Cloning, cDNA analysis and prolactin-dependent expression of a marsupial alpha-lactalbumin

The gene for alpha-lactalbumin has been cloned from a tammar wallaby (Macropus eugenii) mammary gland cDNA library. Tammar alpha-lactalbumin has approximately 50 and 30% homology to the alpha-lactalbumins of eutherians at the levels of nucleotide and protein sequence respectively. Comparison of the inferred tammar polypeptide sequence with the sequence of the eutherian proteins reveals extensive divergence at almost all of the non-essential amino acid residues. However, the hydropathy plots of the tammar protein are almost identical to those of eutherian alpha-lactalbumins, suggesting that protein conformation is conserved. The tammar gene encodes a transcript of approximately 975 bases. Northern blot analysis of hormone-stimulated mammary gland explants shows that maximal induction of alpha-lactalbumin mRNA is dependent on prolactin and that expression is not modulated by other hormones that play a role in the initiation of lactation in eutherians.