Complete sequence analysis of cDNA clones encoding rat whey phosphoprotein: homology to a protease inhibitor.

Abstract

Lactoprotein clones were isolated from a rat mammary gland recombinant library of c[complementary]DNA plasmids. Clones p-Wp 52 and p-WP 47 were shown by hybrid selection, in vitro translation and immunoprecipitation to represent a cloned DNA sequence encoding rat whey phosphoprotein. The nucleotide sequence of the cDNA insert of p-Wp 52 is reported; it encodes the complete whey phosphoprotein sequence. The encoded sequence shows a high content of half-Cys, Glu, Asp and Ser but an absence of Tyr. The half-Cys appear in unique arrangements and are repeated in 2 domains of the protein. The 2nd domain has striking similarities with the 2nd domain of the red sea turtle protease inhibitor. Clone p-Wp 52 allowed the study of expression of whey phosphoprotein mRNA during functional differentiation of rat mammary gland and in mammary tumors. The whey phosphoprotein mRNA is detected during midpregnancy and lactation in the rat mammary gland but is barely detected in mammary tumors in which other milk protein mRNA are expressed. The whey phosphoprotein gene in these tumors is hypermethylated, correlating with the reduced expression of this gene.