Posttranslational Modifications of Dentin Sialophosphoprotein and Dentin Matrix Protein 1

Objective: Mouse and human genetic studies have demonstrated the pivotal functions of dentin sialophosphoprotein (DSPP) and dentin matrix protein 1 (DMP1) in dentin and/or bone formation. While the exact mechanisms by which DSPP and DMP1 participate in dentinogenesis and/or osteogenesis are unclear, we believe that their posttranslational modifications are important for controlling the initiation and rate of mineralization of these two tissues. Specifically, we believe that posttranslational proteolytic processing of DSPP and DMP1 is an activation step that converts precursors from inactive forms to active fragments. To test this hypothesis, we carried out a series of studies. Methods: The methods used in our study include: protein chemistry, immunohistochemistry and in situ hybridization, generation and characterization of transgenic mice, and in vitro mineralization. Results: We have obtained the following results: 1) DSPP and DMP1 are present in the ECM of dentin and/or bone as NH2-terminal and COOH-terminal fragments; 2) the NH2-terminal fragments of DSPP and DMP1 are highly glycosylated, with biochemical features distinct from those of their respective COOH-terminal fragments; 3) proteolytic processing of DSPP and DMP1 involves the cleavage of selected X-Asp bonds; 3) substitutions of Asp residues at key cleavage sites block the proteolytic processing of DSPP and DMP1; 5) the highly phosphorylated COOH-terminal fragments of DSPP and DMP1 promote mineralization in vitro; 6) the NH2-terminal fragments of DSPP and DMP1 show distribution profiles different from those of their respective COOH-terminal fragments; 7) re-expression of the COOH-terminal fragment of DMP1 is capable of rescuing the mineralization defects in Dmp1-null mice. Conclusions: The above findings support our hypothesis that proteolytic processing of DSPP and DMP1 is an activation step that converts precursors from inactive forms to fragments, which plays crucial roles in dentinogenesis and osteogenesis. Supported by NIH grant DE005092 (CQ).