Chaperone function in organic co-solvents: experimental characterization and modeling of a hyperthermophilic chaperone subunit from Methanocaldococcus jannaschii.

Molecular chaperones play a central role in maintaining protein structure within a cell. Previously, we determined that the gene encoding a molecular chaperone, a thermosome, from the hyperthermophilic archaeon Methanocaldococcus jannaschii is upregulated upon lethal heat shock. We have recombinantly expressed this thermosome (rTHS) and show here that it is both stable and fully functional in aqueous solutions containing water-miscible organic co-solvents. Based on circular dichroism the secondary structure of rTHS was not affected by one-hour exposures to a variety of co-solvents including 30% v/v acetonitrile (ACN) and 50% methanol (MeOH). By contrast, the secondary structure of a mesophilic homologue, GroEL/GroES (GroE), was substantially disrupted. rTHS reduced the aggregation of ovalbumin and citrate synthase in 30% ACN, assisted refolding of citrate synthase upon solvent-inactivation, and stabilized citrate synthase and glutamate dehydrogenase in the direct presence of co-solvents. Apparent total turnover numbers of these enzymes in denaturing solutions increased by up to 2.5-fold in the presence of rTHS. Mechanistic models are proposed to help ascertain specific conditions that could enhance or limit organic solvent-induced chaperone activity. These models suggest that thermodynamic stability and the reversibility of enzyme unfolding play key roles in the effectiveness of enzyme recovery by rTHS.