The gene for a novel member of the whey acidic protein family encodes three four-disulfide core domains and is asynchronously expressed during lactation

Secretion of whey acidic protein (WAP) in milk throughout lactation has pre viously been reported for a limited number of species, including the mouse, rat, rabbit, camel, and pig. We report here the isolation of WAP from the milk of a marsupial, the tammar wallaby (Macropus eugenii). Tammar WAP (tWA P) was isolated by reverse-phase HPLC and migrates in SDS-polyacrylamide ge l electrophoresis at 29.9 kDa. tWAP is the major whey protein, but in contr ast to eutherians, secretion is asynchronous and occurs only from approxima tely days 130 through 240 of lactation. The full-length cDNA codes for a ma ture protein of 191 amino acids, which is comprised of three four-disulfide core domains, contrasting with the two four-disulfide core domain arrangem ent in all other known WAPs. A three dimensional model for tWAP has been co nstructed and suggests that the three domains have Little interaction and c ould function independently. Analysis of the amino acid sequence suggests t he protein belongs to a family of protease inhibitors; however, the predict ed active site of these domains is dissimilar to the confirmed active site for known protease inhibitors. This suggests that any putative protease lig and may be unique to either the mammary gland, milk, or gut of the pouch yo ung. Examination of the endocrine regulation of the tWAP gene showed consis tently that the gene is prolactin-responsive but that the endocrine require ments for induction and maintenance of tWAP gene expression are different d uring lactation.