ABSTRACT
The three-dimensional chalice-like crystal structure of initiation factor 2 IF2/eIF5B from Methanobacterium thermoautotrophicum represents a novel fold and domain architecture in which the N-terminal G domain and the C-terminal C domain are separated by an approximately 40 A alpha-helix. Homologous Thermus thermophilus initiation factor 2 (IF2wt), G (IF2G), and C (IF2C) domains were successfully overexpressed and purified which enabled us to perform a thermodynamic analysis and to asses the role of the domain architecture in this atypical fold. Circular dichroism in the far-UV region demonstrated that the proteins are well-folded and that the secondary structure content resembles that of IF2 from M. thermoautotrophicum. IF2wt and IF2G are monomeric proteins, while IF2C has a tendency to form dimeric species as shown by sedimentation velocity studies on analytical ultracentrifugation and differential scanning calorimetry scan analysis. Thermal denaturation studies of multidomain IF2wt reveals an exceptionally high reversibility (>90%) of the transition with a melting temperature of 94.5 degrees C. Melting temperature of IF2wt may be further increased in the presence of its physiological ligand GDP and the GTP analogue, GppNHp. The high reversibility of denaturation is achieved by the modular structure of the protein and by the high reversibility of the thermal denaturation of IF2G. On the other hand, hydrophobic IF2C aggregates during the thermal transition, and the aggregation is suppressed by guanidine hydrochloride. Isothermal denaturation demonstrates that both IF2G and IF2C have comparable stabilities of 46 and 33 kJ/mol, respectively. The apparent cooperative unfolding of the full-length protein has an unusually small denaturant m value. This together with the phase diagram method of analysis indicates the presence of intermediate(s) due to the independent unfolding of IF2G and IF2C. Despite an absence of apparent interactions between the domains in vitro, IF2G plays a role in IF2C reversibility in thermal denaturation. In conclusion, interactions between the domains of folded IF2wt in vivo are likely mediated by their alpha-helix connection and/or by a conformational change on the ribosome.
Subject(s)
Bacterial Proteins/chemistry , Prokaryotic Initiation Factor-2/chemistry , Thermus thermophilus , Amino Acid Sequence , Bacterial Proteins/metabolism , Calorimetry, Differential Scanning , Circular Dichroism , Methanobacterium/metabolism , Models, Molecular , Molecular Sequence Data , Prokaryotic Initiation Factor-2/metabolism , Protein Denaturation , Protein Structure, Secondary , Protein Structure, Tertiary , Sequence Homology, Amino Acid , Temperature , Thermodynamics , UltracentrifugationABSTRACT
The homodimeric wild-type elongation factor Ts, EF-Ts(wt), and its C190A mutant, EF-Ts(C190A), from Thermus thermophilus goes through thermal denaturation in a way consistent with a two state irreversible model with a relatively high activation energy, approximately 530 kJ/mol (Supplemental materials provides a list of 98 activation energies from 54 proteins in various solvent conditions). Removing the intermonomeric disulfide bond by substituting alanine for cysteine 190 affects the rate constant of the irreversible thermal transition. At physiological temperatures, the half-life of the native conformations was estimated to be approximately 21 days for wt and 1.3 days for C190A. Thermally denatured EF-Ts refolds into a molten-globule-like state as indicated by its native-like circular dichroism spectrum in the far UV region and the enhanced fluorescence of the hydrophobic probe, 1-anilinonaphtalene-8-sulphonate. The residual secondary structure observed in the thermally denatured state of EF-Ts at high temperatures affects its apparent temperature of thermal transition, T(trs), independent of the presence or absence of the intermonomeric disulfide bond. The effect of the GdmHCl concentration on the activation energy, E(a), and the temperature, T*, i.e., the temperature at which the rate of the irreversible step is 1 min(-1), indicates that the intermonomeric disulfide bond contributes to the irreversibility of thermal transition of EF-Ts.
Subject(s)
Cystine/chemistry , Peptide Elongation Factors/chemistry , Thermus thermophilus/chemistry , Algorithms , Anilino Naphthalenesulfonates/chemistry , Calorimetry, Differential Scanning , Circular Dichroism , Cystine/genetics , Guanidine/chemistry , Hot Temperature , Mutation/genetics , Peptide Elongation Factors/genetics , Protein Denaturation , Recombinant Proteins/chemistry , Spectrometry, Fluorescence , Thermodynamics , Thermus thermophilus/geneticsABSTRACT
The initiation of protein translation in bacteria requires in addition to mRNA, fMet-tRNA, and ribosomal subunits three protein factors, the initiation factor 1 (IF1), initiation factor 2 (IF2), and initiation factor 3 (IF3). The genes coding for IF1 and IF3 from Thermus thermophilus have been identified and cloned into pET expression vector and were expressed as soluble proteins in Escherichia coli. IF1 was purified by a DEAE-cellulose chromatography, followed by heat denaturation, chromatography on Hydroxylapatit, and gel permeation chromatography using Sephacryl 200HR. For the purification of IF3, a heat denaturation step is followed by anion-exchange chromatography on Q-Sepharose FF and gel permeation chromatography on Sephacryl 200HR. Using these procedures we obtained chromatographically pure and biologically active preparations of both T. thermophilus IF1 and IF3.
