Identification of secreted and membrane proteins in the rat incisor enamel organ using a signal-trap screening approach.

The secretome represents the subset of proteins that are targeted by signal peptides to the endoplasmic reticulum. Among those, secreted proteins play a pivotal role because they regulate determinant cell activities such as differentiation and intercellular communication. In calcified tissues, they also represent key players in extracellular mineralization. This study was carried out to establish a secretome profile of rat enamel organ (EO) cells. A functional genomic technology, based on the signal trap methodology, was applied, starting with a library of 5'-enriched cDNA fragments prepared from rat incisor EOs. A total of 2,592 clones were analyzed by means of macroarray hybridizations and DNA sequencing. Ninety-four unique clones encoding a signal peptide were retrieved. Among those were 84 matched known genes, many not previously reported to be expressed by the EO. Most importantly, 10 clones were classified as being novel, with EO-009 identified as the rat homolog of human APin protein. These data indicate that many secreted and membrane-embedded EO proteins still remain to be identified, some of which may play crucial roles in regulating processes that create an optimal environment for the formation and organization of apatite crystals into a complex three-dimensional calcified matrix.