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J. Biol. Chem., Vol. 276, Issue 2, 904-909, January 12, 2001

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Identification and Characterization of the Carboxyl-terminal Region of Rat Dentin Sialoprotein^*

Chunlin Qin^§, Richard G. Cook^¶, Ralph S. Orkiszewski, and William T. Butler

From the  Department of Basic Sciences, The University of Texas-Houston Health Science Center, Dental Branch, Houston, Texas 77030 and ^¶ Department of Immunology and  Protein Chemistry Core Laboratory, Baylor College of Medicine, Houston, Texas 77030

Two acidic proteins, dentin sialoprotein (DSP) and dentin phosphoprotein (DPP), are present in the extracellular matrix of dentin but not in bone. These two proteins are expressed in odontoblasts and preameloblasts as a single cDNA transcript coding a large precursor protein termed dentin sialophosphoprotein (DSPP). DSPP is specifically cleaved into two unique proteins, DSP and DPP. However, the cleavage site(s) of DSPP and the mechanisms for regulating the cleavages are unknown. To identify the specific site(s) of DSPP that are cleaved when the initial translation product is converted to DSP and DPP, we performed a detailed analysis (Edman degradation and mass spectrometry) on selected tryptic peptides of a size originating from the COOH-terminal region of rat DSP. After cleavage with trypsin, the DSP fragments were separated by a two-dimensional method (size-exclusion chromatography followed by reversed phase high performance liquid chromatography). We characterized 13 peptides from various regions of DSP. The analyses showed that peptide Ile⁴⁰⁹-Tyr⁴²¹ was the major COOH-terminal fragment, ending at Tyr⁴²¹ only 9 residues from the NH₂ terminus of DPP. Peptide Gln³⁸⁵-His⁴⁰⁶ represented a second, minor COOH-terminal peptide that terminated at His⁴⁰⁶. Both of these residues are well beyond the COOH terminus predicted previously by two independent studies estimating that rat DSP contained 360-370 amino acids. Careful studies on two peptides showed that, among 9 potential casein kinase II phosphorylation sites, 2 serines were phosphorylated. We found that rat DSP was heterogeneous with respect to phosphorylation, because this same peptide sequence eluted in two discrete peaks, one with 2 phosphoserines and the other having 1. The finding that 3 lysines just preceding the COOH termini were modified by a 43-Da substituent (possibly a carbamoyl substituent) suggests that the lysines in this region were particularly susceptible to attachment of this substituent.

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