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 matrix-associated protein implicated in the development and plasticity of neuronal connections of brain. Binding to cell surface heparan sulfate is indispensable for the biological activity of HB-GAM. In the present paper we have studied the structure of recombinant HB-GAM using heteronuclear NMR. These studies show that HB-GAM contains two beta-sheet domains connected by a flexible linker. Both of these domains contain three antiparallel beta-strands. In addition to this domain structure, HB-GAM contains the N- and C-terminal lysine-rich sequences that lack a detectable structure and appear to form random coils. Studies using CD and NMR spectroscopy suggest that HB-GAM undergoes a conformational change upon binding to heparin, and that the binding 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 homologous to the thrombospondin type I repeat (TSR). Sequence comparisions show that the beta-sheet structures found previously in midkine, a protein homologous with HB-GAM, also correspond to the TSR motif. We suggest that the TSR sequence motif found in various extracellular proteins defines a beta-sheet structure similar to that found in HB-GAM and midkine. In addition to the apparent structural similarity, a similarity in biological functions is suggested by the occurrence of the TSR sequence motif in a wide variety of proteins that mediate cell-to-extracellular matrix and cell-to-cell interactions, in which the TSR domain mediates specific cell surface binding.
Kilpelainen
I
I
NMR Laboratory, Institute of Biotechnology, Laboratory of Molecular Neurobiology, Department of Biosciences, University of Helsinki, Helsinki FIN-00014, Finland. ikka.kilpelainen@helsinki.fi
Kaksonen
M
M
Kinnunen
T
T
Avikainen
H
H
Fath
M
M
Linhardt
R J
RJ
Raulo
E
E
Rauvala
H
H
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D004926:Escherichia coli; D016326:Extracellular Matrix Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D006133:Growth Substances / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006493:Heparin / Q000378:metabolism*; D008969:Molecular Sequence Data; D011485:Protein Binding; D017510:Protein Folding; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D019699:Thrombospondins / Q000235:genetics* / Q000378:metabolism
D016326:Extracellular Matrix Proteins; D006133:Growth Substances; D011994:Recombinant Proteins; D019699:Thrombospondins; D006493:Heparin