The human eye is an intricate organ, allowing us to visualize the world around us from birth to death in exquisite detail. The lens of the eye is responsible for the transmission of light, which it achieves via a dense array of critical proteins (1,2). Remarkably, in order to maintain the transparency required for vision, lens cells lose their organelles during the development process (3). This means that the proteins produced in the lens in utero persist throughout the entire human lifespan to maintain proper vision.

When we zoom in to the molecular level, there is a key set of players that facilitate the phenomenal feat of vision, crystallin lens proteins (2). Crystallin proteins play a vast set of roles, including acting as protein chaperones, structural proteins, and redox proteins. While these aspects are important, one of the most fascinating features of certain crystallin proteins is that they are created prior to birth and remain stable, folded, and functional for the entire lifespan of a human! This is a remarkable feat, given that most proteins in the human body are thought to degrade within a few days of synthesis (4). For this reason, crystallins are fascinating to study at the biochemical level in order to better understand this unique stability. Additionally, understanding these proteins at the molecule level provides important insights into the process of crystallin aggregation, which can cause age-related cataracts and subsequent blindness (5).

New work published by Ishara Mills-Henry et al. in the Biophysical Journal sought to understand the molecular source of stability for two specific crystallin proteins, γD-crystallin and γS-crystallin. Both of these crystallins have defined N- and C-terminal domains. Each domain contains two structural folds, referred to as Greek key domains, featuring 4 β-strands. The Greek key domains in certain crystallin proteins are thought to contribute to their overall stability (6).

Building on their previous work investigating the kinetic stability of these proteins (7), the authors sought to characterize the relative stability of the N- and C-terminal domains of these proteins in the absence of protein denaturant. By performing a combination of protein folding and unfolding experiments at various guanidinium hydrochloride (protein denaturant) concentrations, the authors were able to extrapolate the kinetic folding parameters in the absence of denaturant. They especially wanted to understand the stabilities of the N- and C-terminal domains of γD-crystallin and γS-crystallin both alone and in the context of the full-length proteins under native conditions.

While previous work had suggested that the N-terminal domains of both crystallins exhibited lower stability than the C-terminal domains, this new set of analyses indicated this may not always be the case. Specifically, the authors demonstrated that in the full-length proteins, the N-terminal domains exhibit greater kinetic stability than the C-terminal domains; however, upon addition of denaturing chemicals, the stability of the respective domains reverses. Thus, as the protein begins to unfold, the N-terminal region loses its kinetic stability. As seen in Figure 2, this data suggests a model in which the barrier for unfolding the N-terminal domain is considerably higher in the context of the full-length crystallin as compared to the isolated domains.

Figure 2

Taken together, these data suggest that the N-terminal domain is stabilized by the interface with the C-terminal domain, indicating that this interface may have evolved to support the requirement for high kinetic stability over a lifetime in crystallin proteins. Interestingly, the double-domain γ-crystallin evolved via gene duplication and fusion (8). While the presence of the two domains certainly enhances the stability of γ-crystallins, this most recent work from Mills-Henry and colleagues demonstrates that it is specifically the interface of these two domains that may be responsible for this remarkable, high stability protein.

- Abigail Powell, Biophysical Journal Social Media Contributor