THE TRANSLATION INITIATION FACTOR-EIF-4B CONTAINS AN RNA-BINDING REGION THAT IS DISTINCT AND INDEPENDENT FROM ITS RIBONUCLEOPROTEIN CONSENSUS SEQUENCE

eIF-4B is a eukaryotic translation initiation factor that is required for the binding of ribosomes to mRNAs and the stimulation of the helic ase activity of eIF-4A. It is an RNA-binding protein that contains a r ibonucleoprotein consensus sequence (RNP-CS)/RNA recognition motif (RR M). We examined the effects of deletions and point mutations on the ab ility of eIF-4B to bind a random RNA, to cooperate with eIF-4A in RNA binding, and to enhance the helicase activity of eIF-4A. We report her e that the RNP-CS/RRM alone is not sufficient for eIF-4B binding to RN A and that an RNA-binding region, located between amino acids 367 and 423, is the major contributor to RNA binding. Deletions which remove t his region abolish the ability of eIF-4B to cooperate with eIF-4A in R NA binding and the ability to stimulate the helicase activity of eIF-4 A. Point mutations in the RNP-CS/RRM had no effect on the ability of e IF-4B to cooperate with eIF-4A in RNA binding but significantly reduce d the stimulation of eIF-4A helicase activity. Our results indicate th at the carboxy-terminal RNA-binding region of eIF-4B is essential for eIF-4B function and is distinct from the RNP-CS/RRM.