Coupled chaperone action in folding and assembly of hexadecameric Rubisco.

Form I Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase), a complex of eight large (RbcL) and eight small (RbcS) subunits, catalyses the fixation of atmospheric CO(2) in photosynthesis. The limited catalytic efficiency of Rubisco has sparked extensive efforts to re-engineer the enzyme with the goal of enhancing agricultural productivity. To facilitate such efforts we analysed the formation of cyanobacterial form I Rubisco by in vitro reconstitution and cryo-electron microscopy. We show that RbcL subunit folding by the GroEL/GroES chaperonin is tightly coupled with assembly mediated by the chaperone RbcX(2). RbcL monomers remain partially unstable and retain high affinity for GroEL until captured by RbcX(2). As revealed by the structure of a RbcL(8)-(RbcX(2))(8) assembly intermediate, RbcX(2) acts as a molecular staple in stabilizing the RbcL subunits as dimers and facilitates RbcL(8) core assembly. Finally, addition of RbcS results in RbcX(2) release and holoenzyme formation. Specific assembly chaperones may be required more generally in the formation of complex oligomeric structures when folding is closely coupled to assembly.

Structure and function of RbcX, an assembly chaperone for hexadecameric Rubisco.

After folding, many proteins must assemble into oligomeric complexes to become biologically active. Here we describe the role of RbcX as an assembly chaperone of ribulose-bisphosphate carboxylase/oxygenase (Rubisco), the enzyme responsible for the fixation of atmospheric carbon dioxide. In cyanobacteria and plants, Rubisco is an approximately 520 kDa complex composed of eight large subunits (RbcL) and eight small subunits (RbcS). We found that cyanobacterial RbcX functions downstream of chaperonin-mediated RbcL folding in promoting the formation of RbcL(8) core complexes. Structural analysis revealed that the 15 kDa RbcX forms a homodimer with two cooperating RbcL-binding regions. A central cleft specifically binds the exposed C-terminal peptide of RbcL subunits, enabling a peripheral surface of RbcX to mediate RbcL(8) assembly. Due to the dynamic nature of these interactions, RbcX is readily displaced from RbcL(8) complexes by RbcS, producing the active enzyme. The strategies employed by RbcX in achieving substrate specificity and efficient product release may be generally relevant in assisted assembly reactions.

Chaperonin TRiC promotes the assembly of polyQ expansion proteins into nontoxic oligomers.

Aberrant folding and fibrillar aggregation by polyglutamine (polyQ) expansion proteins are associated with cytotoxicity in Huntington's disease and other neurodegenerative disorders. Hsp70 chaperones have an inhibitory effect on fibril formation and can alleviate polyQ cytotoxicity. Here we show that the cytosolic chaperonin, TRiC, functions synergistically with Hsp70 in this process and is limiting in suppressing polyQ toxicity in a yeast model. In vitro reconstitution experiments revealed that TRiC, in cooperation with the Hsp70 system, promotes the assembly of polyQ-expanded fragments of huntingtin (Htt) into soluble oligomers of approximately 500 kDa. Similar oligomers were observed in yeast cells upon TRiC overexpression and were found to be benign, in contrast to conformationally distinct Htt oligomers of approximately 200 kDa, which accumulated at normal TRiC levels and correlated with inhibition of cell growth. We suggest that TRiC cooperates with the Hsp70 system as a key component in the cellular defense against amyloid-like protein misfolding.

Attempts to delineate the relative contributions of changes in hydrophobicity and packing to changes in stability of ribonuclease S mutants.

While the hydrophobic driving force is thought to be a major contributor to protein stability, it is difficult to experimentally dissect out its contribution to the overall free energy of folding. We have made large to small substitutions of buried hydrophobic residues at positions 8 and 13 in the peptide/protein complex, RNase-S, and have characterized the structures by X-ray crystallography. The thermodynamics of association of these mutant S peptides with S protein was measured in the presence of different concentrations of methanol and ethanol. The reduction in the strength of the hydrophobic driving force in the presence of these organic solvents was estimated from surface-tension data as well as from the dependence of the DeltaC(p) of protein/peptide binding on the alcohol concentration. The data indicated a decrease in the strength of the hydrophobic driving force of about 30-40% over a 0-30% range of the alcohol concentration. We observe that large to small substitutions destabilize the protein. However, the amount of destabilization, relative to the wild type, is independent of the alcohol concentration over the range of alcohol concentrations studied. The data clearly indicate that decreased stability of the mutants is primarily due to the loss of packing interactions rather than a reduced hydrophobic driving force and suggest a value of the hydrophobic driving force of less than 18 cal mol(-)(1) A(2).