The correct expression and assembly of multisubunit proteins (oligomers) and monomers containing multidomain structures depends on the cell's ability to prevent inappropriate aggregation and misfolding of newly synthesized polypeptides. In recent years, experimental evidence has accumulated which suggests that protein factors, termed molecular chaperons, aid in this process by preventing detrimental aggregation through transient binding events. The exact molecular details involved in both the recognition and release of bound polypeptide are unknown at present. These molecular chaperons have been identified as intrinsic heat shock proteins (levels of these proteins increase under conditions of heat stress) and are, thus far, found to be ubiquitous in all living organisms characterized to date. These chaperons are absolutely required by the cell and deletion of such factors is lethal. The development of appropriate assays using model protein systems is essential for assessing and elucidating the molecular role these molecular chaperons play during in vivo protein folding and assembly. To this end, a series experiments were designed to compare protein refolding between near-physiological and nonphysiological solution conditions using dissociated/unfolded E. coli glutamine synthetase (GS) oligomer and unfolded monomeric carbonic anhydrase. Under conditions of physiological ionic strength, pH, temperature, and appropriate cofactor concentrations (in this case, physiological metal ion concentrations), these proteins refold and/or reassociate with low yields and activity. The use of non-physiological solution conditions such as high ionic strengths, inclusion of small amounts of protein denaturants such as urea or guanidine HCl, large concentrations of metal ions, and low temperature were found to decrease the rates of inappropriate aggregation and resulted in higher yields of renatured and active proteins.
