The Matrix Biochemistry Section focuses its research on the functions of five major noncollagenous proteins first found associated with the mineralized matrix of bones and teeth but that we later showed are also made by many metabolically active ductal epithelial cells. The five proteins are bone sialoprotein (BSP), osteopontin (OPN), dentin matrix protein-1 (DMP1), dentin sialophosphoprotein (DSPP), and matrix extracellular phosphoglycoprotein (MEPE). We have made a strong case for the genetic relatedness of these seemingly different proteins and there is increasing acceptance of the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family concept. The genes encoding these proteins are all clustered in a tandem fashion within a short (400,000 base pairs) region of human chromosome 4 and similarly on all other mammals studied to date. After comparing the intron-exon structures and conserved motifs of their respective protein-encoding exons, we proposed that the five genes might be the result of ancient gene duplication and subsequent divergence. This year we have looked more deeply into the evolution of the most acidic protein made by mammals, DSPP. We have presented evidence that DSPP was likely the result of a gene duplication of the DMP1 gene about the time that reptiles and mammals diverged from their shared ancestor more than 300 million years ago. Detailed analyses of representative mammalian and reptilian DSPP genes showed that different copies of the duplicated DMP1 genes independently amplified different serine-rich phosphorylation motifs resulting in the convergent evolution of a highly repetitive, phosphate-rich, calcium-binding DSPP-like proteins perhaps corresponding to their independent evolution of more sophisticated and complex enamel-covered dentin tooth structures. The evidence included the use of specific serine-encoding codons as well as the convergent use of different positively charged amino acids to interrupt the otherwise negatively charged repeat domain. Our current research involves a focus on the mechanism(s) causing the dominant negative affects of all DSPP mutations in both dentin dysplasia and dentinogenesis imperfecta.
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