A heat-stable protein synthesis initiation factor from wheat germ.

A protein synthesis initiation factor of molecular weight 21,500 has been purified to homogeneity from extracts of wheat germ. The factor, referred to as D2d, is stable to heating for 5 min at 75 degrees C and loses less than 15% of its activity upon a similar treatment at 90 degrees C. In contrast, all other wheat germ protein synthesis factors are inactivated by heating for 5 min at 65 degrees C. Formation of 80 S ribosome . Met-tRNAiMet complexes from 40 S ribosome . Met-tRNAiMet complexes, as measured by the binding of [3H]Met-tRNAiMet, is almost completely dependent upon the presence of component D2d. The shift in sedimentation of the ribosome subunits to the 80 S ribosome region, however, occurs in the absence of factor D2d. Thus, the role of the factor is not to join the ribosomal subunits, but rather to keep the Met-tRNAiMet bound to the ribosome. The formation of 40 S ribosome . Met-tRNAiMet complexes is increased 2-fold by the presence of factor D2d. Fixing this complex with glutaraldehyde results in the binding of more Met-tRNAIMet than a nonfixed incubation in the presence of D2d. These results suggest that the primary interaction with the ribosomal subunit can occur in the absence of factor D2d, but that the presence of the factor stabilizes the ribosome-bound Met-tRNAiMet.