[Primary structure of the casein macropeptide of porcine and human kappa caseins]. / Structure primaire du caséinomacropeptide des caséines kappa porcine et humaine

The amino acid sequence of porcine and human caseinomacropeptides (CMP), the C-terminal glycopeptide released from kappa-casein by chymosin at the initial step of milk coagulation, have been investigated. The complete amino acid sequence of porcine CMP and that of the first 59 amino acid residues of human CMP have been determined. Porcine and human CMPs contain 71 and likely 65 amino acid residues respectively. The extra hexapeptide 38-43 found in porcine CMP arises obviously from the duplication of the DNA fragment coding for the 6 preceding amino acids.

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.

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.

N-terminal sequences of uteroglobin and its precursor.

Translation of uteroglobin mRNA in wheat-germ extract has yielded a precursor protein (pre-uteroglobin) containing an N-terminal extension of 21 amino acid residues. The sequence of this extension and that of the 50 N-terminal amino acid residues of uteroglobin have been determined.