Cyclic AMP-receptor proteins in the enamel matrix

Cyclic AMP receptor proteins (cARP) are present in a variety of cell types. Intracellularly, they are the regulatory (R) subunits of type II cyclic AMP-dependent protein kinase (PKA: E.C.2.7.1.37). Additionally, cARP are secretory products of several cell types.[1] That cARP are present in and secreted by ameloblasts into the enamel matrix of the rat incisor was demonstrated by photoaffinity labeling, Western blotting and immunogold cytochemistry. Gold particles were present over cytoplasmic regions including Tomes' Processes of secretory ameloblasts, secretory granules and in the Golgi region. Specific RII labeling was seen in the enamel matrix, but not in dentin. The enamel matrix was more reactive during early maturation compared to the secretory stage of amelogenesis. Nuclear labeling with the RII antibody showed higher intensity in maturation than in secretory ameloblasts. These results demonstrate that cARP are expressed in ameloblasts and secreted into the enamel matrix. The role(s) of cARP in enamel matrix mineralization and the involvement of PKA-regulated pathways in enamel protein synthesis and secretion remain to be determined.