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 cas ein micelles. Although marsupial milk contains casein micelles, kappa -case in 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 k appa -casein is a 158 amino acid peptide that shares low amino acid sequenc e identity (20-30%) with that of eutherian kappa -caseins. In the gut of su ckling young, casein micelles clot when kappa -casein is cleaved by chymosi n at a specific site. Eutherian kappa -casein sequences are classified acco rding to the sequence of the chymosin cleavage site: Phe-Met, Phe-Ile or Ph e-Leu. Possum kappa -casein appears to form a separate class, with a putati ve 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 molecu le, are conserved in the possum sequence. The kappa -casein gene was expres sed 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 contro lling expression of all three genes in the possum.