The function of Nopp140 in nucle(ol)ar organization and inherited disease

Abstract

Little is known about regulation of intranuclear transport and its co-ordination with other metabolic processes in the cell. We use Nopp140 as a model for understanding these processes. It is a resident protein of the nucleolus and coiled bodies thought to shuttle between the nucleolus and the cytoplasm on tracks observed by immunoelectron microscopy and proposed to be a chaperone in the transport of pre-ribosomal particles.;We determined that it is a relatively fast, actively shuttling protein which moves between the nucleolus, coiled bodies and the cytoplasm. Investigating a dominant negative construct of Nopp140, we show that it associates with H/ACA and C/D box snoRNPs which are involved in processing and modification of ribosomal RNA. They are the putative cargo of Nopp140. Our data suggest that Nopp140 may be a chaperone for snoRNps, involved in their nuclear transport or biosynthesis.;The exogenous expression of the repeat domain of Noppl40 caused the formation of unique concentric rings of endoplasmic reticulum within the nucleus. They resemble the nucleolar channel system of human endometrium during the secretory phase of the menstrual cycle. Differential expression of Nopp140 in the endometrium may thus induce the nucleolar channel system. Major nuclear disturbances caused by the expression of either the carboxyl or the repeat domain of Nopp140 suggest a function in maintaining nuclear organization.;Treacher Collins syndrome (TCS) is a disorder of craniofacial development caused by mutations in the gene TCOF1. Its gene product, treacle, contains a repeat domain related to Nopp140. We established treacle to be a highly phosphorylated nucleolar protein like Nopp140, with probably overlapping but distinct functions. Quantitation of treacle by indirect immunofluorescence and in western blots demonstrated that the amount of treacle varies less than two-fold among fibroblasts and lymphoblasts of patient and control individuals, and independent of the phenotype. Therefore cells of TCS patients possess a mechanism to maintain wild type levels of full-length treacle from a single allele. Thus TCS may be explained by haploinsufficiency or dominant negative effects occurring only in specific embryonic structures during development.