Mrp4, a new mitogen-regulated protein/proliferin gene; Unique in this genefamily for its expression in the adult mouse tail and ear

Mitogen-regulated proteins (also known as proliferin; mrp /plf) are nonclas sical members of the PRL/GH family. They are expressed at high levels durin g midgestation when they are thought to induce angiogenesis and uterine gro wth. There are between four and six mr/plf genes, and three different compl ementary DNAs have been cloned. Here we identify a fourth mrp /plf gene (mr p4) that we have cloned and characterized. MRP4 is 91% identical in amino a cid sequence with the other MRP/PLF proteins but is missing two glycosylati on sites that are present in the other forms. Consistent with the loss of t wo of three glycosylation sites, the expressed form of MRP4 has a lower app arent molecular weight compared with other MRP/PLFs. In vivo, mrp4 is expre ssed in the placenta and the adult skin. Expression of mrp4 messenger RNA p eaks in the placenta on day 12. In the skin, mrp4 expression is specific to the ears and tails of mice. Our results suggest that, as well as having gr owth and angiogenic effects during pregnancy, the MRP/PLFs may have functio ns in nonreproductive tissues. Unique among the members of the mrp/plf fami ly for its expression in the hair follicles of the tail and ear, MRP4 is ex pected to have a singular role in the growth and development of these folli cles.