A newly identified N-terminal amino acid sequence of human eIF4G binds poly(A)-binding protein and functions in poly(A)-dependent translation

Most eukaryotic mRNAs possess a 5' cap and a 3' poly(A) tail, both of which are required for efficient translation. In yeast and plants, binding of eI F4G to poly(A)-binding protein (PABP) was implicated in poly(A)-dependent t ranslation, In mammals, however, there has been no evidence that eIF4G bind s PABP, Using 5' rapid amplification of cDNA, we have extended the known hu man eIF4GI open reading frame from the N-terminus by 156 amino acids. Co-im munoprecipitation experiments showed that the extended eIF4GI binds PABP, w hile the N-terminally truncated original eIF4GI cannot. Deletion analysis i dentified a 29 amino acid sequence in the new N-terminal region as the PABP -binding site. The 29 amino acid stretch is almost identical in eIF4GI and eIF4GII, and the full-length eIF4GII also binds PABP, As previously shown f or yeast, human eIF4G binds to a fragment composed of RRM1 and RRM2 of PABP , In an in vitro translation system, an N-terminal fragment which includes the PABP-binding site inhibits poly(A)-dependent translation, but has no ef fect on translation of a deadenylated mRNA, These results indicate that, in addition to a recently identified mammalian PABP-binding protein, PAIP-1, eIF4G binds PABP and probably functions in poly(A)-dependent translation in mammalian cells.