Characterization of initiation factor 3 from wheat germ. 1. Effects of proteolysis on activity and subunit composition.

Wheat germ initiation factor 3 (eIF-3) is a large (15 S) particle containing 10 subunits with molecular weights ranging from 28 000 to 116 000. Two forms of wheat germ eIF-3 which differ in ability to support polypeptide synthesis in vitro have been obtained by chromatography on carboxymethyl-Sephadex (CM-Sephadex). The less active form is not retained on CM-Sephadex in 50 mM KCl and contains lower amounts of two subunits, the 116 000-dalton polypeptide (pp116) and the 36 000-dalton polypeptide (pp36). The more active form is retained on CM-Sephadex in 50 mM KCl and is eluted by 150 mM KCl. Treatment of the more active form with small amounts of trypsin results in a rapid degradation of four of the subunits (pp116, pp107, pp87, and pp36) and in a rapid loss in the ability to support polypeptide synthesis. Trypsin treatment also diminishes the ability of eIF-3 to support the binding of mRNA to 40S ribosomal subunits. These findings indicate that pp116, pp107, pp87, and pp36 are in exposed positions in the eIF-3 particle and that pp116 and/or pp36 are essential for activity.