Identification of Glyceraldehyde-3-phosphate Dehydrogenase as a Cellular Protein that Binds to the Hepatitis B Virus Posttranscriptional Regulatory Element

The hepatitis B virus posttranscriptional regulatory element (PRE) is an RNAcis-element that is required for high-level expression of viral surface gene transcripts and appears to function by activating mRNA export to the cytoplasm. We have previously shown that multiple fragments of the PRE bind to two cellular proteins of approximately 35 and 55 kDa in molecular mass and that this binding correlates with function. By a combination of column chromatographic techniques and SDS-polyacrylamide gel electrophoresis, we have been able to purify the smaller protein. Amino-terminal sequencing of the purified protein shows identity to glyceraldehyde-3-phosphate dehydrogenase (GAPDH), an RNA-binding glycolytic enzyme that has been implicated in the export of tRNA. Immunoprecipitation analysis reveals that GAPDH is indeed present in the protein-RNA complex resulting from incubation of crude nuclear extracts with a functional region of the PRE. Furthermore, binding of the cellular 35 kDa protein to the PRE fragment is blocked by NAPDH, as would be expected for RNA binding by GAPDH. Finally, purified commercial GAPDH also binds specifically to this RNA fragment. Therefore, GAPDH is one of the cellular proteins that binds to the PRE, and may be involved in the posttranscriptional regulation of hepatitis B virus gene expression.