IBSP (integrin-binding sialoprotein; also BSP or bone sialoprotein (II)) is a 55‑75 kDa, secreted, variably glycosylated, monomeric noncollagenous member of the SIBLING family of extracellular matrix (ECM) proteins (1‑3). It is principally associated with the early stages of bone mineralization. BSP is synthesized as a 317 amino acid (aa) precursor that contains a 16 aa signal sequence and a 301 aa mature region (4‑6). The mature segment is divided into a basic N-terminus (aa 17‑62), a central region (aa 63‑233), and an acidic C‑terminus (aa 234‑317) (7).
Functional segments associated with the mature molecule include a type I collagen binding domain (aa 19‑46), two non‑RGD cell binding sites (aa 30‑57 and 261‑281), an RGD alpha v beta 3 integrin‑binding site (aa 286‑288) and two potential hydroxyapatite (HAp) nucleation domains (aa 76‑83 and 151‑158) (3, 4, 8‑11). HAp formation requires a BSP nucleation site composed of at least eight consecutive glutamic acid residues and, likely, a contribution from a BSP‑associated co‑nucleator (10, 12). BSP is highly glycosylated, sulfated, and phosphorylated. Phosphorylation may impact HAp growth, while carbohydrate may regulate cell adhesion (1, 3, 13). Mature human BSP is 70%, 72%, 78%, and 72% aa identical to porcine, rat, canine, and mouse BSP, respectively. BSP is synthesized by megakaryocytes/platelets, osteoblasts, osteocytes, odontoblasts, osteoclasts, and bone marrow stromal cells (14‑17).
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