Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), a cellular enzyme involved in glycolysis, binds specifically to several viral RNAs, but the functional significance of this interaction is uncertain. Both GAPDH and polypyrimidine tract binding protein (PTB) bind to overlapping sites in stem-loop IIIa of the internal ribosome entry site (IRES) of Hepatitis A virus (HAV), a picornavirus. Since the binding of GAPDH destabilizes the RNA secondary structure, we reasoned that GAPDH may suppress the ability of the IRES to direct cap-independent translation, making its effects antagonistic to the translation-enhancing activity of PTB (D. E. Schultz, C. C. Hardin, and S. M. Lemon, J. Biol. Chem. 271:14134-14142, 1996). To test this hypothesis, we constructed plasmids containing a dicistronic transcriptional unit in which the HAV IRES was placed between an upstream GAPDH-coding sequence and a downstream Renilla luciferase (RLuc) sequence. Transfection with this plasmid results in overexpression of GAPDH and in RLuc production as a measure of IRES activity. RLuc activity was compared with that from a control, null-expression plasmid that was identical except for a frameshift mutation within the 5 GAPDH coding sequence. In transfection experiments, GAPDH overexpression significantly suppressed HAV IRES activity in BSC-1 and FRhK-4 cells but not in Huh-7 cells, which have a significantly greater cytoplasmic abundance of PTB. GAPDH suppression of HAV translation was greater with the wild-type HAV IRES than with the IRES from a cell culture-adapted virus (HM175/P16) that has reproducibly higher basal translational activity in BSC-1 cells. Stem-loop IIIa RNA from the latter IRES had significantly lower affinity for GAPDH in filter binding experiments. Thus, the binding of GAPDH to the IRES of HAV suppresses cap-independent viral translation in vivo in African green monkey kidney cells. The enhanced replication capacity of cell culture-adapted HAV in such cells may be due in part to reduced affinity of the viral IRES for GAPDH.Hepatitis A virus (HAV) is a plus-strand RNA virus that is classified within the genus Hepatovirus of the family Picornaviridae. The genomic RNA of HAV contains a single long open reading frame encoding a polyprotein which is proteolytically processed by a virus-encoded proteinase (27). Like other picornavirus RNAs, it has a relatively lengthy 5Ј-terminal nontranslated RNA segment (5ЈNTR) and a shorter 3ЈNTR with a 3Ј terminal poly(A) tail. As in all picornaviruses, the 5ЈNTR of HAV forms a highly ordered RNA structural element (the internal ribosome entry site [IRES]) that is responsible for directing the cap-independent translation of the polyprotein (5,23,32). This occurs by a unique mechanism that involves internal entry of the ribosome on the RNA hundreds of bases downstream of the 5Ј end and is absolutely dependent on the secondary and probably the tertiary structure of the 5ЈNTR. Although HAV can be propagated in conventional cell culture systems, the replication of wild-type (wt) virus is slow and ...