A 19 KDA PROTEIN SECRETED BY THE ENDOMETRIUM OF THE MARE IS A NOVEL MEMBER OF THE LIPOCALIN FAMILY

Large quantities of an unusual 19 kDa protein (p19) are secreted into the lumen of the uterus of the mare (Equus caballus) during the oestro us cycle and early pregnancy. p19 associates strongly with the acellul ar capsule that surrounds the young horse conceptus and is believed to be important in maintaining pregnancy. Here we report the complete cD NA sequence encoding p19, its expression patterns in horse tissues and a Southern blot analysis of the gene in horse DNA. The predicted amin o acid sequence of the p19 cDNA demonstrated a signal peptide of 18 re sidues and a mature protein of 162 residues, giving a predicted molecu lar mass of 18.8 kDa for the secreted protein. Analysis of the predict ed amino acid sequence showed that p19 belongs to the lipocalin family of proteins, which are classified on the basis of three conserved ami no acid sequence motifs and an eight-stranded anti-parallel beta-barre l conformation. Overall, p19 was most similar to the mouse major urina ry protein (MUP) lipocalins (30-34% identity). Secondary structural pr edictions and structural modelling using the MUP 1 crystal structure c oordinates gave convincing evidence that the three-dimensional structu re of p19 closely resembles that of the lipocalins. However, p19 has s everal unusual substitutions in one of the three conserved lipocalin m otifs and therefore seems to be a novel member of the family. Lipocali ns perform many different functions but most bind small hydrophobic mo lecules and the majority act as transport proteins. The function of p1 9 might therefore be as a carrier of a maternal factor needed,to susta in the developing embryo during pregnancy, or it could be incorporated into the embryonic capsule and perform some other function. Northern blot analysis demonstrated that expression of p19 is confined to the e ndometrial lining of the mare's uterus, and hybridization studies in s itu showed that the mRNA for p19 is localized to the glandular and lum inal epithelia of the endometrium. A Southern blot analysis of horse D NA indicated a single gene for p19 that seems to be at least 4.5 kb in size.