Differential expression of the whey acidic protein gene during lactation in the brushtail possum ( Trichosurus vulpecula)

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The whey acidic protein (WAP) is a whey protein found in the milk of a number of species. We have isolated and characterised a WAP cDNA clone from the brushtail possum ( Trichosurus vulpecula) and examined its expression in the mammary gland. The amino acid sequences of WAP from the possum and another marsupial, the tammar wallaby, share 69% identity, however, less sequence identity exists between the marsupial and eutherian WAP sequences (30–37%). The possum and tammar WAP genes consist of three four-disulphide core (4-DSC) domains, with a WAP motif at the beginning of each domain. In contrast, the eutherian WAP sequences consist of two 4-DSC domains with the WAP motif only present in the second domain. This WAP motif is also present in a number of protease inhibitors found in a wide range of species. Phylogenetic analysis of marsupial and eutherian WAP sequences suggests that the ancestral WAP gene has three domains and that one of the domains has been deleted from the eutherian gene. The profile of WAP gene expression in the possum mammary gland changed throughout lactation, with WAP mRNA levels reaching a peak between days 106 and 177 of lactation. The level of WAP mRNA in the mammary gland appeared to be correlated with the level of circulating prolactin in the lactating female and was different to that observed for several other whey protein genes. Overlapping expression of the WAP and early lactation protein genes, both of which are putative protease inhibitors, may provide protection of milk immunoglobulins that are required for the prolonged period of passive immune transfer to the marsupial pouch young.