CRYSTAL-STRUCTURE OF THE NK1 FRAGMENT OF HUMAN HEPATOCYTE GROWTH-FACTOR AT 2.0 ANGSTROM RESOLUTION

Background: Hepatocyte growth factor (HGF) is a mitogen for hepatocyte s and has also been implicated as an epithelial morphogen in tumor inv asion. HGF activates its specific cellular receptor, c-met, through an aggregation mechanism potentiated by heparan sulfate glycosaminoglyca ns. HGF consists of an N-terminal (N) domain, four kringle domains (th e first of which carries receptor-binding determinants), and an inacti ve serine-protease-like domain. NK1, a naturally occurring fragment of HGF, acts as an antagonist of HGF in the absence of heparin. Results: The N domain of NK1 consists of a central five-stranded antiparallel beta sheet flanked by an alpha helix and a two-stranded beta ribbon. T he overall N domain structure in the context of the NK1 fragment is si milar to the structure of the isolated domain; two lysines and an argi nine residue coordinate a bound sulfate ion. The NK1 kringle domain is homologous to kringle 4 from plasminogen, except that the lysine-bind ing pocket is altered by the insertion of a glycine residue. Here, a H EPES molecule is bound in the pocket, The asymmetric unit of the cryst al contains a 'head-to-tail' NK1 dimer. We use this dimer to propose a model of the NK2 fragment of HGF. Conclusions: A cluster of exposed l ysine and arginine residues in or near the hairpin-loop region of the N domain might form part of the NK1 heparin-binding site. In our NK2 m odel, both kringle domains pack loosely against the N domain, and a lo ng, positively charged groove lines the interface. This groove might b e involved in glycosaminoglycan binding. The HGF receptor-binding dete rminants are clustered near the binding pocket of the first kringle do main, opposite the N domain.