PYRIMIDINE TRACT BINDING-PROTEIN STRONGLY STIMULATES IN-VITRO ENCEPHALOMYOCARDITIS VIRUS-RNA TRANSLATION AT THE LEVEL OF PREINITIATION COMPLEX-FORMATION

Cellular protein p57/58, now known to be identical to polypyrimidine t ract binding protein (PTB), has earlier been shown to specifically bin d to the internal ribosome entry sites (IRES) of encephalomyocarditis virus (EMCV) and some other picornaviral RNAs. To elucidate its releva nce to the internal initiation, the effect of cloned purified PTB on E MCV IRES directed translation was studied in cytoplasmic extracts of K rebs-2 ascites carcinoma cells partially depleted of endogenous PTB. A ddition of PTB to such extracts resulted in a strong stimulation of tr anslation of a beta-glucuronidase (GUS) reporter cistron fused to the EMCV IRES, but had no effect on translation of capped mRNAs, such as b eta-globin, and tobacco mosaic virus (TMV) RNAs. PTB was found to exer t its effect at the level of 48S pre-initiation complex formation.