Increased ribosomal affinity for mRNA causes resistance to edeine in a mutant of Saccharomyces cerevisiae

Crystal structures of complexes of the small ribosomal subunit with tetracycline, edeine and IF3

The small ribosomal subunit is responsible for the decoding of genetic information and plays a key role in the initiation of protein synthesis. We analyzed by X-ray crystallography the structures of three different complexes of the small ribosomal subunit of Thermus thermophilus with the A-site inhibitor tetracycline, the universal initiation inhibitor edeine and the C-terminal domain of the translation initiation factor IF3. The crystal structure analysis of the complex with tetracycline revealed the functionally important site responsible for the blockage of the A-site. Five additional tetracycline sites resolve most of the controversial biochemical data on the location of tetracycline. The interaction of edeine with the small subunit indicates its role in inhibiting initiation and shows its involvement with P-site tRNA. The location of the C-terminal domain of IF3, at the solvent side of the platform, sheds light on the formation of the initiation complex, and implies that the anti-association activity of IF3 is due to its influence on the conformational dynamics of the small ribosomal subunit.

An in vitro system from Plasmodium falciparum active in endogenous mRNA translation

An in vitro translation system has been prepared from Plasmodium falciparum by saponin lysis of infected-erythrocytes to free parasites which were homogeneized with glass beads, centrifuged to obtain a S-30 fraction followed by Sephadex G-25 gel filtration. This treatment produced a system with very low contamination of host proteins (<1%). The system, optimized for Mg2+ and K+, translates endogenous mRNA and is active for 80 min which suggests that their protein factors and mRNA are quite stable.

Involvement of a 50-kDa mRNP protein from Saccharomyces cerevisiae in mRNA binding to ribosomes

A yeast 50-kDa mRNA-binding protein (50mRNP) is found selectively associated with the 48S and 80S initiation complexes. This protein is structurally related to the translational elongation factor EF-1alpha. The protein reacts with antibodies directed against EF-1alpha and, similarly, EF-1alpha recognizes antibodies against the 50mRNP protein. This is evidence that they share at least one epitope which allows a similar antigenic behavior. In addition, both proteins show similar cleavage patterns upon treatment with the endoproteinase Lys-C. A murine antibody raised against 50mRNP inhibits both 48S and 80S initiation complex formation. The inhibitory effect is relieved by preincubating anti-50mRNP with EF-1alpha. Antibody to EF-1alpha manifests a similar inhibitory pattern for the formation of 48S and 80S complexes. These data strongly suggest that 50mRNP is an EF-1alpha-like polypeptide essential for the formation of the above complexes.

Association of ribosomal subunits. A new functional role for yeast EF-1 alpha in protein biosynthesis

A yeast ribosomal subunit association factor (AF) has been purified from a high-salt ribosomal wash. The purified enzyme is a thermostable protein that associates ribosomal subunits at low Mg2+ concentration without requiring energy. It appears to be an aggregate of trimers or dimers (molecular mass 125 or 79 kDa) which on sodium dodecyl sulfate gels shows the presence of a major protein band whose estimated molecular mass is 43 kDa. Evidence also indicates the existence of a 50-kDa polypeptide which seems to be unstable since with freezing and thawing it gives rise to the 43-kDa polypeptide. It was shown that the labelled factor interacts with 80S ribosomes and with 40S ribosomal subunits. The purified polypeptide reacts with antibodies directed against EF-1 alpha, this last protein recognizing the antibodies raised against AF. Likewise, both EF-1 alpha and AF associate ribosomal subunits in the same way. When EF-1 is heated, it not only maintains its association activity, but also behaves like a 43-kDa polypeptide in an SDS electrophoresis run. These observations strongly suggest that AF originates from EF-1 alpha, which implies that the well-known elongation factor may also play a role in the initiation step of protein synthesis.

Importance of polysomal mRNA-associated polypeptides for protein synthesis initiation in yeast

The polysomal mRNA from the cell-free system of the yeast Saccharomyces cerevisiae, in the absence of exogenous energy, binds to the 40S ribosomal subunit thus forming a 48S preinitiation complex which, with energy added, is converted into 80S initiation complex. By using ribosomes with a high affinity to polysomal mRNA (pmRNA) from an edeine-resistant mutant of S. cerevisiae in place of wild-type ribosomes, increased quantities of the 48S preinitiation complex are obtained. The pmRNA is found associated with several polypeptides having molecular masses of 115-98 kDa, 72 kDa, 60 kDa and 51 kDa. These polypeptides, labelled with 125I, interact with 40S and 80S ribosomes and are essential for the formation of the 48S and 80S initiation complexes inasmuch as deproteinized pmRNA alone cannot initiate the process. In addition, other polypeptides present in the cytosol are required to carry out the above-mentioned steps of protein synthesis.

An altered ribosomal protein in an edeine-resistant mutant of Saccharomyces cerevisiae

The r-proteins of an edeine-resistant mutant of Saccharomyces cerevisiae were compared to those of the wild-type strain by using two different two-dimensional electrophoretic techniques: (1) the Kaltschmidt-Wittmann method and, (2) the Kaltschmidt-Wittmann system, in the first dimension and the Na Dodecyl-SO4 system in the second. With the first technique, the results indicate that the patterns of basic ribosomal proteins are similar in the two strains. However, the pattern of acidic ribosomal proteins of the mutant revealed an additional protein band with respect to the normal one. Using the other technique, the patterns of basic and acidic ribosomal proteins of the mutant demonstrated a similarity to the corresponding pattern of the wild-type strain. The data disclose that an acidic ribosomal protein of the mutant may have two forms with different electrophoretic mobilities and similar molecular weights.