Crystallins are separated into two classes: taxon-specific, or
enzyme, and ubiquitous. The latter class constitutes the major
proteins of vertebrate eye lens and maintains the transparency
and refractive index of the lens. Since lens central fiber cells
lose their nuclei during development, these crystallins are made
and then retained throughout life, making them extremely stable
proteins. Mammalian lens crystallins are divided into alpha,
beta, and gamma families; beta and gamma crystallins are also
considered as a superfamily. Alpha and beta families are further
divided into acidic and basic groups. Seven protein regions
exist in crystallins: four homologous motifs, a connecting
peptide, and N- and C-terminal extensions. Gamma-crystallins are
a homogeneous group of highly symmetrical, monomeric proteins
typically lacking connecting peptides and terminal extensions.
They are differentially regulated after early development. Four
gamma-crystallin genes (gamma-A through gamma-D) and three
pseudogenes (gamma-E,