THE MIDKINE (MK) FAMILY OF GROWTH DIFFERENTIATION FACTORS - STRUCTUREOF AN MK-RELATED SEQUENCE IN A PSEUDOGENE AND EVOLUTIONARY RELATIONSHIPS AMONG MEMBERS OF THE MK FAMILY/

Midkine (MK) is a novel heparin-binding growth/differentiation factor coded by a retinoic acid-responsive gene. MK cDNA probe reacts with tw o bands, a 4 kb one and a 3 kb one, upon Southern blot analysis of Hil t dill fragments of mouse genomic DNA: the midkine gene (Mdk) is on th e 4 kb fragment. Sequence analysis of the 3 kb fragment revealed that it has an Mdk-related sequence (Mdk-rs) highly homologous to MK cDNA, three mouse Alu-equivalent repeats and seven A+T-rich segments. The Md k-rs carried an inserted microsatellite DNA, is flanked by imperfect d irect repeats observed in many retroposons, and lacks introns. Intersp ecific hybrid analysis revealed that Mdk-rs is located on chromosome 1 1, while Mdk is known to be on chromosome 2. The evolutional velocity of Mdk-rs was calculated to be 11 times higher than that of mouse Mdk. These features suggest that Mdk-rs is a processed pseudogene generate d in the mouse genome. The 3 kb fragment with Mdk-rs, which is rich in inserted DNA sequences probably due to the presence of A+ T-rich segm ents, may be a hot spot for amplification and evolution of genomic DNA . Mdk-rs was estimated to have been generated about 19.1 million years ago. A chicken protein retinoic acid-induced, heparin-binding protein (RIHB), is highly homologous to MK, and its divergence from human MK was estimated to have occurred about 250 million years ago, suggesting that RIHB is the chicken homolog of MK. Thus, so far there are only t wo established protein members, MK and heparin-binding, growth-associa ted molecule (HB-GAM)/pleiotrophin (PTN) in the MK family.