Biochemical Evidence Supporting a Mechanism for Cap-independent and Internal Initiation of Eukaryotic MRNA

The role of eukaryotic initiation factor (eIF)4B in translation is somewhat uncertain, although it appears to stimulate a variety of activities of eIF-4A and eIF-4F. Using the model RNA-dependent ATP hydrolysis assay, the ability of eIF-4B to stimulate eIF-4A and eIF-4F was investigated. The most dramatic effect of eIF-4B is to increase the affinity of eIF-4A for RNA; no effect is seen on the affinity of eIF-4A for ATP. This is not the case for eIF-4F where stimulation occurs primarily through an increase in Vmax and not a change in the affinity for RNA. The finding that eIF-4A and eIF-4B can bind to an mRNA (lacking in secondary structure), with essentially the same degree of effectiveness and affinity as would occur for natural mRNAs in the presence of eIF-4A, eIF-4B, and eIF-4F, suggests a possible role for eIF-4A and eIF-4B in both cap-independent and internal initiation.