title: Gfi1 ubiquitination and proteasomal degradation is inhibited by the ubiquitin ligase Triad1.
abstract: Growth factor independence 1 (Gfi1) is a transcriptional repressor essential for the function and development of many different hematopoietic lineages. The Gfi1 protein expression is regulated by the ubiquitin-proteasome system. In granulocytes, Gfi1 is rapidly degraded by the proteasome, while it is more stable in monocytes. How the ubiquitination and degradation of Gfi1 is regulated is unclear. Here, we show that the ubiquitin ligase Triad1 interacts with the DNA-binding domain of Gfi1. Unexpectedly, we found that Triad1 inhibited Gfi1 ubiquitination, resulting in a prolonged half-life. Down-regulation of endogenous Triad1 by siRNAs resulted in increased Gfi1 ubiquitination. In U937 cells, Triad1 caused an increase in endogenous Gfi1 protein levels and slowed cell proliferation in a similar manner when Gfi1 itself was expressed. A Triad1 mutant that lacks the Gfi1-binding domain did not affect Gfi1 levels and proliferation. Because neither proteasome-ubiquitin nor Triad1 ubiquitin ligase activity was required for the inhibition of Gfi1 ubiquitination, these data suggest that Triad1 competes for Gfi1 binding with as yet to be identified E3 ubiquitin ligases that do mark Gfi1 for proteasomal degradation. The fine-tuning of Gfi1 protein levels regulated by Triad1 defines an unexpected role for this protein in hematopoiesis.
authors.affiliation: Central Hematology Laboratory, Radboud University Nijmegen Medical Centre, Nijmegen Centre for Molecular Life Sciences, 6500 HB Nijmegen, the Netherlands.
authors.forename: Jurgen A F
authors.initials: JA
authors.lastname: Marteijn
authors.forename: Laurens T
authors.initials: LT
authors.lastname: van der Meer
authors.forename: Liesbeth
authors.initials: L
authors.lastname: van Emst
authors.forename: Simon
authors.initials: S
authors.lastname: van Reijmersdal
authors.forename: Willemijn
authors.initials: W
authors.lastname: Wissink
authors.forename: Theo
authors.initials: T
authors.lastname: de Witte
authors.forename: Joop H
authors.initials: JH
authors.lastname: Jansen
authors.forename: Bert A
authors.initials: BA
authors.lastname: Van der Reijden
mesh_terms: D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D004268:DNA-Binding Proteins / Q000378:metabolism*; D015536:Down-Regulation; D006410:Hematopoiesis / Q000502:physiology; D006801:Humans; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D011485:Protein Binding; D011499:Protein Processing, Post-Translational; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism / Q000502:physiology*; D054875:Ubiquitination; D016335:Zinc Fingers
chemical_list: D004268:DNA-Binding Proteins; C498275:GFI1 protein, human; C111977:GFI1B protein, human; D011518:Proto-Oncogene Proteins; D012097:Repressor Proteins; D014157:Transcription Factors; C506696:ARIH2 protein, human; D044767:Ubiquitin-Protein Ligases; D046988:Proteasome Endopeptidase Complex