Hemangiopoietin (HAPO) is a novel growth factor stimulating the proliferation of hematopoietic and endothelial progenitor cells in vitro and in vivo. The native protein is a 294-amino acid multimodular protein. The N-terminus constitutes of two somatomedin B (SMB) homology domains that contain 14 cysteines. The central region is a putative heparin-binding domain (pHBD) and the C-terminus contains mucin-like repeats. In the present study, we demonstrated that prokaryotic recombinant human HAPO

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... hHAPO) self-associates into a multimeric form with a mass weight of ~129 kDa, suggesting a homologous tetramer. rhHAPO in its multimeric form was found to be more stable and more potent in promoting HESS-5 cell adhesion. Multimeric rhHAPO had a higher affinity to heparin compared with its dimeric form, although there was no significant conformational change. C-terminal repeats-truncated rhHAPO (rhHAPOΔmucin) was also found to be assembled into a multimer, while deletion of pHBD (rhHAPOΔmucin-pHBD) caused the protein to remain in a dimeric form, demonstrating that SMB domains participate in self-aggregation of the molecule and that the pHBD region promotes the tetramerization.