Abstract
Human γD-crystallin (HγD) is an abundant two-domain protein concentrated in the core region of the eye lens. Destabilizing mutations and post-translational modifications in the N-terminal domain of γ-crystallins are linked to onset of aggregation that causes cataract disease (lens turbidity). WT HγD greatly accelerates aggregation of the cataract-related W42Q variant, without itself aggregating. The mechanism of this “inverse prion” catalysis of aggregation remained unknown. Here we provide evidence that it proceeds via an early unfolding intermediate with an opened domain interface, which enables transient dimerization of WT and mutant, or mutant and mutant, HγD molecules at their C-terminal domains. This dimerization deprives the N-terminal domain of intramolecular stabilization by the domain interface and thus promotes its conversion to a distinct, aggregation-prone partially unfolded intermediate. This mechanism can be generalized to explain how surprising reactions, such as conformational catalysis of misfolding, may arise from simple domain-domain interactions in multidomain proteins.
Significance Most known proteins in nature consist of multiple domains. Interactions between domains may lead to unexpected folding and misfolding phenomena. This study of human γD-crystallin, a two-domain protein in the eye lens, revealed one such surprise: conformational catalysis of misfolding via intermolecular domain interface “stealing.” An intermolecular interface between the more stable domains outcompetes the native intramolecular domain interface. Loss of the native interface in turn promotes misfolding and subsequent aggregation, especially in cataract-related γD-crystallin variants. This phenomenon is likely a contributing factor in the development of cataract disease, the leading worldwide cause of blindness. However, interface stealing likely occurs in many proteins composed of two or more interacting domains.