Identification of an isozyme form of protein synthesis initiation factor 4F in plants

We showed previously that wheat germ extracts contain two forms of protein synthesis initiation factor 4F that have very similar functional properties (Browning, K. S., Lax, S. R., and Ravel, J. M. (1987) J. Biol. Chem. 262, 11228-11232). One form, designated eIF-4F, is a complex containing two subunits, p220 and p26. The other form, designated eIF-(iso)4F, is a complex containing two subunits, p82 and p28, which are antigenically distinct from the subunits of eIF-4F. Both the p26 subunit of eIF-4F and the p28 subunit of eIF-(iso)4F are m7G cap-binding proteins. In this investigation, affinity-purified antibodies to the p220 and p26 subunits of wheat germ eIF-4F and to the p82 and p28 subunits of wheat germ eIF-(iso)4F were used to determine if isozyme forms of eIF-4F are present in maize and cauliflower. Extracts from wheat germ, maize root tips, and cauliflower inflorescences were partially purified by adsorption on m7GTP-Sepharose and elution with m7GTP (MGS eluate). Analysis by sodium dodecyl sulfate gel electrophoresis and immunoblotting with antibodies to the subunits of the wheat germ factors showed that the MGS eluate from maize contains polypeptides that react with antibodies to the p82 and p28 subunits of wheat eIF-(iso)4F, as well as polypeptides that react with antibodies to the p220 and p26 subunits of wheat eIF-4F. The MGS eluate from cauliflower also contains polypeptides that reacted with antibodies to the subunits of wheat eIF-(iso)4F. These results indicate that both maize and cauliflower contain the isozyme form of eIF-4F. In addition, it was found that the factors in the MGS eluate from maize support polypeptide synthesis in a system from wheat deficient in eIF-4F and eIF-(iso)4F, whereas the factors in the MGS eluate from cauliflower support polypeptide synthesis only to a small extent.