Conditional knockout of nucleolin in DT40 cells reveals the functional redundancy of its RNA-binding domains.

Nucleolin is a major nucleolar protein which is highly expressed in rapidly dividing cells and cancer cell lines. This protein is claimed to be multifunctional and could play a role in rRNA (ribosomal RNA) synthesis, as well as in cell division or response to cellular stresses. Therefore, how nucleolin influences cell proliferation remained elusive so far. We have generated conditional nucleolin-knockout cells using the chicken B lymphocyte cell line DT40. Our results indicate that nucleolin is absolutely required for the proliferation and for the survival of these cells. Depletion of nucleolin drastically inhibits rDNA (ribosomal DNA) transcription while only slightly affecting pre-rRNA processing. This inhibition is accompanied by modifications of the shape and the structure of the nucleolus. The analysis of mutants of nucleolin, which lack two or three RNA-binding domains, shows that these domains harbour redundant functions and that nucleolin's roles in transcription, rRNA maturation and nucleolar shape can be partially uncoupled. The function of nucleolin in ribosomal synthesis could account for its effect on cell division and survival, but this vital role does not seem to be linked to sequence-specific RNA binding.