DSP-PP Precursor Protein Processing

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Abstract: Dentin sialoprotein (DSP) and phosphophoryn (PP) are the two most abundant noncollagenous proteins in dentin, and have more recently been found in bone, kidney and salivary glands, suggesting that the DSP-PP gene may participate in a variety of processes during organogenesis. DSP and PP coding sequences are derived from a single DSP-PP gene, yet in dentin there exists a 1:6 ratio of DSP:PP instead of the expected 1:1 ratio. To date it has not been possible to study DSP-PP post-translational processing and cleavage because no DSP- PP precursor protein has been identified in any cell or tissue sample, leaving unanswered such questions as where DSP-PP cleavage occurs (i.e., intracellularly or extracellularly) and what cleavage enzyme(s) may be involved. To answer these DSP-PP protein-processing questions, we utilized a baculovirus expression system to produce recombinant DSP-PP precursor proteins from a DSP-PP240 cDNA, which represents one of several endogenous DSP-PP transcripts believed to play different roles during dentin mineralization. Our recent publication in the J. Biol. Chemistry, and our Preliminary Results, demonstrate that DSP-PP240 precursor proteins are produced by this system, and are capable of self-processing to yield both DSP and PP proteins. This application proposes a series of studies to better define and characterize DSP-PP precursor protein processing. The Specific Aims of this proposal are to utilize a variety of different recombinant DSP-PP precursor protein compositions to investigate DSP-PP processing in various expression systems (Aim 1); to determine the first DSP-PP cleavage site using site mutagenesis (Aim 2); to examine functional effects that DSP-PP cleavage defective mutants may have on dental pulp cell mineralization (Aim 3); and in Aim 4 we will investigate proteolytic activity of DSP-PP and PP using kinetic studies and protease inhibitors, and determine how residues within specific PP domains may affect PP proteolysis. We expect that this proposal will potentially lead to the characterization of a new class of protease. We also expect that this proposal will lead to the identification of specific single nucleotide polymorphisms in PP that may be associated with dental related abnormalities.