Particulate aminoacyl-tRNA synthetases are retained on heparin bound to Sepharose

Purification of the yeast mitochondrial methionyl-tRNA synthetase. Common and distinctive features of the cytoplasmic and mitochondrial isoenzymes

Yeast-mitochondrial methionyl-tRNA synthetase was purified 1060-fold from mitochondrial matrix proteins of Saccharomyces cerevisiae using a four-step procedure based on affinity chromatography (heparin-Ultrogel, tRNA(Met)-Sepharose, Agarose-hexyl-AMP) to yield to a single polypeptide of high specific activity (1800 U/mg). Like the cytoplasmic methionyl-tRNA synthetase (Mr 85,000), the mitochondrial isoenzyme is a monomer, but of significantly smaller polypeptide size (Mr 65,000). In contrast, the corresponding enzyme of Escherichia coli is a dimer (Mr 152,000) made up of identical subunits. The measured affinity constants of the purified mitochondrial enzyme for methionine and tRNA(Met) are similar to those of the cytoplasmic isoenzyme. However, the two yeast enzymes exhibit clearly different patterns of aminoacylation of heterologous yeast and E. coli tRNA(Met). Furthermore, polyclonal antibodies raised against the two proteins did not show any cross-reactivity by inhibition of enzymatic activity and by the highly sensitive immunoblotting technique, indicating that the two enzymes share little, if any, common antigenic determinants. Taken together, our results further support the belief that the yeast mitochondrial and cytoplasmic methionyl-tRNA synthetases are different proteins coded for by two distinct nuclear genes. Like the yeast cytoplasmic aminoacyl-tRNA synthetases, the mitochondrial enzymes displayed affinity for immobilized heparin. This distinguishes them from the corresponding enzymes of E. coli. Such an unexpected property of the mitochondrial enzymes suggests that they have acquired during evolution a domain for binding to negatively charged cellular components.

Particulate protein-synthesis factors associated with translatable mRNA in mouse hybridoma cells

Besides of mRNA, the postribosomal pellet of mouse hybridoma cells contains RNA species which become labeled more rapidly than rRNA. Their synthesis is inhibited by actinomycin D. Density-gradient centrifugation of the postribosomal pellet yielded fractions of approx. 55-60 and 90S, synthesizing after the addition of both ribosomal subunits and energy-sources light and heavy chains of immunoglobulin, as demonstrated by indirect immunoprecipitation. Analysis of translation products by electrophoresis indicated the presence of precursors of mRNAs for immunoglobulin chains in these particles. Postribosomal pellets thus apparently contain different particles composed of similar polypeptide chains and containing protein-synthesis factors associated with translatable mRNA.

The postribosomal particle of rabbit liver contains protein-synthesis factors and serum albumin mRNA

As demonstrated by indirect immunoprecipitation and polyacrylamide gel electrophoresis, an 85S particle separated by sucrose density-gradient centrifugation from the postribosomal pellet of rabbit liver, is able to synthesize serum albumin if supplemented with both ribosomal subunits and sources of energy. It is retained on heparin bound to Sepharose 4B, contains translatable mRNA and apparently all protein factors required for translation. This particle may represent a highly organized protein synthesizing machinery, the combination of which with ribosomes results in formation of new protein molecules.

Biosynthesis of rabbit serum albumin in a heterologous fractionated subcellular system

As demonstrated by a simple procedure based on indirect immunoprecipitation, proteins retained on heparin-Sepharose 4B from postmitochondrial supernatants of rat liver and Zajdela hepatoma catalyse the translation of rabbit serum albumin mRNA in the presence of ribosomal subunits from rat liver, Zajdela hepatoma or rabbit reticulocytes. The albumin synthesis shows an optimum at 1.5 mM MgCl2 and 25 mM KCl and requires ATP and GTP. It is significantly stimulated by tRNA and proceeds for more than 2 h, suggesting a high rate of reinitiation. At the optimum ribosomes:mRNA ratio of 13:1, the immunoprecipitable radioactivity exceeds 15-20-times the blank values. Fluorography of polyacrylamide slabs after electrophoresis of immunoprecipitates revealed the presence of only complete full-size serum albumin without any smaller peptides resulting from premature terminations of polypeptide chains, demonstrating faithful translation. In stained gels only, both heavy and light chains of immunoglobulin G were found, indicating that the assay procedure is highly specific and reliable. The fractionated heterologous protein-synthesizing system described in this paper may be generally useful for studies on the synthesis of specific proteins and factors affecting their rates since, unlike comparable translation assays, a precise calculation of the balance of newly synthesized proteins is possible.