RECOGNITION OF PICORNAVIRUS INTERNAL RIBOSOME ENTRY SITES WITHIN CELLS - INFLUENCE OF CELLULAR AND VIRAL-PROTEINS

The ability of different picornavirus internal ribosome entry site (IR ES) elements to direct initiation of protein synthesis has been assaye d in different cell lines in the presence and absence of viral proteas es that inhibit cap-dependent protein synthesis. Reporter plasmids tha t express dicistronic mRNAs, containing different IRES elements, with the general structure CAT/IRES/LUC, have been assayed. In each plasmid , the CAT sequence encodes chloramphenicol acetyl transferase and the LUC sequence encodes luciferase. The poliovirus (PV) 2A protease and t he foot-and-mouth disease virus (FMDV) Lb protease induce the cleavage of the translation initiation factor elF4G and hence inhibit the acti vity of the cap-binding complex, elF4F. In human osteosarcoma (HTK-143 ) cells, each of the various IRES elements functioned efficiently. In these cells, the co-expression of the viral proteases severely inhibit ed the expression of CAT, but the proteases had little effect on the a ctivities of the various IRES elements. In contrast, in baby hamster k idney (BHK) cells, the efficiencies of the different IRES elements var ied significantly, whereas, in normal rat kidney (NRK) cells, each of the IRES elements was relatively inefficient. In both BHK and NRK cell s, the activities of those IRES elements that functioned inefficiently were strongly stimulated by the co-expression of the PV 2A or FMDV Lb proteases. This stimulation was independent of the loss of cap-depend ent protein synthesis and was not achieved by the co-expression of the C-terminal fragment of elF4G. The results suggest that the PV 2A and FMDV Lb proteases induce the cleavage of another cellular protein, in addition to elF4G, which influences IRES function.