Lee-Ho Wang

The mineralization process associated with the conversion of predentin to dentin is believed to be initiated and controlled by a set of acidic regulatory noncollagenous proteins (NCPs) which include phosphophoryn, the major NCP in dentin. Phosphophoryn binds tightly to collagen and is believed to initiate the formation of apatite crystals which play a central role in the mineralization process. During the process of analyzing the 3' end of an odontoblast-specific cDNA which codes for dentin sialoprotein (Ritchie, H. H., Hou, H., Veis, A., and Butler, W. T. (1994) J. Biol. Chem. 269, 3698-3702), we discovered a 801-base pair open reading frame. This downstream open reading frame encodes a putative leader sequence and a very acidic mature protein sequence having a deduced amino acid composition containing high percentages of both Ser (43%) and Asp (31%) residues which closely coincides with the amino acid composition of phosphophoryns from human, bovine, rat, and rabbit (i. e. Asp (30-40%) and Ser (38-50%)). This newly identified cDNA therefore encodes a protein with characteristics similar to phosphophoryn. Here we present the cDNA sequence, the deduced amino acid sequence, and the prospective Ser residue-specific casein kinase I and II phosphorylation sites for this putative phosphophoryn.

Two different genomic genes for the COUP-transcription factor, COUP-TF I and COUP-TF II, have been isolated from a human cosmid genomic library using a [32P]-labeled cDNA probe. Data obtained from Southern blot analysis of these cosmid clones indicated that two closely related genes exist in the human genome and have a similar genomic organization. The genes are similar in the hormone and DNA binding domains but diverge from one another in the N-terminal region. Using DNA sequencing and polymerase chain reaction (PCR) techniques we have determined that the structure of COUP-TF I consists only of three exons and two introns. Surprisingly, both zinc fingers (i.e., F1 and F2) are located in the first exon. Therefore, COUP-TF I is unique among the members of the steroid/thyroid hormone receptor superfamily which have been described to date.

Hydroperoxides induce formation of a tyrosyl radical on Tyr385 in prostaglandin H synthase (PGHS). The Tyr385 radical initiates hydrogen abstraction from arachidonic acid, thereby mechanistically connecting the peroxidase and cyclooxygenase activities. In both PGHS isoforms the tyrosyl radical undergoes a time-dependent transition from a wide doublet to a wide singlet species; pretreatment with cyclooxygenase inhibitors results in a third type of signal, a narrow singlet [Tsai, A.-L.; Kulmacz, R. J. (2000) Prost. Lipid Med. 62, 231-254]. These transitions have been interpreted as resulting from Tyr385 ring rotation, but could also be due to radical migration from Tyr385 to another tyrosine residue. PATHWAYS analysis of PGHS crystal structures identified four tyrosine residues with favorable predicted electronic coupling: residues 148, 348, 404, and 504 (ovine PGHS-1 numbering). We expressed recombinant PGHS-2 proteins containing single Tyr --> Phe mutations at the target residues, a quadruple mutant with all four tyrosines mutated, and a quintuple mutant, which also contains a Y385F mutation. All mutants bind heme and display appreciable peroxidase activity, and with the exception of the quintuple mutant, all retain cyclooxygenase activity, indicating that neither of the active sites is significantly perturbed. Reaction of the Y148F, Y348F, and Y404F mutants with EtOOH generates a wide singlet EPR signal similar to that of native PGHS-2. However, reaction of the Y504F and the quadruple mutants with peroxide yields persistent wide doublets, and the quintuple mutant is EPR silent. Nimesulide pretreatment of Y504F and the quadruple mutant results in an abnormally small amount of wide doublet signal, with no narrow singlet being formed. Therefore, the formation of an alternative tyrosine radical on Tyr504 probably accounts for the transition from a wide doublet to a wide singlet in native PGHS-2 and for formation of a narrow singlet in complexes of PGHS-2 with cyclooxygenase inhibitors.