TRANSLATION INITIATION-FACTORS EIF-ISO4G AND EIF-4B INTERACT WITH THEPOLY(A)-BINDING PROTEIN AND INCREASE ITS RNA-BINDING ACTIVITY

The 5'-cap and the poly(A) tail act synergistically to increase the tr anslational efficiency of eukaryotic mRNAs, which suggests that these two mRNA elements communicate during translation, We report here that the cap-associated eukaryotic initiation factors (eLFs), i.e, the two isoforms of the cap-binding complex (eIF-4F and eLF-iso4F) and eIF-4B, bind to the poly(A)-binding protein (PABP) both in the presence and a bsence of poly(A) RNA, The interactions between PABP and eIF-4F, eIF-i so4F, and eIF-4B were measured in the absence of poly(A) RNA using far Western analysis and confirmed by direct fluorescence titration studi es, The functional consequence of the interaction between these initia tion factors and PABP was examined using RNA binding assays and RNA mo bility shift analysis, eIF-4F, eIF-iso4F, and eIF-4B promoted PARR act ivity through a shift in its equilibrium affinity for poly(A), eIF-iso 4G, the large subunit of eIF-iso4F, was the subunit responsible for th e interaction between eLF-iso4F and PABP and was the subunit that prom oted PABP RNA binding activity, Truncation analysis of eIF-iso4G indic ated that a domain close to its N-terminal end appeared to be involved in binding PARR, These results suggest that the interaction between P ABP and eIF-4B and eIF-iso4G may be involved in mediating the function al co-dependence observed between the cap and the poly(A) tail during translation.