A novel protein localized to the fibrillar compartment of the nucleolus and to the brush border of a secretory cell

Abstract

We report the identification and molecular characterization of a novel abundant nucleolar protein of the dipteran Chironomus tentans. As shown by Western blot analysis, this protein is present in nuclear extracts in a phosphorylated form with a mobility corresponding to 100 kDa. Therefore, the protein has been termed Chironomus tentans p100, or p100 for short. Analysis of the cDNA-derived primary structure of p100 indicates a protein that contains a combination of structural domains which could be involved in interactions with proteins and nucleic acids: twelve alternating acidic and basic repeats, a glycine-arginine-rich domain and a region with two zinc fingers of the C4-type. Acidic and basic repeats are typical for a group of nonribosomal nucleolar proteins. The best-studied representatives of this group are Nopp140 and nucleolin, proteins with structural and regulatory functions in rDNA transcription. Immunocytology and immunoelectron microscopy of Chironomus tentans salivary gland cells have shown that the p100 protein is located in the fibrillar compartment of the nucleolus, while it is almost absent from the granular compartment and from the nucleoplasm. The p100 protein remains in the nucleolus after removal of RNA and DNA by digestion with nucleases. This indicates that p100 might be a constituent of the nucleolar proteinaceous framework. Remarkably, p100 is also localized in the brush border in the apical part of the salivary gland cell. The presence of p100 both in the nucleolus and at the apical plasma membrane suggests that it could be involved in coordination of the level of protein production and export from the cell through regulation of the level of rRNA production in the nucleolus.