Lim, J., Yao, S., Graf, M., Winkler, C., and Yang, D. (2013) Structure-function analysis of full-length midkine reveals novel residues important for heparin-binding and zebrafish embryogenesis. The Biochemical journal. 451(3):407-15.
Midkine is a heparin-binding di-domain growth factor, implicated in many biological processes as diverse as angiogenesis, neurogenesis and tumorigenesis. Elevated midkine levels reflect poor prognosis for many carcinomas, yet the molecular and cellular mechanisms orchestrating its activity remain unclear. Presently, the individual structures of isolated half domains of human midkine are known and its functionally active C-half domain remains a popular therapeutic target. Here, we determined the structure of full length zebrafish midkine and show that it interacts with fondaparinux (a synthetic, highly sulfated pentasaccharide) and natural heparin through a previously uncharacterized but highly conserved hinge region. Mutating six consecutive residues in the conserved hinge to glycine residues strongly abates heparin-binding and midkine embryogenic activity. In contrast to previous in vitro studies, we find that the isolated C-half domain is not active in vivo in embryos. Instead, we demonstrate that the N-half domain is needed to enhance heparin-binding and mediate midkine embryogenic activity surprisingly in both heparin-dependent and -independent manners. Our findings provide new insights into the structural features of full length midkine relevant for embryogenesis, and unravel additional therapeutic routes targeting the N-half domain and conserved hinge.