Midkine (MK)

Midkine, a product of a retinoic acid responsive gene, is a secreted 13 kDa protein belonging to the family of heparin binding growth/differentiation factors. MK shares 45% sequence identity with other member of this family called Pleiotrophin (HB-GAM).

Midkine is composed of two domains held together by disulfide linkages. The C-terminally located domain contains two heparin binding sites and is usually responsible for midkine activity. Part of the MK activity is enhanced by dimerization of MK. Midkine has been found in vertebrates from human to zebrafish and is most strongly expresed in midgestation. In the adult MK expression is restricted.