Folding and self-assembly of the domains of betaB2-crystallin from rat eye lens.

betaB2-Crystallin from vertebrate eye lens forms domain-swapped dimers, with subunits consisting of two all-beta domains connected by an eight-residue extended linker peptide. Topologically, the two domains show great similarity; however, they differ widely in their stability. As shown by urea-induced equilibrium unfolding experiments, the isolated monomeric C-terminal domain is more stable than complete betaB2. In contrast, the N-terminal domain exhibits marginal stability only in its dimeric state; upon subunit dissociation, at low protein concentration, unfolding takes place. The folding and association of intact betaB2 follows a sequential uni-bimolecular mechanism according to N2 <==> 2 I <==> 2U, whereas the isolated domains may be quantitatively described by the two-state model (N <==> U).