Pressure-induced dissociation of tight couple ribosomes

AbstractRibosomes from Escherichia coli have been shown to undergo subunit dissociation at elevated hydrostatic pressure. This holds for both crude and highly purified ribosomes. No inhibitory effect could be detected by addition of either the S100 supernatant, or tRNA, polyuridylic acid, and spermine. Light scattering experiments at pressures up to 1000 bar reveal different susceptibility of tight couple and loose couple ribosomes toward pressure dissociation. Tight couples are subjected to EF-Tu-catalyzed binding of aminoacyl-tRNA, thus yielding a model system of the elongating ribosome before the peptidyl transfer step. High pressure dissociation of this compound suggests that enzymatic binding converts tight couples into loose couples. A hypothesis referring to conformational changes during the elongation cycle is presented

THERMODYNAMIC AND KINETIC ASPECTS OF THE FOLDING AND SELFASSEMBLY OF PROTEINS

The acquisition of the spatial structure of proteins may be described as a hierarchical condensation reaction. Starting from next-neighbor interac tions (involved in secondary structure formation), the native tertiary and quaternary structure are generated by the merging and docking of domains and subunits. The overall kinetics of the folding and assembly of multi subunit oligomeric proteins consist of consecutive unimolecular and bimo lecular steps which may be distinguished by concentration-dependent recon stitution experiments. In order to reach the native state, both folding and association must be tuned such that the formation of the correct subunit recognition sites precedes association. Beyond a limiting concentration, "wrong aggregation” (in vitro) or precipitation of "inclusion bodies" (in overexpressing strains of bacteria) are found to outrun proper structure formation. Other variables determining the yield and rate of in vitro reconstitution are the temperature, pH, ionic strength and specific ligands. Apart from the physi co-chemical parameters, folding in vivo may be affected by codon usage, amino-acid pools and genome organization