RESUMO
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.
