Fractionation and partial characterization of the protein synthesis system of wheat germ. II. Initiation factors D1...

This report characterizes three components of the fractionated wheat germ protein synthesizing system, factors D1, D2c, and D2d. FActor D1 has a molecular weight of about 420,000 and is comprised of 7-10 components varying in molecular mass between 28,000 and 105,000 daltons. The factor is absolutely required for the formation of both 40-S ribosome.Met-tRNAiMet and 80 S ribosome.Met-tRNAiMet complexes except at low Mg2+ concentration, where significant 40 S ribosome complex formation can be obtained without D1, provided the reaction products are fixed with glutaraldehyde. Factor D1 can be shown to interact with the 40-S ribosome subunit in two ways: the change in sedimentation of the ribosome subunits that occurs when ribosomes are incubated with the factor under dissociating conditions and the prevention of the reformation of 80 S ribosomes by a prior incubation of ribosome subunits with the factor. Factor D2c has a molecular mass of about 100,000 daltons and is absolutely required for the conversion of 40 S ribosome.Met-tRNAiMet complexes to 80 S ribosome.Met-tRNAiMet. Factor D2d strongly augments the D2c-dependent formation of 80S ribosome.Met-tRNAiMet complexes, with the requirement for D2d most apparent when the complexes are fixed with glutaraldehyde, a treatment that labilizes the 80 S ribosome-bound Met-tRNAiMet. Addition of factor D2d to a 40 S ribosome.Met-tRNAiMet synthesizing system also increases the amount of complex obtained 2-5-fold. Consideration of these characteristics establishes components D1, D2c, and D2d as wheat germ eucaryotic initiation factors 3, 5, and 4C, respectively.