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Masking of a nuclear signal motif by monoubiquitination leads to mislocalization and degradation of the regulatory enzyme cytidylyltransferase.

Monoubiquitination aids in the nuclear export and entrance of proteins into the lysosomal degradative pathway, although the mechanisms are unknown. Cytidylyltransferase (CCTalpha) is a proteolytically sensitive lipogenic enzyme containing an NH(2)-terminal nuclear localization signal (NLS). We show here that CCTalpha is monoubiquitinated at a molecular site (K(57)) juxtaposed near its NLS, resulting in disruption of its interaction with importin-alpha, nuclear exclusion, and subsequent degradation within the lysosome. Cellular expression of a CCTalpha-ubiquitin fusion protein that mimics the monoubiquitinated enzyme resulted in cytoplasmic retention. A CCTalpha K(57R) mutant exhibited an extended half-life, was retained in the nucleus, and displayed proteolytic resistance. Importantly, by using CCTalpha-ubiquitin hybrid constructs that vary in the intermolecular distance between ubiquitin and the NLS, we show that CCTalpha monoubiquitination masks its NLS, resulting in cytoplasmic retention. These results unravel a unique molecular mechanism whereby monoubiquitination governs the trafficking and life span of a critical regulatory enzyme in vivo.

Pubmed ID: 19332566


  • Chen BB
  • Mallampalli RK


Molecular and cellular biology

Publication Data

June 13, 2009

Associated Grants

  • Agency: NHLBI NIH HHS, Id: R01 HL068135
  • Agency: NHLBI NIH HHS, Id: R01 HL081784
  • Agency: NHLBI NIH HHS, Id: R01 HL097376

Mesh Terms

  • Amino Acid Motifs
  • Amino Acid Sequence
  • Animals
  • Cell Nucleus
  • Choline-Phosphate Cytidylyltransferase
  • Endosomes
  • Enzyme Stability
  • Half-Life
  • Lysine
  • Lysosomes
  • Mice
  • Molecular Sequence Data
  • Mutant Proteins
  • Nuclear Localization Signals
  • Protein Binding
  • Protein Processing, Post-Translational
  • Protein Transport
  • Rats
  • Structure-Activity Relationship
  • Tumor Necrosis Factor-alpha
  • Ubiquitination
  • alpha Karyopherins