The Importin-Alpha/Nucleophosmin Switch Controls Taspase1 Protease Function

Authors

  • Carolin Bier,

    1. Molecular and Cellular Oncology/MAINZ SCREENING CENTER (MSC), University Hospital of Mainz, Langenbeckstrasse 1, 55101 Mainz, Germany
    Search for more papers by this author
  • Shirley K. Knauer,

    Corresponding author
    1. Centre for Medical Biotechnology (ZMB), Institute for Molecular Biology, University Duisburg-Essen, Universitätsstraße, 45117 Essen, Germany
    Search for more papers by this author
  • Dominic Docter,

    1. Molecular and Cellular Oncology/MAINZ SCREENING CENTER (MSC), University Hospital of Mainz, Langenbeckstrasse 1, 55101 Mainz, Germany
    Search for more papers by this author
  • Günter Schneider,

    1. II. Medizinische Klinik, Technische Universität München, Ismaninger Strasse 22, 81675 München, Germany
    Search for more papers by this author
  • Oliver H. Krämer,

    1. Centre for Molecular Biomedicine (CMB), Institute for Biochemistry and Biophysics, Friedrich-Schiller-University Jena, Hans-Knöll-Str. 2, 07745 Jena, Germany
    Search for more papers by this author
  • Roland H. Stauber

    Corresponding author
    1. Molecular and Cellular Oncology/MAINZ SCREENING CENTER (MSC), University Hospital of Mainz, Langenbeckstrasse 1, 55101 Mainz, Germany
    Search for more papers by this author

Shirley K. Knauer, shirley.knauer@uni-due.de and Roland H. Stauber, rstauber@uni-mainz.de

Abstract

Taspase1 is a threonine protease suspected to process (patho)biologically relevant nuclear and cytoplasmic substrates, such as the mixed lineage leukemia protein. However, neither the mechanisms regulating Taspase1's intracellular localization nor their functional consequences are known. Analysis of endogenous and ectopically expressed Taspase1 detected the protease predominantly in the nucleus accumulating at the nucleolus. Microinjection and ectopic expression studies identified an evolutionarily conserved bipartite nuclear import signal (NLS) (amino acids 197KRNKRKLELA ERVDTDFMQLKKRR220) interacting with importin-α. Notably, an NLS-mutated, import-deficient Taspase1 was biologically inactive. Although the NLS conferred nuclear transport already of the proenzyme, Taspase1's nucleolar localization required its autoproteolytic processing, triggering its interaction with the nucleolar shuttle protein nucleophosmin. In contrast, (auto)catalytically inactive Taspase1 mutants neither accumulated at the nucleolus nor bound nucleophosmin. Active nuclear import and interaction with nucleophosmin was found to be required for the formation of proteolytically active Taspase1 ensuring to efficiently process its nuclear targets. Intriguingly, coexpression of pathological nucleophosmin variants increased the amount of cytoplasmic Taspase1. Hence, Taspase1 appears to exploit the nuclear export activity of nucleophosmin to gain transient access to the cytoplasm required to also cleave its cytoplasmic substrates. Collectively, we here describe a hitherto unknown mechanism regulating the biological activity of this protease.

Ancillary