Heparin-binding growth-associated molecule contains two heparin-binding beta-sheet domains that are homologous to the thrombospondin type I repeat

Heparin-binding growth-associated molecule (HB-GAM) is an extracellular mat rix-associated protein implicated in the development and plasticity of neur onal connections of brain. Binding to cell surface heparan sulfate is indis pensable for the biological activity of HB-GAM, In the present paper we hav e studied the structure of recombinant HB-GAIM using heteronuclear NMR. The se studies show that HB-GARI contains two beta-sheet domains connected by a flexible linker. Both of these domains contain three antiparallel beta-str ands, In addition to this domain structure, HB-GAM contains the Nand C-term inal lysine-rich sequences that lack a detectable structure and appear to f orm random coils. Studies using CD and NMR spectroscopy suggest that HB-GAI M undergoes a conformational change upon binding to heparin, and that the b inding occurs primarily to the beta-sheet domains of the protein. Search of sequence data bases shows that the beta-sheet domains of HB-GAM are homolo gous to the thrombospondin type I repeat (TSR), Sequence comparisions show that the beta-sheet structures found previously in midkine, a protein homol ogous with HB-GAM, also correspond to the TSR motif, We suggest that the TS R sequence motif found in various extracellular proteins defines a beta-she et structure similar to that found in HB-GAM and midkine, In addition to th e apparent structural similarity, a similarity in biological functions is s uggested by the occurrence of the TSR sequence motif in a wide variety of p roteins that mediate cell-to-extracellular matrix and cell-to-cell interact ions, in which the TSR domain mediates specific cell surface binding.