A novel marsupial protein expressed by the mammary gland only during the early lactation and related to the Kunitz proteinase inhibitors

A novel whey protein has been found in marsupial milk, the early lactation protein (ELP). The whey of Trichosurus vulpecula, the Australian common brush-tailed possum, contains two forms of ELP, estimated by protein electrophoresis at 8 and 16 kDa, The 16-kDa form contains approximately 60% N-linked carbohydrate. The ELP cDNA was obtained by the screening of an early lactation cDNA library with an early lactation total cDNA probe and random selection of strongly positive clones. The full-length cDNA sequence of 306 bp codes for an 82 amino acid residue mature protein and a 20-residue secretory signal peptide. The mature protein has a calculated molecular weight of 9325.4 and a pI of 8.1. Protein and RNA analysis show that the expression of the ELP is restricted only to the early lactation phase. The ELP has amino acid sequence homologies with the Kunitz proteinase inhibitor family and the whey acidic proteins. (C) 1996 Academic Press, Inc.