Posttranslational modifications to human bone sialoprotein determined by mass spectrometry

Bone sialoprotein (BSP) is an acidic 301 amino acid protein expressed by os teoblasts and at a low level by hypertrophic chondrocytes. Its expression i s highest during early stages of bone formation, and it is particularly abu ndant in the cells lining the surface of newly formed trabeculae. BSP conta ins numerous substituents which are anionic in nature and apparently essent ial for the function of the protein. Thus, the proposed role of BSP in hydr oxyapatite nucleation and growth may depend on such modifying groups. The p osttranslational. modifications include several acidic oligosaccharides as well as phosphate and sulfate groups. This work combines matrix-assisted la ser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry with selective enzyme treatment of BSP to provide new information on the precis e distribution and structure of oligosaccharides, sulfate, and phosphate gr oups in BSP isolated from human bone. The results provide a high level of d etail in the location of these modifying groups toward the end of providing a basis for further understanding the function of BSP in bone nucleation.