Amino terminal sequence of the precursor of ovine alpha-lactalbumin

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.

Molecular cloning and nucleotide sequence of a bovine alpha-lactalbumin cDNA

A cDNA clone for the bovine milk protein, alpha-lactalbumin (alpha LA), has been identified using a rat cDNA probe. The bovine cDNA clone is 703 nucleotides (nt) long, contains 8 nt of 5'-untranslated sequence and 269 nt of 3'-untranslated sequence. When compared with previously reported sequences, the bovine alpha LA mRNA sequence has 74% similarity with rat alpha LA mRNA, 79% similarity with human mRNA and 74% similarity with guinea pig mRNA.

The differential actions of cortisol on the synthesis and turnover of alpha-lactalbumin and casein and on accumulation of their mRNA in mouse mammary gland in organ culture

Cortisol was previously shown to exert different, concentration-dependent, effects on the accumulation of casein and alpha-lactalbumin in mammary glands from mid-pregnant mice cultured in the presence of insulin and prolactin [Ono & Oka (1980) Cell 19, 473-480]. The present study demonstrated that the addition of 30nM-cortisol to the medium containing insulin and prolactin resulted in a marked enhancement of the rate of synthesis of both alpha-lactalbumin and casein in cultured tissue. The addition of 3 microM-cortisol in combination with insulin and prolactin caused a marked decrease in the rate of alpha-lactalbumin synthesis, but increased casein synthesis substantially. Similar changes were also observed in the amount of translatable mRNA for alpha-lactalbumin and casein in mammary explants cultured with insulin, prolactin and the two concentrations of cortisol. The study of the turnover of the milk proteins in cultured explants showed that virtually all of the casein synthesized remained intact in tissue explants cultured with 3 microM cortisol, whereas about 45% of casein disappeared in 40h from explants cultured with 30nM-cortisol. In contrast, the two concentrations of cortisol did not differentially affect the disappearance of alpha-lactalbumin, which was about 55% in 40h. These results indicate that the concentration-dependent differential actions of cortisol on the accumulation of alpha-lactalbumin and casein are exerted through its effects on the rate of synthesis and turnover of the two proteins as well as on the accumulation of their mRNA species.

Amino terminal sequence, processing, and biological activity of porcine pre-alpha-lactalbumin

Polyadenylated RNAs isolated from bound polysomes of a lactating sow's mammary gland, were translated in a cell-free system and in vitro synthesized alpha-lactalbumin was immunoprecipitated and radiosequenced. The translation product was found to contain an amino terminal extension of 19 amino acid residues, very similar to its ovine counterpart, that was selectively removed when translation was carried out in the presence of rabbit mammary microsomal membranes. Assays of porcine pre-alpha-lactalbumin for activity on galactosyltransferase showed that the preprotein can also interact with and modify the specificity of the enzyme, as indicated by de novo synthesis of lactose.

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

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

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

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