Developmentally-regulated expression of a putative protease inhibitor gene in the lactating mammary gland of the tammar wallaby, Macropus eugenii.

A novel milk protein, which is secreted only in the early stage of lactation, has been identified in the whey fraction of milk from the tammar wallaby (Macropus eugenii). The amino acid sequence currently available suggests the protein comprises 71 amino acids. The protein migrates at 18 kDa when analysed by SDS polyacrylamide gel electrophoresis but has a calculated molecular weight of 8 kDa. A partial cDNA clone of 153 bp has been isolated by reverse transcriptase PCR. Northern analysis of mammary gland RNA extracted from various stages throughout the entire lactation period showed a messenger RNA transcript of approximately 500 bp present only in the first third of lactation. The protein shares 74.5% similarity at the amino acid level with early lactation protein (ELP) from the brush-tailed possum (Trichosurus vulpecula) and 37% with bovine colostrum trypsin inhibitor, a member of the Kunitz family of protease inhibitors. We hypothesise that the expression of this gene may be controlled by changes in the sucking patterns of the dependent pouch young.