The reaction between N-ethylmaleimide and ribosomes. A re-examination of some common assumptions

Characterization and site of action of a soluble protein that stimulates peptide-bond synthesis

A recently identified soluble protein, named EF-P, stimulates peptide bond synthesis from ribosomal-bound N-formylmethionyl-tRNA and the aminoacyl-tRNA analog, puromycin. Using this model of peptide bond formation we have purified this activity approximately 100-fold from ribosome-free extracts of Escherichia coli. In order to study the mechanism by which the EF-P factor stimulates peptide bond formation, we examined and compared the requirements and site of action of the spontaneous and the EF-P-mediated synthesis of peptide bonds. We find that "enzymic" peptide bond synthesis (+EF-P) is characterized by relatively broad temperature and NH4Cl optima, a sharp Mg2+ optimum at 12 mM, and an apparent pKa of approximately 8.5. The characteristics of enzymic peptide bond synthesis closely resemble those reported for native peptidyl-puromycin formation rather than other models of peptide synthesis. Factor EF-P requires both 30-S and 50-S subunits for activity. The 30-S particle is inactive by itself and may function in the reaction merely to bind the fMet-tRNA substrate. Both the peptidyl transferase and the EF-P binding site may be part of the 50-S subunit. Unlike all other propagation factors, EF-P does not require the 50-S ribosomal proteins L7 and L12 and may therefore occupy a different ribosomal site.

Reaction of N-(3-pyrene)maleimide with thiol groups of reticulocyte ribosomes

The reaction of N-(3-pyrene)maleimide with thiol groups of rabbit reticulocyte ribosomes offers a possible fluorescent probe for studying ribosomal structure and conformation. At relatively low concentrations of N-(3-pyrene)maleimide a group of 30-40 readily reactive sulfhydryl residues is derivatized. The major ribosomal proteins containing these thiol groups are identified as S2 + S3, S5, S7, S8, S29, S31, S32, L1, L5, L6, L10 + L14, L15, L18 + L19, and L36. Ribosomal activity, as measured by the nonenzymic binding of phenylalanyl-tRNA and polyphenylalanine synthesis, is inhibited by this degree of reaction with N-(3-pyrene)maleimide. The inhibition is relieved by the prior binding of polyuridylic acid to the ribosomes while the extent of derivatization by N-(3-pyrene)-maleimide is diminished only slightly. The average relative polarization of the fluorescence of the ribosomal bound N-(3-pyrene)maleimide changes significantly with the degree of derivatization of ribosomal thiol groups or with the binding of polyuridylic acid, indicating the value of such a fluorescent thiol-derivatizing agent as a probe of ribosomal structure.

Identification of a soluble protein that stimulates peptide bond synthesis

A soluble protein factor was isolated, free of elongation factor (EF)-T and EF-G, based on its ability to stimulate the synthesis of peptide bonds using ribosomal bound 70S-AUG-N-formyl-[35S]methionyl-tRNA complex and added puromycin as substrates. Over 90% of this activity was found in the ribosome-free cytoplasm of Escherichia coli extracts. Otherfeatures such as molecular weight, purification properties, and catalytic activities distinguish this factor from ribosomal proteins and known activators of translation. The factor requires all components needed for peptide bond synthesis and is inhibited by antibiotics known to specifically block the peptidyl transferase activity of ribosomes. The factor increases the binding affinity of the ribosome for the aminoacyl-tRNA analog puromycin about 10-fold. We suggest that this extraribosomal factor modulates the intrinsic activity of ribosomes to catalyze peptide-bond synthesis, and regard it as a new factor required for peptide chain elongation, which we call EF-P.