Recombinant hemangiopoietin promotes cell adhesion and binds heparin in its multimeric form

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 (rhHAPO) 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.