Expression of glutamine/glutamic acid-rich proteins in rat submandibular glands.

The expression of GRP transcripts was found to be highly specific to the rat submandibular gland. GRP cross-reactive species were detected in the saliva of both inbred and outbred rat strains. There was no evidence of GRP transcripts in RNA prepared from bovine, ovine, porcine or murine submandibular glands. Thus the GRPs differ from the family of PRPs that are expressed in several species. The restriction of GRP expression to the rat suggests a relatively recent origin for a functional GRP gene, presumably after rat-mouse divergence. The ontogeny of the relative steady-state levels of GRP transcripts was assessed by dot blot analysis. Maximal levels of RNA-encoding GRP were detected at 6 months; there was a significant age-related decline at both 12 and 18 months. There was, however, no significant age-related alteration in the size of transcripts which encode this protein family.