Characterization of an Avian Bone Sialoprotein (BSP) cDNA: Comparisons to Mammalian BSP and Identification of Conserved Structural Domains

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Abstract
Bone sialoprotein (BSP) is one of the major noncollagenous proteins found in mineralized vertebrate tissue. It is an acidic glycoprotein containing a high sialic acid content and is phosphorylated on several of its Ser and Thr residues. While it has been extensively characterized from various mammalian species, little is known about its sequence or expression in lower vertebrates. The cloning and characterization of several cDNAs encoding the chicken bone sialoprotein are reported here. A partial cDNA clone encoding the carboxyl terminus of the protein was initially isolated from a λGT11 expression library using a polyclonal antibody against BSP purified from chicken bone matrix. Subsequently, several additional clones were obtained by further screening and by reverse transcription polymerase chain reaction (RT‐PCR). Three overlapping clones encompassing about 1 kb, which included the complete coding sequence for BSP, were analyzed. The deduced amino acid sequence revealed that chicken BSP contains 276 amino acid residues. Although the overall identity between chicken and mammalian BSP is only ∼39%, the diversity in amino acid sequence occurs mostly between the major functional domains of this molecule. These domains include: (1) three acidic poly‐Glu regions; (2) two tyrosine‐rich domains, which may be sites for protein sulfation; (3) several casein kinase II phosphorylation sites; (4) an Asn glycosylation site; and (5) an RGD cell‐binding motif. Of interest in the chicken BSP is the identification of two additional RGD motifs within the avian sequence, unlike the mammalian forms of BSP which has only one. The existence of several consecutive gly‐x‐y triplets in the molecule suggest that it may bind to collagen, which is consistent with the fact that BSP protein is tightly associated with the matrix. Northern blot analysis demonstrated that chicken BSP was expressed specifically in bone tissue, and its expression was detected as early as Day 12 during chicken embryogenesis. BSP expression preceded osteopontin (OPN) expression, reaching maximal levels during embryogenesis in contrast to that of OPN, which only reaches its highest level of expression several weeks after hatching.

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