Nucleolus

About: Nucleolus is a research topic. Over the lifetime, 5873 publications have been published within this topic receiving 232435 citations. The topic is also known as: GO:0005730 & cell nucleolus.

Interaction of a plant virus-encoded protein with the major nucleolar protein fibrillarin is required for systemic virus infection

Abstract: The nucleolus and specific nucleolar proteins are involved in the life cycles of some plant and animal viruses, but the functions of these proteins and of nucleolar trafficking in virus infections are largely unknown. The ORF3 protein of the plant virus, groundnut rosette virus (an umbravirus), has been shown to cycle through the nucleus, passing through Cajal bodies to the nucleolus and then exiting back into the cytoplasm. This journey is absolutely required for the formation of viral ribonucleoprotein particles (RNPs) that, themselves, are essential for the spread of the virus to noninoculated leaves of the shoot tip. Here, we show that these processes rely on the interaction of the ORF3 protein with fibrillarin, a major nucleolar protein. Silencing of the fibrillarin gene prevents long-distance movement of groundnut rosette virus but does not affect viral replication or cell-to-cell movement. Repressing fibrillarin production also localizes the ORF3 protein to multiple Cajal body-like aggregates that fail to fuse with the nucleolus. Umbraviral ORF3 protein and fibrillarin interact in vitro and, when mixed with umbravirus RNA, form an RNP complex. This complex has a filamentous structure with some regular helical features, resembling the RNP complex formed in vivo during umbravirus infection. The filaments formed in vitro are infectious when inoculated to plants, and their infectivity is resistant to RNase. These results demonstrate previously undescribed functions for fibrillarin as an essential component of translocatable viral RNPs and may have implications for other plant and animal viruses that interact with the nucleolus.

A nuclear localization signal binding protein in the nucleolus.

Abstract: We used functional wild-type and mutant synthetic nuclear localization signal peptides of SV-40 T antigen cross-linked to human serum albumin (peptide conjugates) to assay their binding to proteins of rat liver nuclei on Western blots. Proteins of 140 and 55 kD (p140 and p55) were exclusively recognized by wild-type peptide conjugates. Free wild-type peptides competed for the wild-type peptide conjugate binding to p140 and p55 whereas free mutant peptides, which differed by a single amino acid from the wild type, competed less efficiently. The two proteins were extractable from nuclei by either low or high ionic strength buffers. We purified p140 and raised polyclonal antibodies in chicken against the protein excised from polyacrylamide gels. The anti-p140 antibodies were monospecific as judged by their reactivity with a single nuclear protein band of 140 kD on Western blots of subcellular fractions of whole cells. Indirect immunofluorescence microscopy on fixed and permeabilized Buffalo rat liver (BRL) cells with anti-p140 antibodies exhibited a distinct punctate nucleolar staining. Rhodamine-labeled wild-type peptide conjugates also bound to nucleoli in a similar pattern on fixed and permeabilized BRL cells. Based on biochemical characterization, p140 is a novel nucleolar protein. It is possible that p140 shuttles between the nucleolus and the cytoplasm and functions as a nuclear import carrier.

Human autoantibody to RNA polymerase I transcription factor hUBF. Molecular identity of nucleolus organizer region autoantigen NOR-90 and ribosomal RNA transcription upstream binding factor.

Abstract: In dividing eukaryotic cells, nucleoli disperse before mitosis and reform in daughter cells at sites of ribosomal RNA (rRNA) gene clusters that are at the secondary constrictions of chromosomes, called nucleolus organizer regions (NORs). In this study, cDNA clones for a NOR autoantigen (NOR-90) were selected using a specific human autoantibody probe and were subsequently identified to encode an alternative form of the reported human upstream binding factor (hUBF). Results from immunoprecipitation showed that anti-NOR-90 antibodies recognized both forms of hUBF/NOR-90. Our data therefore showed that UBF, a critical factor in the regulation of rRNA transcription, was tightly bound to NOR during mitosis even when rRNA synthesis was thought to be minimal. Furthermore, we identified a nucleolar transcription factor as a novel target for human autoimmune response.