Differential splicing of pre-messenger RNA produces multiple forms of mature caprine alpha(s1)-casein.
The identity of multiple forms of caprine alpha(s1)-casein in variants A, B, and C has been determined by structural characterisation using mass spectrometry, automated Edman degradation and peptide mapping. Mature goat alpha(s1)-casein exists as a mixture of at least four molecular species which differ in peptide chain length. The main component corresponds to the 199-residues form already described. The other three, in lesser amounts, were shorter forms of alpha(s1)-casein and differed for the deleted peptides 141-148, as shown previously for ovine alpha(s1)-casein, peptide 110-117, or Gln78. Analysis of alpha(s1)-casein mRNA from milk somatic cells demonstrated that these forms originated from skipping events at the level of exon 13 (codifying for peptide 110-117) and 16 (codifying for peptide 141-148) and from the presence of a cryptic splice site within exon 11 (whose first CAG triplet encodes Gln78) during primary transcript processing. The finding of these splicing abnormalities in the three common variants A, B, and C suggests that this is a general feature of alpha(s1)-casein in goat. A further source of heterogeneity of caprine alpha(s1)-casein was identified in the discrete phosphorylation of seryl residues. Eight serine residues (at positions 44, 46, 64 to 68 and 75) are fully phosphorylated (except in variant A because of the replacement Glu77-->Gln which prevents phosphorylation of Ser75). Conversely, Ser115 and Ser41 are phosphorylated only to about 50% and 20%, respectively. Ser12, although located in a consensus triplet, is never phosphorylated, similarly to the ovine alpha(s1)-casein variants. These results confirm that there are stabilised mechanisms of simultaneous synthesis of alpha(s1)-casein at different length and of post-translational modification in both caprine and ovine species.
Ferranti
P
P
Dipartimento di Scienza degli Alimenti, Universià degli Studi di Napoli Federico II, Portici, Italy. ferranti@unina.it
Addeo
F
F
Malorni
A
A
Chianese
L
L
Leroux
C
C
Martin
P
P
D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002364:Caseins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics*; D002417:Cattle; D005091:Exons; D014644:Genetic Variation; D006041:Goats; D013058:Mass Spectrometry; D010766:Phosphorylation; D016133:Polymerase Chain Reaction; D011110:Polymorphism, Genetic; D012322:RNA Precursors / Q000235:genetics* / Q000378:metabolism*; D017384:Sequence Deletion; D012694:Serine / Q000737:chemistry; D012756:Sheep; D013045:Species Specificity
D002364:Caseins; D012322:RNA Precursors; D012694:Serine