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 chaperonins, 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 polypeptides are unknown at present. One set of molecular chaperonins found in E. coli has been identified as intrinsic heat shock proteins (expression levels increase during heat stress). These components have homologous counterparts in eucaryotic mitochondrial systems and are absolutely required by the cell. In order to gain insight into the mechanism of enhanced renaturation of proteins by chaperonins, we are continuing to examine the chaperonin- assisted folding and assembly of dodecameric E. coli glutamine synthetase (GS). We have found that the rates of renaturation are not significantly accelerated when refolding and assembly of GS are initiated in the presence of the chaperonin proteins. However, the amount of correctly assembled GS increases when chaperonins are present. Using nondenaturing gradient pore electrophoresis, we have found that the time dependent accumulation of correctly folded GS appears to be dependent on an increased steady state concentration of monomers that are competent to form higher order oligomers. The assembly of GS occurs first through the collision of monomers to form correctly formed dimers. Based on immunoblot analysis of the nondenaturing gel system, it appears that the formation of the properly formed dimer is the rate limiting step prior to the formation of tetramers and higher. The higher order even-numbered oligomers form as a result of collision with dimers. We have experimentally determined that there exists an order of interaction difference that is dependent on whether MgATP or unfolded GS monomers are initially complexed with groEL. If the gro-EL-GS complex is formed first, the renaturation rate profile is significantly slower than the rates observed if MgATP is initially complexed with groEL prior to initiating the renaturation of unfolded GS. Furthermore, we have found that the small chaperonin oligomer, groES, will increase GS release rates from groEL if groES is present when MgATP is added to the preformed EL-GS complex.
