Conspectus
Crystallins are transparent, refractive proteins that contribute
to the focusing power of the vertebrate eye lens. These proteins are
extremely soluble and resist aggregation for decades, even under crowded
conditions. Crystallins have evolved to avoid strong interprotein
interactions and have unusual hydration properties. Crystallin aggregation
resulting from mutation, damage, or aging can lead to cataract, a
disease state characterized by opacity of the lens.
Different
aggregation mechanisms can occur, following multiple
pathways and leading to aggregates with varied morphologies. Studies
of variant proteins found in individuals with childhood-onset cataract
have provided insight into the molecular factors underlying crystallin
stability and solubility. Modulation of exposed hydrophobic surface
is critical, as is preventing specific intermolecular interactions
that could provide nucleation sites for aggregation. Biophysical measurements
and structural biology techniques are beginning to provide a detailed
picture of how crystallins crowd into the lens, providing high refractivity
while avoiding excessively tight binding that would lead to aggregation.
Despite the central biological importance of refractivity, relatively
few experimental measurements have been made for lens crystallins.
Our work and that of others have shown that hydration is important
to the high refractive index of crystallin proteins, as are interactions
between pairs of aromatic residues and potentially other specific
structural features.
This Account describes our efforts to understand
both the functional
and disease states of vertebrate eye lens crystallins, particularly
the γ-crystallins. We use a variety of biophysical techniques,
notably NMR spectroscopy, to investigate crystallin stability and
solubility. In the first section, we describe efforts to understand
the relative stability and aggregation propensity of different γS-crystallin
variants. The second section focuses on interactions of these proteins
with the holdase chaperone αB-crystallin. The third, fourth,
and fifth sections explore different modes of aggregation available
to crystallin proteins, and the final section highlights the importance
of refractive index and the sometimes conflicting demands of selection
for refractivity and solubility.