland was almost totally inhibited by 0.5 mM 7-methylguanosine-5'-phosphate in the wheat-germ cell-free system. This inhibition was maintained during the preparation of the 9S whey-protein mRNA and of the 12S and 15S casein mRNAs. Chemical decapping of these mRNAs caused a similar reduction of their activity. Although a large fraction of milk-protein mRNAs have been reported to lack 3'-polyadenylation, these results show that the mRNAs in the mammary gland do contain a 5'-terminal 7-methylguanosine cap.

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.

Structure and expression of mouse VL30 genes

DNA sequencing and blot hybridization analyses have been used to study the structure of a mouse VL30 gene and the molecular nature of VL30-related RNA which is induced upon the stimulation of cultured AKR mouse embryo cells with defined peptide growth factors. An integrated mouse VL30 gene was found to contain identical 601-base-pair long terminal repeats (LTRs) which were themselves terminated in short inverted repeats. The entire VL30 gene was flanked by a 4-base-pair direct repeat of cellular DNA. Thus, VL30 genes are structurally analogous to integrated forms of retrovirus proviruses and certain other classes of mobile genetic elements. The LTR sequence was found to contain putative promoter and polyadenylation signals and generally exhibited little sequence homology to murine leukemia virus proviral LTRs. Certain short regions of sequence conservation, however, were evident, including the inverted terminal repeat, LTR-adjacent regions corresponding to origins of murine leukemia virus proviral DNA synthesis, and a 36-base-pair direct repeat bearing homology to the 72-base-pair direct repeat (enhancer sequence) of the murine leukemia virus-related Moloney sarcoma virus. Upon mitogenic stimulation of quiescent cells with epidermal growth factor and insulin, a major 5.5-kilobase VL30-specific RNA complementary to both LTR and non-LTR sequences was rapidly induced. We conclude that a complete VL30 gene(s) is highly regulated by peptide growth factor binding to specific membrane receptors in these cells.

Guinea-pig casein A cDNA. Nucleotide sequence analysis and comparison of the deduced protein sequence with that of bovine alphas2 casein

The nucleotide sequence (1036 bases) of guinea-pig casein A mRNA has been determined. Two cDNA recombinant plasmids contained a total of 993 base pairs, including part of the 5' noncoding region, and the complete coding and 3' noncoding region. The remaining 5' noncoding sequence was obtained by primer extension. The deduced 223-amino-acid-coding sequence of guinea-pig pre-casein A exhibited 30% homology with bovine alpha s2 casein, the most striking similarities being in the locations of potential phosphorylation sites.