SOLUTION STRUCTURE OF MIDKINE, A NEW HEPARIN-BINDING GROWTH-FACTOR

Midkine (MK) is a 13 kDa heparin-binding polypeptide which enhances ne urite outgrowth, neuronal cell survival and plasminogen activator acti vity. MK is structurally divided into two domains, and most of the bio logical activities are located on the C-terminal domain, The solution structures of the two domains were determined by NMR. Both domains con sist of three antiparallel beta-strands, but the C-terminal domain has a long flexible hairpin loop where a heparin-binding consensus sequen ce is located. Basic residues on the beta-sheet of the C-terminal doma in form another heparin-binding site, Measurement of NMR signals in th e presence of a heparin oligosaccharides verified that multiple amino acids in the two sites participated in heparin binding. The MK dimer h as been shown to be the active form, giving signals to endothelial cel ls and probably to neuronal cells, We present a head-to-head dimer mod el of MK. The model was supported by the results of cross-linking expe riments using transglutaminase, The dimer has a fused heparin-binding site at the dimer interface of the C-terminal domain, and the heparin- binding sites on MK fit the sulfate group clusters on heparin. These f eatures are consistent with the proposed stronger heparin-binding acti vity and biological activity of the dimer.