Introduction

Since Kepler and Descartes first investigated the optics of the eye, the central role of the lens in light refraction has been appreciated. The lens must be extremely dense to refract light in the aqueous media in which it is suspended. The necessary density is achieved by the presence of the crystallins, proteins that accumulate to concentrations of 450 mg/ml or higher in the lens fiber cell cytoplasm (Fagerholm et al., 1981; Huizinga et al., 1989; Siezen et al., 1988). Since most proteins would aggregate and strongly scatter light long before accumulating to these high concentrations, the crystallins are believed to have a number of special properties that allow for the creation of the short range order necessary for lens transparency (Tardieu and Delaye, 1988). In the past 50 years, our understanding of the molecular nature of crystallins has increased exponentially, and now much is known about the structure, function and evolutionary origin of these proteins. Before the advent of molecular biology, proteins would be designated as crystallins if their concentration in the lens was sufficient to create a major peak on a size exclusion column, a band on a SDS-PAGE gel, or a spot on a two-dimensional protein gel. Practically, this working definition designated a protein as a crystallin if its concentration in the lens reached about 5% of the total water soluble protein (de Jong et al., 1994).