A tissue-specific small nuclear ribonucleoprotein and the regulated splicing of the calcitonin/calcitonin gene-related protein transcript.

Based on a correlation between expression patterns of an abundant tissue-specific small nuclear ribonucleoprotein (N protein) and the calcitonin gene-related protein (CGRP) splicing choice, a small nuclear ribonucleoprotein (the N factor) has been hypothesized to potentially function as a trans-acting factor involved in the regulation of the alternative splicing of the calcitonin/CGRP transcript. RNA analysis indicated that most rat, human, and simian cell lines and tissues making the CGRP mRNA splicing choice expressed the N factor mRNA. These data led us to address the effect of ectopic expression of the N factor in HeLa cells, which exhibit a calcitonin splicing choice when expressing the calcitonin/CGRP gene. Expression of the N factor exerts no effect on the calcitonin/CGRP splicing choice in HeLa cells. Furthermore, several cell lines such as the human 293 cell line make the CGRP mRNA splicing choice in the absence of any detectable level of the mRNA encoding the N factor. Together, these data reveal that the N protein is neither sufficient nor required for the tissue-specific CGRP splicing decision and that the N protein is not the trans-acting factor regulating the alternative splicing of the calcitonin/CGRP gene.