Ubiquitin-independent degradation of hepatitis C virus F protein

J Virol. 2009 Jan;83(2):612-21. doi: 10.1128/JVI.00832-08. Epub 2008 Oct 29.


Hepatitis C virus (HCV) F protein is encoded by the +1 reading frame of the viral genome. It overlaps with the core protein coding sequence, and multiple mechanisms for its expression have been proposed. The full-length F protein that is synthesized by translational ribosomal frameshift at codons 9 to 11 of the core protein sequence is a labile protein. By using a combination of genetic, biochemical, and cell biological approaches, we demonstrate that this HCV F protein can bind to the proteasome subunit protein alpha3, which reduces the F-protein level in cells in a dose-dependent manner. Deletion-mapping analysis identified amino acids 40 to 60 of the F protein as the alpha3-binding domain. This alpha3-binding domain of the F protein together with its upstream sequence could significantly destabilize the green fluorescent protein, an otherwise stable protein. Further analyses using an F-protein mutant lacking lysine and a cell line that contained a temperature-sensitive E1 ubiquitin-activating enzyme indicated that the degradation of the F protein was ubiquitin independent. Based on these observations as well as the observation that the F protein could be degraded directly by the 20S proteasome in vitro, we propose that the full-length HCV F protein as well as the F protein initiating from codon 26 is degraded by an ubiquitin-independent pathway that is mediated by the proteasome subunit alpha3. The ability of the F protein to bind to alpha3 raises the possibility that the HCV F protein may regulate protein degradation in cells.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Artificial Gene Fusion
  • Cell Line
  • Genes, Reporter
  • Green Fluorescent Proteins / genetics
  • Green Fluorescent Proteins / metabolism
  • Humans
  • Mice
  • Proteasome Endopeptidase Complex / metabolism
  • Protein Binding
  • Protein Interaction Domains and Motifs
  • Protein Interaction Mapping
  • Recombinant Fusion Proteins
  • Sequence Deletion
  • Two-Hybrid System Techniques
  • Ubiquitin / physiology
  • Viral Core Proteins / genetics
  • Viral Core Proteins / metabolism*


  • Recombinant Fusion Proteins
  • Ubiquitin
  • Viral Core Proteins
  • hepatitis C protein F, Hepatitis C virus
  • Green Fluorescent Proteins
  • PSMA3 protein, human
  • Proteasome Endopeptidase Complex