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POST-TRANSLATIONAL MODIFICATIONS OF SIBLING PROTEINS AND THEIR ROLES IN OSTEOGENESIS AND DENTINOGENESIS
C. Qin*,
O. Baba and
W.T. Butler
The Department of Endodontics and Periodontics, the University of Texas-Houston Health Science Center, Dental Branch, 6516 M.D. Anderson Blvd., DBB, Rm 375, Houston, TX 77030;
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Figure 1a. H&E staining of a bucco-lingual section from the mandible of a three-week-old rat. Osteoblasts (ob) secrete unmineralized bone matrix, termed ‘osteoid’ (os, doubleheaded arrow), which is subsequently mineralized and becomes bone (mm, long arrow) when apatite crystals are deposited. Small arrows indicate osteocytes buried in the mineralized bone matrix. Bar = 40 µm.
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Figure 1b. H&E staining of a bucco-lingual section from a mandibular incisor of a five-week-old rat. Similar to osteoblasts, odontoblasts (od) secrete unmineralized predentin (pd, doubleheaded arrow), which is subsequently mineralized and becomes dentin (d, long arrow) when apatite crystals are deposited. The long columnar odontoblasts form processes that extend into dentin. The short arrows indicate dental tubules that house odontoblast processes. Bar = 40 µm.
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Figure 2a. Full-length DMP1 is an inactive precursor. It is proteolytically processed into 37K and 57K fragments, originating from the NH2-terminal and COOH-terminal regions of the DMP1 amino acid sequence, respectively. The 57K fragments contain more phosphates (P) than the 37K fragments.
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Figure 2b. Amino acid sequences surrounding the cleavage sites of DMP1 and DSPP. Vertical arrows indicate cleavages sites. In DMP1, all 4 cleavages occur at the NH2-terminus of the Asp residue (Qin et al., 2003a). Among the 3 cleavage sites in DSPP, 2 major ones are at the NH2-terminus of the Asp residue, while the minor one, as indicated by a smaller arrow, is at the His-Ser bond (Qin et al., 2001b). Amino acid residues are numbered (written on the top of corresponding residues), starting from the NH2-terminus of full-length DMP1 or DSPP, excluding signal peptide.
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Figure 2c. Full-length DSPP is an inactive precursor. It is proteolytically processed into DSP and DPP that are from the NH2-terminal and COOH-terminal regions of DSPP, respectively. DSP is rich in carbohydrates (CHO) but contains fewer phosphates (P), whereas DPP is devoid of glycosylation but contains an unusually large number of phosphates. The phosphates on DPP are mainly present on one side of the protein backbone (George et al., 1996) and are postulated to sequester calcium ions (Ca++).
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Figure 3. Hypothetical processing of DMP1 and DSPP by PHEX protein. DMP1 and DSPP are converted to functional fragments by PHEX enzyme on the surfaces of osteoblasts, osteocytes, and odontoblasts. PHEX protein, one of the M13 endopeptidases, has a short cytoplamic NH2-terminal region, a single transmembrane domain, and a long extracytoplasmic domain that contains the active site of the enzyme (Crine et al., 1997). After DMP1 and DSPP are processed by cleavage of selected X-Asp bonds, their fragments—namely, 37K, 57K, DSP, and DPP—are transported to the ECM of bone and dentin, where they influence mineralization.
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Critical Reviews in Oral Biology & Medicine, Vol. 15, No. 3,
126-136 (2004)
DOI: 10.1177/154411130401500302
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