Cloning of a marsupial kappa-casein cDNA from the brushtail possum (Trichosurus vulpecula).
The main role of kappa-casein in milk is to stabilize the formation of casein micelles. Although marsupial milk contains casein micelles, kappa-casein had not been identified in any species. In these experiments, the first marsupial kappa-casein has been prepared as enriched casein fractions and the cDNA cloned from the brushtail possum (Trichosurus vulpecula). Possum kappa-casein is a 158 amino acid peptide that shares low amino acid sequence identity (20-30%) with that of eutherian kappa-caseins. In the gut of suckling young, casein micelles clot when kappa-casein is cleaved by chymosin at a specific site. Eutherian kappa-casein sequences are classified according to the sequence of the chymosin cleavage site: Phe-Met, Phe-Ile or Phe-Leu. Possum kappa-casein appears to form a separate class, with a putative chymosin cleavage site of Phe-Ala, which is different from that found in eutherian mammals. Other features of kappa-caseins, such as the location of the N-terminal cysteine, solubility in the presence of calcium, and the O-glycosylation sites on threonine residues in the C-terminus of the molecule, are conserved in the possum sequence. The kappa-casein gene was expressed throughout lactation in the mammary gland, and although mRNA levels of kappa-, alpha- and beta-casein varied between animals there appeared to be a correlation in the expression of these genes within an individual animal. This suggests that a common transcription regulatory region may be controlling expression of all three genes in the possum.
Stasiuk
S X
SX
Reproductive Technologies Group, AgResearch Ruakura, Hamilton, New Zealand.
Summers
E L
EL
Demmer
J
J
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D002364:Caseins / Q000235:genetics* / Q000378:metabolism; D002417:Cattle; D012085:Chymosin / Q000378:metabolism; D003001:Cloning, Molecular; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics*; D005260:Female; D014644:Genetic Variation; D006801:Humans; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D009893:Opossums / Q000235:genetics* / Q000378:metabolism; D010802:Phylogeny; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity
D002364:Caseins; D017931:DNA Primers; D018076:DNA, Complementary; D012085:Chymosin