title: Accumulation of c-Myc and proteasomes at the nucleoli of cells containing elevated c-Myc protein levels.
abstract: c-Myc is a predominantly nuclear transcription factor that is a substrate for rapid turnover by the proteasome system. Cancer-related mutations in c-Myc lead to defects in its degradation and thereby contribute to the increase in its cellular level that is associated with the disease. Little is known about the mechanisms that target c-Myc to the proteasomes. By using a GFP fusion protein and live analysis we show that c-Myc shuttles between the nucleus and cytoplasm and thus it could be degraded in either compartment. Strikingly, at elevated levels of expression c-Myc accumulates at nucleoli in some cells, consistent with saturation of a nucleolus-associated degradation system in these cells. This idea is further supported by the observation that proteasome inhibitor treatment causes accumulation of c-Myc at the nucleoli of essentially all cells. Under these conditions c-Myc is relatively stably associated with the nucleolus, as would be expected if the nucleolus functions as a sequestration/degradation site for excess c-Myc. Furthermore, during elevated c-Myc expression or proteasome inhibition, nucleoli that are associated with c-Myc also accumulate proteasomes. c-Myc and proteasomes co-localise in intranucleolar regions distinct from the dense fibrillar component of the nucleolus. Based on these results we propose a model for c-Myc downregulation where c-Myc is sequestered at the nucleoli. Sequestration of c-Myc is accompanied by recruitment of proteasomes and may lead to subsequent degradation.
authors.affiliation: Natural Sciences Section, Södertörns University College, S-141 89 Huddinge, Sweden. azadeh.arabi@sh.se
authors.forename: Azadeh
authors.initials: A
authors.lastname: Arabi
authors.forename: Cecilia
authors.initials: C
authors.lastname: Rustum
authors.forename: Einar
authors.initials: E
authors.lastname: Hallberg
authors.forename: Anthony P H
authors.initials: AP
authors.lastname: Wright
mesh_terms: D021581:Active Transport, Cell Nucleus; D000818:Animals; D019556:COS Cells; D002466:Cell Nucleolus / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D006801:Humans; D008954:Models, Biological; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011480:Protease Inhibitors / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013347:Subcellular Fractions / Q000378:metabolism
chemical_list: D009097:Multienzyme Complexes; D011480:Protease Inhibitors; D016271:Proto-Oncogene Proteins c-myc; D011993:Recombinant Fusion Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex