Global suppression of protein folding defects and inclusion body formation

Independent domain folding of Pseudomonas exotoxin and single-chain immunotoxins: influence of interdomain connections

We have studied the refolding of completely unfolded and reduced Pseudomonas exotoxin (PE) and of recombinant single-chain immunotoxins made with monoclonal antibody B3 that are composed of a heavy-chain variable region connected by a flexible linker to the corresponding light-chain variable region (Fv), which is in turn fused to a truncated form of PE. We have found by direct activity assays that different functional domains of these multifunctional proteins fold independently with different kinetics. The ADP-ribosylation domain of PE and of the recombinant immunotoxin fold rapidly, whereas the assembly of the binding and/or translocation domains is regained more slowly. The complete refolding of native PE occurs more rapidly than the refolding of the recombinant immunotoxins. To determine the influence of the connector region between the B3(Fv) moiety and the toxin on the folding process of the recombinant immunotoxin B3(Fv)-PE38KDEL, we have made two different mutations in the peptide that connects the single-chain Fv domain to domain II of PE. These molecules show different folding kinetics, differences in their propensity to aggregate, and different yields of correctly folded molecules. A mutation that decreases aggregation increases the rate of formation and the yield of active immunotoxin molecules.

Optimizing protein folding to the native state in bacteria

A correctly folded protein is usually both active and soluble. This review focuses on novel ways to improve the folding of recombinant proteins during production in bacteria and includes a few tips for refolding proteins. Major results in correlating protein primary structure with proper folding and stability, and the production of viral antigens and antibodies in bacteria are also discussed.

Protein stability and molecular adaptation to extreme conditions

Proteins, due to the delicate balance of stabilizing and destabilizing interactions, are only marginally stable. Adaptation to extreme environments tends to shift the 'mesophilic' characteristics of proteins to the respective extremes of temperature, hydrostatic pressure, pH and salinity, such that, under the mutual physiological conditions, the molecular properties are similar regarding overall topology, flexibility and solvation. Enhanced intrinsic stability requires only minute local structural changes so that general strategies of stabilization cannot be established. Apart from mutative changes of amino-acid sequences, extrinsic factors (or cellular components) may be involved in 'extremophilic adaptation'. The molecular basis of acidophilic, alkalophilic and barophilic adaptation is still obscure. Mechanisms of enhanced thermal stability involve improved packing density, as well as specific local interactions. In halophiles, water and salt binding of the intrinsically stable protein inventory is accomplished by favoring acidic over basic amino acid residues and decreased hydrophobicity. General limits of viability are: (a) the susceptibility of the covalent structure of the polypeptide chain toward hydrolysis or hydrothermal degradation; (b) the competition of extreme solvent parameters with the weak electrostatic and hydrophobic interactions involved in protein stabilization; (c) perturbations of the folding and assembly of proteins; and (d) 'dislocation' of biochemical pathways due to effects of extreme conditions on the intricate network of metabolic reactions.

Construction of a synthetic gene for an R-plasmid-encoded dihydrofolate reductase and studies on the role of the N-terminus in the protein

R67 dihydrofolate reductase (DHFR) is a novel protein that provides clinical resistance to the antibacterial drug trimethoprim. The crystal structure of a dimeric form of R67 DHFR indicates the first 16 amino acids are disordered [Matthews et al. (1986) Biochemistry 25, 4194-4204]. To investigate whether these amino acids are necessary for protein function, the first 16 N-terminal residues have been cleaved off by chymotrypsin. The truncated protein is fully active with kcat = 1.3 s-1, Km(NADPH) = 3.0 microM, and Km(dihydrofolate) = 5.8 microM. This result suggests the functional core of the protein resides in the beta-barrel structure defined by residues 27-78. To study this protein further, synthetic genes coding for full-length and truncated R67 DHFRs were constructed. Surprisingly, the gene coding for truncated R67 DHFR does not produce protein in vivo or confer trimethoprim resistance upon Escherichia coli. Therefore, the relative stabilities of native and truncated R67 DHFR were investigated by equilibrium unfolding studies. Unfolding of dimeric native R67 DHFR is protein concentration dependent and can be described by a two-state model involving native dimer and unfolded monomer. Using absorbance, fluorescence, and circular dichroism techniques, an average delta GH2O of 13.9 kcal mol-1 is found for native R67 DHFR. In contrast, an average delta GH2O of 11.3 kcal mol-1 is observed for truncated R67 DHFR. These results indicate native R67 DHFR is 2.6 kcal mol-1 more stable than truncated protein. This stability difference may be part of the reason why protein from the truncated gene is not found in vivo in E. coli.