PROPERTIES OF THE (DSS)(N) TRIPLET REPEAT DOMAIN OF RAT DENTIN PHOSPHOPHORYN

Phophophoryns (PPs) are unique aspartic acid and phosphoserine-rich pr oteins present in all species of dentin. Rat incisor odontoblast cDNA libraries contain messages encoding several acidic phosphorylated, ser ine-rich proteins. At least two of these share a common C-terminal dom ain coding region sequence. The polypeptide sequences in the N-termina l direction immediately adjacent to the conserved C-terminal domains o f these two proteins (DMP2, DMP3) are distinctly different. In this do main, the DMP2 has extensive sequences of (DSS)(n) repeats with n as l arge as 24. DMP3 has fewer and shorter triplet seqnences, n = 3,4. The major rat incisor PPs (90-95 kDa) probably have the (DSS)(n>3). We pr opose that the name phosphophoryn be reserved for the extracellular ma trix proteins with these extended repeats. DMP1, although strongly aci dic, does not fit this category. If the S residues are phosphorylated and n>3, conformational energy minimization computations show the (DSS )(n) sequence to assume a unique extended structure with parallel arra ys of carboxylate and phosphate groups which may function as Ca2+ ion interaction edges. The phosphorylation of recombinant DMP2 C-terminal domain by various kinases has been examined. The repeat domains are no t direct substrates for the CK2-like kinases but the kinases act in co ncert, so that the phosphorylation is hierarchical, apparently control led by the presence of specific interruptions between the triplet doma ins.