title: Guinea-pig milk-protein synthesis. Isolation and characterization of messenger ribonucleic acids from lactating mammary gland and identification of caseins and pre-alpha-lactalbumin as translation products in heterologous cell-free systems.
abstract: 1. The major milk proteins synthesized by the lactating mammary gland of the guinea pig were identified and designated as caseins A, B and C and alpha-lactalbumin, with estimated mol.wts. of 28000, 25500, 20500 and 14500 respectively. 2. Antisera to the total casein fraction and to alpha-lactalbumin were prepared from rabbits. The milk proteins were also iodinated with either 131I or 125I. 3. A poly(A)-rich RNA fraction was isolated from lactating guinea-pig mammary glands. Isolation was by affinity chromatography on oligo(dT)-cellulose. 4. Examination of this RNA fraction by electrophoresis on polyacrylamide gels containing formamide indicated three major species 1, 2 and 3, with estimated wol.wts. of 5.4 X 10(5) and 3.3 X 10(5), and the apparent absence of rRNA species. 5. The poly(A)-rich RNA stimulated protein synthesis in heterologous cell-free systems based on wheat germ, Krebs II ascites-tumour cells, and the latter supplemented with an initiation factor-3 fraction from rabbit reticulocyte ribosomes. 6. Between 80 and 90% of the protein synthesis directed by the mRNA was for milk proteins. 7. Analysis of the proteins immunoprecipitated by the alpha-lactalbumin antiserum showed in the wheat-germ system that the product was a protein with a molecular weight greater than that of alpha-lactalbumin, whereas in the ascites-tumour-cell systems both this protein and alpha-lactalbumin were found. When the larger protein was treated with CNBr and the resulting peptides were examined, it was shown that the extra peptide was at the N-terminus. This and other evidence is adduced for the initial translation product of alpha-lactalbumin being a precursor with an addition of about ten amino acids at the N-terminus. 8. Similar analysis of the casein immlnospecific proteins produced under the direction of mRNA indicated that the products had a molecular weight that was apparently a littel smaller than that of the caseins synthesized in vivo. This was not consistent with higher-molecular weight casein precursors. 9. Possible explanations for the results obtained are discussed, especially in terms of the physiological significance of the pre-alpha-lactalbumin as a secretory protein.
mesh_terms: D000818:Animals; D002364:Caseins / Q000096:biosynthesis*; D002474:Cell-Free System; D005260:Female; D005914:Globins; D006168:Guinea Pigs; D007768:Lactalbumin / Q000096:biosynthesis*; D007774:Lactation; D008321:Mammary Glands, Animal / Q000032:analysis*; D051379:Mice; D008894:Milk Proteins / Q000096:biosynthesis*; D008970:Molecular Weight; D011247:Pregnancy; D011498:Protein Precursors / Q000302:isolation & purification; D012333:RNA, Messenger / Q000302:isolation & purification*; D011817:Rabbits; D012156:Reticulocytes / Q000032:analysis; D013925:Thromboplastin / Q000302:isolation & purification
chemical_list: D002364:Caseins; D008894:Milk Proteins; D011498:Protein Precursors; D012333:RNA, Messenger; D005914:Globins; D007768:Lactalbumin; D013925:Thromboplastin