Identification of two cDNA clones encoding small proline-rich proteins expressed in sheep ruminal epithelium.

Small proline-rich (SPRR) proteins are markers frequently associated with squamous cell differentiation. They have been proposed to be a novel group of precursor polypeptides for the cornified envelope in epidermal keratinocytes. A plus/minus screening procedure was used to identify cDNA clones expressed in mature but not in neonatal sheep ruminal epithelium. Two clones encoding SPRR proteins were identified and are reported here. Clone 27 encodes an ovine SPRR protein corresponding to the human type-II SPRR protein. Clone 26 encodes an ovine SPRR protein similar to human type-II SPRR protein, but which also contains an N-terminal His-Pro repeat similar to the paired repeats found in the Drosophila paired proteins. The unique combination of a paired domain and an SPRR protein has not been reported prior to this study. The tissue distribution indicates that specific expression of the genes corresponding to these two clones occurs in the epithelium of the ruminant forestomach, and to a lesser extent in skin epithelium. In situ hybridization demonstrated that the SPRR mRNA for both clones were localized in the stratum granulosum, in support of their putative physiological function, i.e. formation of the cornified envelope. Based on Northern blot analysis, mRNA complementary to the two clones appears in the ruminal epithelium by 1 week of age, corresponding to the formation of the stratum granulosum during ruminal epithelial development. The different patterns of changes in amount of mRNA corresponding to these clones during rumen epithelial development indicate that they play different roles in rumen epithelial development.