Expression studies of the HCV-1a core+1 open reading frame in mammalian cells

The hepatitis C virus (HCV) genome possesses an open reading frame overlapping the core gene in the +1 frame (core+1 ORF). Initial studies, mainly in rabbit reticulocyte lysates, indicated that the HCV-1 core+1 ORF is expressed by a -2/+1 frameshift at codons 8-11 during translation elongation of the viral polyprotein, resulting in a protein known as alternative reading frame protein (ARFP), frameshift (F), or core+1. However, subsequent investigation, based on reporter constructs carrying portions of the core+1 ORF, suggested the function of alternative mechanisms for core+1 expression in mammalian cells, including translation initiation from internal codons 85/87 or 26. Because results from these studies have been variable, we sought to re-evaluate expression of the core+1 ORF using constructs carrying the complete core+1 coding sequence fused to GFP or LUC. We showed here that codons 85/87 serve as the predominant initiation sites for internal translation initiation of core+1 ORF in Huh-7 and Huh-7/T7 mammalian cells, which support nuclear or cytoplasmic transcription, respectively. We also showed that internal translation initiation can occur concomitantly with the expression of the core+1/F protein that is produced artificially in Huh-7 or naturally in Huh-7/T7 cells. Furthermore, translation of core+1 ORF is not significantly affected by the presence of the HCV IRES element. The core+1/S-GFP protein is cytoplasmic and exhibits an ER distribution similar to that of the core+1/F-GFP protein.