Isolation and partial characterization of rate casein proteins

Isolation and characterization of the principal caseins in rat milk

The 4 major caseins, A1, A2, B1, B2, from rat milk have been isolated and analysed. From molecular weight determination, amino acid and phosphorus analyses and N-terminal sequence determination, A1 and A2 are concluded to possess similar peptide chains as do B1 and B2, with the individual fractions within each of these 2 groups differing only in their sialic acid content.

Mol. wts of approximately 22000 and 38000 were found for A1–A2 and B1–B2 respectively. The amino acid sequence of the first 13 residues of A1–A2 has been partly established. Components A1 and A2 appeared to be homologous with bovine β-casein, whereas B1 and B2 were different from any known casein, especially in their molecular weight.

The Nuclear Factor I (NFI) gene family in mammary gland development and function

Mammary gland development and function require the coordinated spatial and temporal expression of a large fraction of the mammalian genome. A number of site-specific transcription factors are essential to achieve the appropriate growth, branching, expansion, and involution of the mammary gland throughout early postnatal development and the lactation cycle. One family of transcription factors proposed to play a major role in the mammary gland is encoded by the Nuclear Factor I (NFI) genes. The NFI gene family is found only in multicellular animals, with single genes being present in flies and worms and four genes in vertebrates. While the NFI family expanded and diversified prior to the evolution of the mammary gland, it is clear that several mammary-gland specific genes are regulated by NFI proteins. Here we address the structure and evolution of the NFI gene family and examine the role of the NFI transcription factors in the expression of mammary-gland specific proteins, including whey acidic protein and carboxyl ester lipase. We discuss current data showing that unique NFI proteins are expressed during lactation and involution and suggest that the NFI gene family likely has multiple important functions throughout mammary gland development.

Pituitary-induced lactation in mammary gland explants from the pregnant tammar (Macropus eugenii): a negative role for cyclic AMP

1. alpha-Lactalbumin and casein have been isolated from tammar milk. 2. alpha-Lactalbumin was induced in mammary explants by culture with anterior pituitary. 3. Casein was induced maximally in the presence of a physiological concentration of prolactin alone. 4. Progesterone did not inhibit the prolactin-induced synthesis of casein, alpha-lactalbumin, galactosyltransferase or fatty acids. 5. Both dibutyryl cAMP and a combination of cholera toxin and IBMX did significantly inhibit the induction of casein and alpha-lactalbumin. 6. Progesterone withdrawal is not a component of the lactogenic trigger in this marsupial but cAMP may be a common intracellular signal for negative control of lactogenesis in both marsupials and eutherians.

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.

Complete sequence analysis of cDNA clones encoding rat whey phosphoprotein: homology to a protease inhibitor

Lactoprotein clones have been isolated from a rat mammary gland recombinant library of cDNA plasmids. Clones p-Wp 52 and p-Wp 47 were shown by hybrid selection, in vitro translation, and immunoprecipitation to represent a cloned DNA sequence encoding rat whey phosphoprotein. We report here the nucleotide sequence of the cDNA insert of p-Wp 52 and shows that it encodes the complete whey phosphoprotein sequence. The encoded sequence shows a high content of half-cystine, glutamic acid, aspartic acid, and serine but an absence of tyrosine. The half-cystines appear in unique arrangements and are repeated in two domains of the protein. The second domain has striking similarities with the second domain of the red sea turtle protease inhibitor. Clone p-Wp 52 has allowed the study of expression of whey phosphoprotein mRNA during functional differentiation of rat mammary gland and in mammary tumors. The whey phosphoprotein mRNA is detected during midpregnancy and lactation in the rat mammary gland but is barely detected in mammary tumors in which other milk protein mRNAs are expressed. The whey phosphoprotein gene in these tumors is hypermethylated, correlating with the reduced expression of this gene.

Purification and characterization of four components of rat caseins

Rat casein components (C1-, C3A-, C3B- and C4-casein) were extensively purified from rat milk, and the properties of these proteins were compared with those of other caseins including rat C2-casein. C1-casein was precipitated by a low concentration of CaCl2 (1.5 mM). Both C3A- and C3B-casein were less sensitive to Ca2+ than were C1- and C2-casein, and the presence of 20 mM CaCl2 was required at 37 degrees C for their precipitation. C4-casein was absolutely insensitive to Ca2+. This protein exhibited the ability to stabilize all of the other rat casein components against Ca2+-dependent precipitation. In addition, C4-casein contained sialic acid, galactose and N-acetylgalactosamine. Therefore, C4-casein appears to be a bovine kappa-casein-like protein.

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.

Purification and partial characterization of a unique group of phosphoproteins from rat milk whey

A group of four electrophoretically distinct but related proteins has been isolated from rat milk whey. These have been classified as P.1 to P.4 in order of their decreasing electrophoretic mobility in an alkaline buffer system. All four members of this group of proteins are immunologically identical apparently constitute a single protein which is differentially phosphorylated in the ratio of 3:2:1:0. Sodium dodecyl sulfate gel electrophoresis yielded constant estimates of molecular weight of approximately 20,700. Enzymatic dephosphorylation of purified P.1 generated the other three members of the group, demonstrating the commonality of the peptide component. This group of proteins in total makes a significant contribution of the total whey proteins being approximately 5 mg/ml in whole rat milk in the approximate proportion of 1:1.5:2:1.5. The P.1, P.2, and P.3 were isolated in sufficient quantities to permit further characterization. The partial amino acid analyses of each of the three forms were similar. They had commonextinction coefficients, E1(1)% cm A280, of 4.9 and A280/A290 ratio of about 1.36.

Purification and characterization of rat alpha-lactalbumins: apparent genetic variants

Rat alpha-lactalbumin, from the milk of Fischer 344 (CDF) rats, was isolated and purified by a combination of gel filtration and diethylaminoethyl-cellulose ion exchange chromatography. Three electrophoretically distinct proteins had alpha-lactalbumin activity. Staining for carbohydrate indicated that at least two of the three forms were glycoproteins. The low molecular weight protein fraction from the wheys of two additional strains of laboratory rat were compared to ascertain whether the composition of this fraction was common in the divergent strains. Outbred Wistar and Long-Evans dams yielded wheys containing up to six forms of alpha-lactalbumin. Either one or both of two groups of three alpha-lactalbumins were in a given milk sample. The two groups of three alpha-lactalbumins appear to represent two genetic variants upon which is imposed a polymorphic character. All forms of alpha-lactalbumin, within and between strains, were immunologically identical.