title: Subcellular distribution of nuclear import-defective isoforms of the promyelocytic leukemia protein.
abstract: BACKGROUND
  The promyelocytic leukemia (PML) protein participates in a number of cellular processes, including transcription regulation, apoptosis, differentiation, virus defense and genome maintenance. This protein is structurally organized into a tripartite motif (TRIM) at its N-terminus, a nuclear localization signal (NLS) at its central region and a C-terminus that varies between alternatively spliced isoforms. Most PML splice variants target the nucleus where they define sub-nuclear compartments termed PML nuclear bodies (PML NBs). However, PML variants that lack the NLS are also expressed, suggesting the existence of PML isoforms with cytoplasmic functions. In the present study we expressed PML isoforms with a mutated NLS in U2OS cells to identify potential cytoplasmic compartments targeted by this protein.
  
  
  RESULTS
  Expression of NLS mutated PML isoforms in U2OS cells revealed that PML I targets early endosomes, PML II targets the inner nuclear membrane (partially due to an extra NLS at its C-terminus), and PML III, IV and V target late endosomes/lysosomes. Clustering of PML at all of these subcellular locations depended on a functional TRIM domain.
  
  
  CONCLUSIONS
  This study demonstrates the capacity of PML to form macromolecular protein assemblies at several different subcellular sites. Further, it emphasizes a role of the variable C-terminus in subcellular target selection and a general role of the N-terminal TRIM domain in promoting protein clustering.
authors.affiliation: Department of Biomedicine, University of Bergen, Jonas Lies Vei 91, 5009 Bergen, Norway.
authors.forename: Asne
authors.initials: A
authors.lastname: Jul-Larsen
authors.forename: Amra
authors.initials: A
authors.lastname: Grudic
authors.forename: Rolf
authors.initials: R
authors.lastname: Bjerkvig
authors.forename: Stig O
authors.initials: SO
authors.lastname: Bøe
mesh_terms: D021581:Active Transport, Cell Nucleus; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism*; D011992:Endosomes / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D008247:Lysosomes / Q000378:metabolism; D009154:Mutation; D009685:Nuclear Envelope / Q000378:metabolism; D009687:Nuclear Proteins / Q000032:analysis* / Q000235:genetics*; D000071156:Promyelocytic Leukemia Protein; D020033:Protein Isoforms / Q000032:analysis / Q000235:genetics; D014157:Transcription Factors / Q000032:analysis* / Q000235:genetics*; D025521:Tumor Suppressor Proteins / Q000032:analysis* / Q000235:genetics*
chemical_list: D009687:Nuclear Proteins; D000071156:Promyelocytic Leukemia Protein; D020033:Protein Isoforms; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; C070576:PML protein, human