Use of purified polysomes from rabbit reticulocytes in a specific test for initiation factors

Determination of eukaryotic peptidyltransferase activity by pseudo-first-order kinetic analysis

We have developed an in vitro system for the determination of peptidyltransferase activity in rabbit reticulocyte ribosomes. Using this system, a detailed kinetic analysis of a model reaction for peptidyltransferase is described, with AcPhe-tRNA as the peptidyl donor and puromycin as the acceptor. The [AcPhe-tRNA-poly(U)-80S ribosome] complex (complex C) is isolated and then reacted with excess puromycin to give AcPhe-puromycin. This reaction (puromycin reaction) follows first-order kinetics at all concentrations of puromycin tested. At saturating concentrations of puromycin, the first-order rate (k3) constant is identical to the catalytic rate constant (kcat) of peptidyltransferase. This k3 of peptidyltransferase is equal to 2.9 min-1 at 37 degrees C. Moreover, the ratio k3/ Ks, which is an accurate measure of peptidyltransferase activity, was increased 80-fold when salt-washed ribosomes were replaced by unwashed ribosomes. Finally, the puromycin reaction was inhibited by several well-known antibiotics acting on the eukaryotic peptidyltransferase.

Isolation and purification of hen oviduct protein synthesis initiation factors A2A and A2B

Two initiation factors, IF-A2A and IF-A2B, required for protein synthesis in a fractionated system have been isolated from hen oviduct. These factors were obtained from a 0.5 M KCl extraction of a nuclear-microsomal fraction of the oviduct. The crude extract inhibited protein synthesis, but, following DEAE-cellulose chromatography, activity was detected. Sephadex G-200 chromatography separated the activity into two active frations, A2A and A2B. These factors have been characterized with respect to their activities in polyphenylalanine polymerization at a low Mg2+ concentration, AUG- and GTP-dependent Met-tRNAf binding to 40S ribosomal subunits, the hydrolysis of GTP, and natural messenger ribonucleic acid (mRNA) dependent protein synthesis. In all of these systems A2A and A2B were able to substitute for rabbit reticulocyte M2A and M2B. The molecular weights of A2A and A2B have been estimated by gel filtration chromatography to be 280 000 and 23 000, respectively. Sedimentation analysis on sucrose gradients showed A2A to have a sedimentation coefficient of 5.2 S. Combining these data, the molecular weight of A2A was calculated to be 125 000. These values are similar to those for corresponding reticulocyte proteins. Finally, in the presence of added ovalbumin mRNA, A2A and A2B stimulated protein synthesis on non-salt-whashed hen oviduct and rabbit reticulocyte polysomes. Moreover, A2A- and A2B-dependent synthesis of ovalbumin was shown to occur on reticulocyte polysomes programmed with ovalbumin mRNA. This supports the conclusion that these factors are initiation factors for protein synthesis and not ribosomal structural proteins.

The binding of Met-tRNAf to isolated 40-S ribosomal subunits and the formation of Met-tRNAf - 80-S-ribosome initiation complexes

Different forms of 40-S ribosomal subunit, distinguishable by their buoyant densities on CsCl equilibrium density gradients, are formed when derived 40-S ribosomal subunits are incubated with partially purified reticulocyte ribosomal wash proteins. One of these subunits, the 1.37-g-cm-3 form is not present in the cell but the other two forms, the 1.40-g-cm-3 and 1.40-g-cm-3 subunits, are present in cell extracts. 35S label is bound to 1.37-g-cm-3 and 1.40-g-cm-s subunits when [35S]Met-tRANf, GTP and poly(A,U,G) are included in the incubations. The 35S-labelled 40-S subunits recovered, and the amount of 35S label bound to them, are changed if the [35S]Met-tRNAf-40-S-subunit-poly(A,U,G) complexes are first purified on sucrose gradients before analysing them on CsCl. The 1.37-g-cm-3 particle is no longer seen and the total quantity of 35S label on the 40-S subunits is 90% lower after sucrose gradient purification. Between 30% and 40% of the 40-S subunits bind [35S]Met-tRNAf when 1 mM GTP, an excess of ribosomal wash proteins and [35S]Met-tRNAf over derived 40-S subunits, and poly(A,U,G) or AUG is included in the incubations. The omission of poly(A,U,G) or AUG from the incubations substantially lowers the amount of subunit-bound 35S label ultimately recovered. With these incubations less than 10% of the 40-S subunits have bound [35S]Met-tRNAf. [35S]Met-tRNAf binding is affected by the nature of the RNA added. The addition of poly(U), rRNA and native 9-S golbin mRNA is without effect, whereas denatured globin mRNA is stimulatory. Maximum binding is obtained however with AUG. Poly(A,U,G) is less stimulatory than AUG but more stimulatory than denatured mRNA, suggesting that the number as well the accessibility of the AUG initiations condons determines the amount of 35S label bound. Similar results are obtained for the ribosomal-wash-dependent binding of [35S]Met-tRNAf to 80-S ribosomes. Contrary to the binding results, the ability of mRNA to stimulate protein synthesis is dependent on the integrity of the mRNA. Thus, native 9-S globin mRNA but not poly(A,U,G) stimulatex protein synthesis in the wheat germ system. HCHO-treated globin mRNA, although stimulatory, is 45% less effective than native mRNA. The addition of AUG, derived 60-S subunits and extra ribosomal wash is required for the formation of [35S]Met-tRNAf-80-S-ribosome complexes from sucrose-gradient-purified [35S]Met-tRNAf-40-S-subunit complexes. The 80-S ribosome complexes are able to form peptide bonds. Thus, if puromycin is added to the full incubations at zero time, no 35S label is present on the 80-S ribosome. 35S label is released as methionyl-puromycin. If the [35S]Met-tRNAf-40-S-subunit complexes are assembled with poly(A,U,G) or AUG in the incubations and then purified, only derived 60-S subunits are required to form [35S]Met-tRNAf-80-S-ribosome complexes. 35S label is not released from them when puromycin is added to the incubations unless extra ribosomal wash is also added.

Nonribosomal proteins associated with eukaryotic native small ribosomal subunits

The native small ribosomal subunit (Sn) from rabbit reticulocytes which is able to initiate translation of globin mRNA in a cell-free system carries additional protein components. The latter can be separated from the subunit in a high salt sucrose gradient yielding a top fraction (T) and a complex fraction (C), sedimenting at about 4 and 15 S, respectively. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate revealed that fraction T contained four dominant polypeptides, while fraction C represents a large protein complex consisting of at least 10 polypeptides. Sn isolated from other sources showed similar patterns of their nonribosomal proteins. Reconstitution experiments revealed that fraction C is absolutely required for protein synthesis, while fraction T enhances protein synthesis only in the presence of C. The adherence of these protein factors to the subunit is not mediated by magnesium ions. Treatment of Sn with EDTA and centrifugation in a magnesium-free sucrose gradient caused unfolding of the subunits and dissociation of several ribosomal proteins, but not of the factors. The unfolded ribosomal subunits sedimented as two distinct peaks. The more slowly sedimenting peak contained proteins of fraction T and the faster sedimenting one contained the 15S complex, indicating heterogeneity of the Sn population with respect to the factors attached to them.

Use of eukaryotic native small ribosomal subunits for the translation of globin messenger RNA

A highly active in vitro system for the translation of globin mRNA, resulting in more than 10 rounds of translation, is described. The reconstituted system consists of native small ribosomal subunits of rabbit reticulocytes (as a source of initiation factors as well as small ribosomal subunits), large subunits derived from rat liver polysomes by the puromycin-KCl procedure, and a pH 5 fraction obtained from a Krebs ascites cell high speed supernatant. In this system no differences were found between globin messenger ribonucleoprotein and globin mRNA.