Nucleolin facilitates nuclear retention of an ultraconserved region containing and accelerates colon cancer cell growth.

Transcribed-ultraconserved regions (T-UCRs), which contain conserved sequences with 100% identity across human, rat and mouse species, are a novel category of functional RNAs. The human gene () encodes a UCR that spans exon 2 (276 bp) and its neighboring introns. Among five spliced RNA variants () transcribed from the gene, only contains the conserved exon 2. is overexpressed in colon cancer cells and accelerates cell growth by blocking the transcription of . However, the mechanisms underlying the overexpression of in colon cancer cells are unknown. Using biotinylated RNA pull-down assays followed by liquid chromatography-mass spectrometric analysis, we identified nucleolin as a -binding protein. Knockdown of nucleolin reduced the nuclear retention of and accelerated its degradation in the cytoplasm, whereas nucleolin overexpression increased levels and its mitogenic activity. Nucleolin directly bound to exon 2 via the glycine/arginine-rich (GAR) domain. Overexpression of GAR-deficient nucleolin failed to increase expression and growth of colon cancer cells. RNA fluorescence hybridization showed that co-localized with nucleolin in nuclei but not with the mutant lacking GAR. Our results suggest that specific interactions between nucleolin and UCR-containing may be associated with abnormal growth of colon cancer cells.