A putative function of the nucleolus in the assembly or maturation of specialized messenger ribonucleoprotein complexes.

The nucleolus is a nuclear subcompartment with a well-defined function in ribosomal RNA transcription and assembly of ribosomal subunits. However, the nucleolus is multifunctional and involved in processes as diverse as regulation of mitosis, cell proliferation and viral infection. There is increasing evidence that, in addition to ribosomes, several different ribonucleoprotein (RNP) particles assemble or mature in the nucleolus, including the signal recognition particle (SRP) or viral RNPs. Here we discuss recent findings from mammalian and yeast cells that specific localized mRNAs or proteins that associate with localized mRNAs can accumulate in the nucleolus under particular conditions. Experimental evidence derived from studies on the nucleolar accumulation of yeast ASH1 mRNA and of cytoplasmic retention of its binding protein She2 are integrated into a hypothetical model that suggests a novel role of the nucleolus in the assembly of specific mRNPs.