Eap1p, a novel eukaryotic translation initiation factor 4E-associated protein in Saccharomyces cerevisiae

Ribosome binding to eukaryotic mRNA is a multistep process which is mediate d by the cap structure [m(7)G(5')ppp(5')N, where N is any nucleotide] prese nt at the 5' termini of all cellular (with the exception of organellar) mRN As. The heterotrimeric complex, eukaryotic initiation factor 4F (eIF4F), in teracts directly with the cap structure via the eIF4E subunit and functions to assemble a ribosomal initiation complex on the mRNA. In mammalian cells , eIF4E activity is regulated in part by three related translational repres sors (4E-BPs), which bind to eIF4E directly and preclude the assembly of eI F4F. No structural counterpart to 4E-BPs exists in the budding yeast, Sacch aromyces cerevisiae. However, a functional homolog (named p20) has been des cribed which blocks cap-dependent translation by a mechanism analogous to t hat of 4E-BPs. We report here on the characterization of a novel yeast eIF4 E-associated protein (Eap1p) which can also regulate translation through bi nding to eIF4E. Eap1p shares limited homology to p20 in a region which cont ains the canonical eIF4E-binding motif. Deletion of this domain or point mu tation abolishes the interaction of Eap1p with eIF4E. Eap1p competes with e IF4G (the large subunit of the cap-binding complex, eIF4F) and p20 for bind ing to eIF4E in vivo and inhibits cap-dependent translation in vitro. Targe ted disruption of the EAP1 gene results in a temperature-sensitive phenotyp e and also confers partial resistance to growth inhibition by rapamycin. Th ese data indicate that Eap1p plays a role in cell growth and implicates thi s protein in the TOR signaling cascade of S. cerevisiae.