Initiation factor promoted reassociation of eukaryotic ribosomal subunits

Protein initiation in eukaryotes: formation and function of a ternary complex composed of a partially purified ribosomal factor, methionyl transfer RNA, and guanosine triphosphate

A protein factor contained in a 1 M KCl extract of L-cell ribosomes and partially purified by chromatography on DEAE-cellulose forms a specific ternary complex with rat-liver Met-tRNA(f) and GTP. The complex is measured by its quantitative retention on nitrocellulose membranes. Complex assembly is optimal at 100 mM KCl and 0.2 mM MgCl(2), and is independent of mRNA and of ribosomes. The GTP requirement can be replaced over 65% by its methylene analogue GDPCH(2)P, indicating that GTP hydrolysis is not involved. Complex formation is inhibited by 10 muM aurintricarboxylic acid, but is unaffected by 100 muM pactamycin, 100 muM fusidic acid, or by excess uncharged methionine tRNA(f). The ternary complex is relatively stable and appears at the void volume during filtration on Sephadex G-100. At 1-3 mM MgCl(2) and in the presence of other factors, the ternary complex is implicated in protein initiation by (i) its capacity to bind to the 40S ribosomal subunit to form a 48S complex; and (ii) the subsequent association of the 48S complex with a 60S subunit to form a functional "80S complex."

Separation of reticulocyte initiation factor M 2 activity into two components

IF-M(2), one of three initiation factors isolated by DEAE-cellulose chromatography from the 0.5 M KCl-wash fraction of rabbit reticulocyte ribosomes, has been separated by Sephadex G-200 chromatography into two components: IF-M(2A) and IF-M(2B). IF-M(2A) elutes near the void-volume, while IF-M(2B), which is much smaller in molecular weight than IF-M(2A), elutes slightly after a hemoglobin marker. In the presence of the other appropriate factors, both IF-M(2A) and IF-M(2B) are required to stimulate poly(U)-directed polyphenylalanine synthesis at low Mg(++) concentration, ApUpG-directed Met-tRNA(F) binding to washed reticulocyte ribosomes, and initiation of globin synthesis from endogenous mRNA. IF-M(2A) stimulates ribosome-dependent GTP hydrolysis, while IF-M(2B) does not; IF-M(2B) stimulates ApUpG-directed fMet-tRNA(F) binding in the presence of IF-M(1), while IF-M(2A) does not. Although IF-M(2A) and IF-M(2B) can be distinguished from each other by size and by activity, a distinct function for IF-M(2B) has not yet been found. Therefore, its precise role in the initiation process remains unclear.

Requirement for GTP in the initiation process on reticulocyte ribosomes and ribosomal subunits

The requirement for GTP in the initiation process on reticulocyte ribosomes and ribosomal subunits has been examined by studying Met-tRNA(F) binding, ribosome-dependent [gamma-(32)P]GTP hydrolysis, and peptide-bond formation with puromycin. Met-tRNA(F) binding can be obtained with the methylene analogue, 5'-guanylylmethylene diphosphonate, as well as GTP, and it is not inhibited by fusidic acid or several other inhibitors of protein synthesis. This reaction can be performed with the 40S subunit and has the same requirements as the Met-tRNA(F)-binding reaction with washed ribosomes. Ribosome-dependent [gamma-(32)P]GTP hydrolysis can be obtained with the initiation factor M(2A) using either washed ribosomes or the 40S subunit. This reaction is also not significantly inhibited by fusidic acid. Peptide-bond formation between puromycin and Met-tRNA(F), however, is inhibited by fusidic acid, and does not occur if the methylene analogue of GTP is substituted for GTP. These data suggest that the binding of the initiator tRNA to the 40S subunit does not require the hydrolysis of GTP, but that at least one GTP hydrolysis event must occur after Met-tRNA(F) binding in order for the first peptide bond to be formed.