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Bone sialoprotein

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Bone sialoprotein (BSP) is a component of mineralized tissues such as bone, dentin, cementum and calcified cartilage. BSP is a significant component of the bone extracellular matrix and has been suggested to constitute approximately 8% of all non-collagenous proteins found in bone and cementum.[1] BSP, a SIBLING protein, was originally isolated from bovine cortical bone as a 23-kDa glycopeptide with high sialic acid content.[2][3]

The human variant of BSP is called bone sialoprotein 2 also known as cell-binding sialoprotein or integrin-binding sialoprotein and is encoded by the IBSP gene.[4]

Structure

Native BSP has an apparent molecular weight of 60-80 kDa based on SDS-PAGE, which is a considerable deviation from the predicted weight (based on cDNA sequence) of approximately 33 kDa.[5] The mammalian BSP cDNAs encode for proteins averaging 327 amino acids, which includes the 16-residue preprotein secretory signal peptide. Among the mammalian cDNAs currently characterized, there is an approximate 45% conservation of sequence identity and a further 10-23% conservative substitution. The protein is highly acidic (pKa of ~ 3.9)[6] and contains a large amount of Glu residues, constituting ~22% of the total amino acid.

Secondary structure prediction and hydrophobicity analyses suggest that the primary sequence of BSP has an open, flexible structure with the potential to form regions of α-helix and some β-sheet.[7] However, the majority of studies have demonstrated that BSP has no α-helical or β-sheet structure by 1D NMR[6][8] and circular dichroism.[9] Analysis of native protein by electron microscopy confirm that the protein has an extended structure approximately 40 nm in length.[10] This flexible conformation suggests that the protein has few structural domains, however it has been suggested that there may be several spatially segmented functional domains including a hydrophobic collagen-binding domain (rattus norvegicus residues 36-57),[11] a hydroxyapatite-nucleating region of contiguous glutamic acid residues (rattus norvegicus residues 78-85, 155-164)[9] and a classical integrin-binding motif (RGD) near the C-terminal (rattus norvegicus residues 288-291).

BSP has been demonstrated to be extensively post-translationally modified, with carbohydrates and other modifications comprising approximately 50% of the molecular weight of the native protein.[12][13] These modifications, which include N- and O-linked glycosylation, tyrosine sulfation and serine and threonine phosphorylation, make the protein highly heterogeneous.

Function

The amount of BSP in bone and dentin is roughly equal,[14] however the function of BSP in these mineralized tissues is not known. One possibility is that BSP acts as a nucleus for the formation of the first apatite crystals.[15] As the apatite forms along the collagen fibres within the extracellular matrix, BSP could then help direct, redirect or inhibit the crystal growth.

Additional roles of BSP are MMP-2 activation, angiogenesis, and protection from complement-mediated cell lysis. Regulation of the BSP gene is important to bone matrix mineralization and tumor growth in bone.[16]

References

  1. ^ Fisher LW, McBride OW, Termine JD, Young MF (February 1990). "Human bone sialoprotein. Deduced protein sequence and chromosomal localization". J. Biol. Chem. 265 (4): 2347–51. PMID 2404984.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  2. ^ Williams PA, Peacocke AR (November 1965). "The physical properties of a glycoprotein from bovine cortical bone (bone sialoprotein)". Biochim. Biophys. Acta. 101 (3): 327–35. doi:10.1016/0926-6534(65)90011-4. PMID 5862222.
  3. ^ Herring GM (February 1964). "Comparison of bovine bone sialoprotein and serum orosomucoid". Nature. 201 (4920): 709. doi:10.1038/201709a0. PMID 14139700.
  4. ^ Kerr JM, Fisher LW, Termine JD, Wang MG, McBride OW, Young MF (August 1993). "The human bone sialoprotein gene (IBSP): genomic localization and characterization". Genomics. 17 (2): 408–15. doi:10.1006/geno.1993.1340. PMID 8406493.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Fisher LW, Whitson SW, Avioli LV, Termine JD (October 1983). "Matrix sialoprotein of developing bone". J. Biol. Chem. 258 (20): 12723–7. PMID 6355090.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ^ a b Stubbs JT, Mintz KP, Eanes ED, Torchia DA, Fisher LW (August 1997). "Characterization of native and recombinant bone sialoprotein: delineation of the mineral-binding and cell adhesion domains and structural analysis of the RGD domain". J. Bone Miner. Res. 12 (8): 1210–22. doi:10.1359/jbmr.1997.12.8.1210. PMID 9258751.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ Shapiro HS, Chen J, Wrana JL, Zhang Q, Blum M, Sodek J (November 1993). "Characterization of porcine bone sialoprotein: primary structure and cellular expression". Matrix. 13 (6): 431–40. doi:10.1016/s0934-8832(11)80109-5. PMID 8309422.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Fisher LW, Torchia DA, Fohr B, Young MF, Fedarko NS (January 2001). "Flexible structures of SIBLING proteins, bone sialoprotein, and osteopontin". Biochem. Biophys. Res. Commun. 280 (2): 460–5. doi:10.1006/bbrc.2000.4146. PMID 11162539.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  9. ^ a b Tye CE, Rattray KR, Warner KJ, Gordon JA, Sodek J, Hunter GK, Goldberg HA (March 2003). "Delineation of the hydroxyapatite-nucleating domains of bone sialoprotein". J. Biol. Chem. 278 (10): 7949–55. doi:10.1074/jbc.M211915200. PMID 12493752.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  10. ^ Oldberg A, Franzén A, Heinegård D (December 1988). "The primary structure of a cell-binding bone sialoprotein". J. Biol. Chem. 263 (36): 19430–2. PMID 3198635.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ Tye CE, Hunter GK, Goldberg HA (April 2005). "Identification of the type I collagen-binding domain of bone sialoprotein and characterization of the mechanism of interaction". J. Biol. Chem. 280 (14): 13487–92. doi:10.1074/jbc.M408923200. PMID 15703183.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  12. ^ Kinne RW, Fisher LW (July 1987). "Keratan sulfate proteoglycan in rabbit compact bone is bone sialoprotein II". J. Biol. Chem. 262 (21): 10206–11. PMID 2956253.
  13. ^ Ganss B, Kim RH, Sodek J (1999). "Bone sialoprotein". Crit. Rev. Oral Biol. Med. 10 (1): 79–98. doi:10.1177/10454411990100010401. PMID 10759428.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. ^ Qin C, Brunn JC, Jones J, George A, Ramachandran A, Gorski JP, Butler WT (April 2001). "A comparative study of sialic acid-rich proteins in rat bone and dentin". Eur. J. Oral Sci. 109 (2): 133–41. doi:10.1034/j.1600-0722.2001.00001.x. PMID 11347657.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. ^ Hunter GK, Goldberg HA (August 1994). "Modulation of crystal formation by bone phosphoproteins: role of glutamic acid-rich sequences in the nucleation of hydroxyapatite by bone sialoprotein". Biochem. J. 302 ( Pt 1) (Pt 1): 175–9. PMC 1137206. PMID 7915111.
  16. ^ Ogata Y (April 2008). "Bone sialoprotein and its transcriptional regulatory mechanism". J. Periodont. Res. 43 (2): 127–35. doi:10.1111/j.1600-0765.2007.01014.x. PMID 18302613.

Further reading

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