Weak Cooperativity in the Core Causes a Switch in Folding Mechanism Between Two Proteins of the cks Family

The human protein ckshs1 (cks1) is a 79 residue α/β protein with low thermodynamic and kinetic stability. Its folding mechanism was probed by mutation at sites throughout the structure. Many of the mutations caused changes in the slope of the unfolding arm of the chevron plot. The effects can be rationalised in terms of either transition-state movement or native-state “breathing”, and in either case, the magnitude of the effect enables the sequence of events in the folding reaction to be determined. Those sites that fold early exhibit a small perturbation, whilst those sites that fold late exhibit a large perturbation. The results show that cks1 folds sequential pairs of β-strands first; β1/β2 and β3/β4. Subsequently, these pairs pack against each other and onto the α-helical region to form the core. The folding process of cks1 contrasts with that of the homologue, suc1. The 113 residue suc1 has the same β-sheet core structure but, additionally, two large insertions that confer much greater thermodynamic and kinetic stability. The more extensive network of tertiary interactions in suc1 provides sufficient enthalpic gain to overcome the entropic cost of forming the core and thus tips the balance in favour of non-local interactions: the non-local, central β-strand pair, β2/β4, forms first and the periphery strands pack on later. Moreover, the greater cooperativity of the core of suc1 protects its folding from perturbation and consequently the slope of the unfolding arm of the chevron plot is much less sensitive to mutation.