Book of Abstracts: Albany 2009
June 16-20 2009
© Adenine Press (2008)
On the Nature of the Protein-Protein Interactions in Cataract
Several mutations in the human γD-crystallin (HGD) gene have been associated with childhood cataract. We have been examining these mutant proteins in order to understand the molecular mechanisms underlying the pathology. We find that mutations (a) alter the interactions among mutant protein molecules (i.e. like-like, homologous interactions) such that protein solubility is compromised, or (b) change the interactions with other crystallins (i.e. like-unlike, heterologous interactions), with both effects leading to increased light scattering and opacity. The common theme that emerges however, is that the global protein fold of the mutant crystallins remains largely intact but some ?sticky? patches are created on the protein surface.
We have published several examples of homologous interactions [1-3], the latest being the P23T mutation in which protein aggregates are formed, held together by net hydrophobic interactions. In this case we have now defined the sticky patches on the surface of the protein that are likely to promote aggregation. In contrast, we find that in other mutations, for example, the E107A mutation, such homologous interactions are not observed. This raises the question as to how such mutations lead to increased light scattering. To address this problem we examined the heterologous interactions between E107A and the molecular chaperone, α-crystallin, and found that the mechanism of light scattering in this case is more complex. Phase diagrams of E107A with α-crystallin at protein concentrations and compositions approaching that in the lens, show clear differences compared to similar mixtures of HGD and α-crystallin. Due to the loss of a negative charge in the protein as a result of the mutation, the net attractive interactions between E107A and α-crystallin increase. These in turn lead to an altered phase diagram. Based on molecular dynamics calculations, Stradner et al.  predicted that increased attractive interactions such as those between E107A and α-crystallin, would lead to an altered phase-diagram and increased light-scattering, due to the thermodynamic instability of these protein mixtures.
Our studies reveal that subtle changes in protein-protein interactions due to genetic mutations rather than global protein unfolding can clearly lead to serious pathological effects.
References and Footnotes
Priya R. Banerjee
Dept. of Chemistry