The N-terminal domain anchors human topoisomerase I at fibrillar centers of nucleoli and nucleolar organizer regions of mitotic chromosomes.
DNA topoisomerase I releases torsion stress created by DNA transcription. In principle, this activity is required in the nucleoplasm for mRNA synthesis and in the nucleoli for rRNA synthesis. Yet, topoisomerase I is mostly a nucleolar protein. Current belief holds that this preference is triggered by the N-terminal domain of the enzyme, which constitutes a nucleolar import signal. Contradicting this view, we show here that nucleolar accumulation of various fragments of topoisomerase I is correlated with their lesser mobility in this compartment and not with the N-terminal domain being intact or present. Therefore, the N-terminal domain is not likely a nucleolar import signal. We show that it rather serves as an adaptor that anchors a subpopulation of topoisomerase I at fibrillar centers of nucleoli and nucleolar organizer regions of mitotic chromosomes. Thus, it provides a steady association of topoisomerase I with the rDNA and with RNA polymerase I, which is maintained in a living cell during the entire cell cycle.
Department of Clinical Chemistry, Medizinische Poliklinik, University of Würzburg, Klinikstrasse 6-8, D-97070 Würzburg, Germany.
Morten O
MO
Christensen
Hans U
HU
Barthelmes
Fritz
F
Boege
Christian
C
Mielke
D015153:Blotting, Western; D002384:Catalysis; D002460:Cell Line; D002466:Cell Nucleolus / Q000648:ultrastructure*; D002467:Cell Nucleus / Q000378:metabolism; D002843:Chromatin / Q000378:metabolism; D002875:Chromosomes / Q000648:ultrastructure*; D004264:DNA Topoisomerases, Type I / Q000737:chemistry* / Q000378:metabolism*; D004275:DNA, Ribosomal / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D049452:Green Fluorescent Proteins; D006801:Humans; D007150:Immunohistochemistry; D008164:Luminescent Proteins / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008938:Mitosis; D009697:Nucleolus Organizer Region / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D012333:RNA, Messenger / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013696:Temperature; D013997:Time Factors
D002843:Chromatin; D004275:DNA, Ribosomal; D008164:Luminescent Proteins; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D004264:DNA Topoisomerases, Type I