HEPARIN-BINDING AND OLIGOMERIZATION OF HEPATOCYTE GROWTH-FACTOR SCATTER FACTOR ISOFORMS - HEPARAN-SULFATE GLYCOSAMINOGLYCAN REQUIREMENT FORMET BINDING AND SIGNALING

Hepatocyte growth factor/scatter factor (HGF/SF) is a heparin-binding polypeptide that stimulates cell proliferation, motility, and morphoge nesis by activation of its receptor, the c-Met tyrosine kinase. HGF/SF consists of a series of structural units, including an amino-terminal segment with a hairpin loop, four kringle domains, and a serine prote ase-like region. In this study, we demonstrate that the amino-terminal (N) domain retains the heparin-binding properties of full-length HGF/ SF. In contrast to a previous hypothesis, selected basic amino acid re sidues in the hairpin loop are not critical for heparin binding, altho ugh alanine substitution at a subset of these sites markedly reduced t he biological activity of the HGF/SF isoform, HGF/NK1. Covalent cross- linking experiments performed with wild-type and heparan sulfate glyco saminoglycan (HSGAG)-deficient Chinese hamster ovary (CHO) cells revea led that Met-HGF/NK1 binding was strongly dependent on HSGAG. Addition of heparin to HSGAG-deficient CHO cells not only restored ligand bind ing, but also increased ligand-dependent Met tyrosine phosphorylation and c-fos expression, Moreover, our results showed that heparin stimul ated ligand oligomerization through an interaction with the N domain, These findings establish the importance of the N domain for heparin-li gand and ligand-ligand interactions, and demonstrate a crucial role fo r HSGAG in receptor binding and signal transduction.