title: Ubiquitin-specific protease 4 inhibits mono-ubiquitination of the master growth factor signaling kinase PDK1.
abstract: BACKGROUND
  Phosphorylation by the phospho-inositide-dependent kinase 1 (PDK1) is essential for many growth factor-activated kinases and thus plays a critical role in various processes such as cell proliferation and metabolism. However, the mechanisms that control PDK1 have not been fully explored and this is of great importance as interfering with PDK1 signaling may be useful to treat diseases, including cancer and diabetes.
  
  
  RESULTS
  In human cells, few mono-ubiquitinated proteins have been described but in all cases this post-translational modification has a key regulatory function. Unexpectedly, we find that PDK1 is mono-ubiquitinated in a variety of human cell lines, indicating that PDK1 ubiquitination is a common and regulated process. Ubiquitination occurs in the kinase domain of PDK1 yet is independent of its kinase activity. By screening a library of ubiquitin proteases, we further identify the Ubiquitin-Specific Protease 4 (USP4) as an enzyme that removes ubiquitin from PDK1 in vivo and in vitro and co-localizes with PDK1 at the plasma membrane when the two proteins are overexpressed, indicating direct deubiquitination.
  
  
  CONCLUSIONS
  The regulated mono-ubiquitination of PDK1 provides an unanticipated layer of complexity in this central signaling network and offers potential novel avenues for drug discovery.
authors.affiliation: CeMM - Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna, Austria.
authors.forename: Iris Z
authors.initials: IZ
authors.lastname: Uras
authors.forename: Thomas
authors.initials: T
authors.lastname: List
authors.forename: Sebastian M B
authors.initials: SM
authors.lastname: Nijman
mesh_terms: D064413:3-Phosphoinositide-Dependent Protein Kinases; D002460:Cell Line; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D015398:Signal Transduction; D043222:Ubiquitin Thiolesterase / Q000378:metabolism / Q000502:physiology*; D064570:Ubiquitin-Specific Proteases; D054875:Ubiquitination
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D008565:Membrane Proteins; C486203:USP4 protein, human; D064413:3-Phosphoinositide-Dependent Protein Kinases; C578562:PDPK1 protein, human; D017346:Protein Serine-Threonine Kinases; D043222:Ubiquitin Thiolesterase; D064570:Ubiquitin-Specific Proteases