Solution structure of midkine, a new heparin-binding growth factor

Area of Science:

• Molecular Biology
• Structural Biology
• Neuroscience

Background:

• Midkine (MK) is a heparin-binding polypeptide involved in neurite outgrowth, neuronal survival, and plasminogen activator activity.
• MK's biological functions are primarily attributed to its C-terminal domain.
• The dimeric form of MK is considered its active state, mediating signaling to endothelial and neuronal cells.

Purpose of the Study:

• To elucidate the structural basis of Midkine's (MK) heparin-binding activity and its dimeric structure.
• To investigate the interaction between MK dimer and heparin oligosaccharides.
• To propose a structural model for the active MK dimer.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was used to determine the solution structures of MK's two domains.
• NMR titration experiments with heparin oligosaccharides were performed to analyze binding interactions.
• Cross-linking experiments using transglutaminase supported the proposed head-to-head dimer model of MK.

Main Results:

• Both N-terminal and C-terminal domains of MK consist of three antiparallel beta-strands.
• The C-terminal domain possesses two distinct heparin-binding sites: a flexible hairpin loop and basic residues on the beta-sheet.
• The MK dimer model reveals a fused heparin-binding site at the dimer interface, which effectively interacts with heparin's sulfate groups, correlating with enhanced activity.

Conclusions:

• The dimeric structure of Midkine (MK) is essential for its biological activity.
• The fused heparin-binding site in the MK dimer, formed at the C-terminal interface, explains its strong affinity for heparin.
• Structural insights into the MK dimer provide a basis for understanding its enhanced signaling and biological functions.