The nature of the primary gene product for the a-phosphophoryn component of rat incisor dentin has been examined by cell-free translation of the total RNA and poly(A+) mRNA from rat maxillary incisors, including pulp cells and odontoblasts. The RNA was extracted by the guanidinium thiocyanate method and translated in a rabbit reticulocyte system. The translated proteins were analyzed by gradient gel electrophoresis, and a-phosphophoryn was identified by isolation on an anti-rat a-phosphophoryn antibody coupled Sepharose column and dot-blot procedures. The major protein identified as a-phosphophoryn had a molecular weight of 153 000 (±5000) and had chromatographic properties similar to those of a-phosphophoryn. Since tissue-isolated rat phosphophoryn has a molecular weight of only ~ 90 000 when fully phosphorylated, it appears that the primary gene product is a prepro-a-phosphophoryn. Thus, a-phosphophoryn in the extracellular space of rat incisor dentin must be the product of one or more posttranslational proteolytic processing steps.
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