title: Bone sialoprotein-collagen interaction promotes hydroxyapatite nucleation.
abstract: In bone, hydroxyapatite (HA) crystals are deposited onto the type I collagen scaffold by a mechanism that has yet to be elucidated. Bone sialoprotein (BSP) is an acidic phosphoprotein that is expressed at high levels in mineralized tissues, capable of binding type I collagen, and nucleating HA. Both bone-extracted and recombinant BSP (rBSP) bind with equal affinity to collagen. The nature of the BSP-collagen interaction and its role in HA nucleation are not known. We have used a solid-phase binding assay and affinity chromatography to characterize the BSP-collagen interaction. rBSP-binding affinities of triple-helical and fibrillar type I collagen were similar (K(D) approximately 13 nM), while that of heat-denatured type I collagen was lower (K(D) approximately 44 nM), indicating the importance of triple-helical structure in binding BSP. Pepsin treatment of collagen had no effect on rBSP binding, demonstrating that the telopeptides of collagen are not involved. The majority of collagen-bound rBSP was eluted by acetonitrile, indicating that hydrophobic interactions are principally responsible for binding. Using an HA-nucleation assay, it was shown that rBSP is ten-fold more potent in reconstituted fibrillar collagen gels than in agarose gels. Nucleating potency of a non-collagen-binding, HA-nucleating peptide [rBSP(134-206)] showed no difference in the two gel systems. The work here shows that optimal binding of rBSP requires collagen to be in a native, triple-helical structure, does not require the telopeptides, and is stabilized by hydrophobic interactions. Upon binding to collagen, rBSP displays an increase in nucleation potency, implying a co-operative effect of BSP and collagen in mineral formation.
authors.affiliation: CIHR Group in Skeletal Development and Remodeling, Department of Biochemistry and Schulich Dentistry, Schulich School of Medicine & Dentistry, University of Western Ontario, London, Ontario, Canada N6A 5C1.
authors.forename: Gurpreet S
authors.initials: GS
authors.lastname: Baht
authors.forename: Graeme K
authors.initials: GK
authors.lastname: Hunter
authors.forename: Harvey A
authors.initials: HA
authors.lastname: Goldberg
mesh_terms: D000818:Animals; D001842:Bone and Bones / Q000378:metabolism; D002118:Calcium / Q000737:chemistry; D002845:Chromatography / Q000379:methods; D003094:Collagen / Q000737:chemistry / Q000378:metabolism*; D005782:Gels; D006882:Hydroxyapatites / Q000737:chemistry*; D058574:Integrin-Binding Sialoprotein; D007700:Kinetics; D008954:Models, Biological; D010710:Phosphates / Q000737:chemistry; D011485:Protein Binding; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry; D012685:Sepharose / Q000737:chemistry; D012795:Sialoglycoproteins / Q000378:metabolism*
chemical_list: D005782:Gels; D006882:Hydroxyapatites; D058574:Integrin-Binding Sialoprotein; D010710:Phosphates; D011994:Recombinant Proteins; D012795:Sialoglycoproteins; D003094:Collagen; D012685:Sepharose; D002118:Calcium