Ultrastructural changes in theSchizosaccharomyces pombe nucleolus following the disruption of thegar2+gene, which encodes a nucleolar protein structurally related to nucleolin

Abstract

The nucleolar protein gar2, from the fission yeast Schizosaccharomyces pombe, is the functional homolog of NSR1 from Saccharomyces cerevisiae, and is structurally related to nucleolin from vertebrates. By immunocytochemistry at the electron microscope level, we show that gar2 co-localizes with RNA polymerase I and the gar1 protein along the dense fibrillar component of the nucleolus in a wild-type strain of S. pombe, suggesting that gar2 is involved in the transcription and/or in the early steps of maturation of the ribosomal RNAs. Since the effects of disruption of the gar2+ gene might also shed light on the role of the gar2 protein, we analyzed the ultrastructure of the nucleolus of a gar2-disruption mutant. The nucleolus of the gar2- mutant is dramatically reorganized when compared with that of the wild-type gar2+ strain: a truncated protein containing the NH2-terminus of the gar2 protein is accumulated in an unusual nucleolar "dense body". Our results also suggest that the NH2-terminus might be sufficient for nucleolar localization via interaction with specific nucleolar components and support the hypothesis that gar2 in wild-type S. pombe interacts with nascent pre-rRNA via its two RNA-binding domains in combination with the glycine/arginine-rich domain. We also report that disruption of the gar2+ gene results in a mutant that is defective in cytokinesis and nuclear division.

The Homologous Putative GTPases Grn1p from Fission Yeast and the Human GNL3L Are Required for Growth and Play a Role in Processing of Nucleolar Pre-rRNA

“…As depicted in Figure 6C, mammalian nucleoli contain fibrillar centers (FC) known to house rDNA genes, surrounded by a layer called the dense fibrillar component (DFC) in which the maturation of pre-rRNA transcripts is said to take place, which in-turn is surrounded by a granular component (GC) wherein the assembly of preribosomes takes place (Carmo-Fonseca et al, 2000). Similar morphological nucleolar subcompartments are found in S. pombe (Leger-Silvestre et al, 1997) and S. cerevisiae (Trumtel et al, 2000). Though the presence of FC and DFC in yeast is currently a debated issue, the existence of a granular zone (GC?)…”

Antiproliferative Activity of G-rich Oligonucleotides Correlates with Protein Binding

“…Another emerging role for nucleolin is as a shuttle protein that transports viral and cellular proteins between the cytoplasm and nucleus/ nucleolus of the cell (41)(42)(43). Nucleolin is also implicated, directly or indirectly, in other roles including nuclear matrix structure (44), cytokinesis, and nuclear division (45) and as an RNA and DNA helicase (46). Its multifunctional nature is reflected in its multidomain structure, consisting of a histonelike N terminus, a central domain containing RNA recognition motifs, and a glycine-and arginine-rich C terminus (47).…”

Association of Yeast RNA Polymerase I with a Nucleolar Substructure Active in Rrna Synthesis and Processing

“…Interestingly, components involved in processing of rRNA, like the small nucleolar RNAs (snoRNAs) U14, U3, and MRP, or the protein Gar1p, were also shown to localize to the dense fibrillar component (Lazdins et al 1997; Leger-Silvestre et al 1997), indicating that processing activities are in close proximity to the transcription apparatus. Furthermore, the nucleolar protein fibrillarin colocalizes with pol I in mouse early embryos (Cuadros-Fernandez and Esponada 1996), but also with nascent rRNA transcripts (Garcia Blanco et al 1995).…”