INTERNAL TRANSLATION INITIATION ON POLIOVIRUS RNA - FURTHER CHARACTERIZATION OF LA FUNCTION IN POLIOVIRUS TRANSLATION IN-VITRO

Initiation of poliovirus RNA translation by internal entry of ribosome s is believed to require the participation of h ans-acting factors. Th e mechanism of action of these factors is poorly defined. The limiting amount of one of these factors, La protein, in rabbit reticulocyte ly sates (RRL) has been postulated to partially explain the inefficient t ranslation of poliovirus RNA in this system. To further characterize L a activity in translation and to identify other potential limiting fac tors, we assayed the ability of La protein as well as purified initiat ion factors, eIF-2, guanine nucleotide exchange factor (GEF), eIF-4A, eIF-4B, eIF-4F, and eIF-3, to stimulate the synthesis of P1, the capsi d precursor protein, in poliovirus type 1 (Mahoney) RNA-programmed RRL . Of the proteins tested, only La, GEF, and to some extent eIF-2 stimu lated the synthesis of p1. The enhanced translation of P1 in response to La occurred concomitantly with the inhibition of synthesis of most aberrant polypeptides, resulting from initiation in the middle of the genome. Deletion of the carboxy-terminal half (214 amino acids) of La did not decrease its binding to the poliovirus 5' untranslated region but abrogated the stimulatory and correcting activity in translation. In contrast to La, GEF and eIF-2 stimulated the overall translation an d increased the synthesis of aberrant products as well as P1. Neither La, GEF, nor any other factor stimulated translation of encephalomyoca rditis virus RNA in RRL. The implications of these findings for the me chanism of internal translation initiation on picornavirus RNAs are di scussed.