γN-crystallin and the evolution of the βγ-crystallin superfamily in vertebrates

The beta and gamma crystallins are evolutionarily related families of proteins that make up a large part of the refractive structure of the vertebrate eye lens. Each family has a distinctive gene structure that reflects a history of successive gene duplications. A survey of gamma-crystallins expressed in mammal, reptile, bird and fish species (particularly in the zebrafish, Danio rerio) has led to the discovery of gamma N-crystallin, an evolutionary bridge between the beta and gamma families. In all species examined, gamma N-crystallins have a hybrid gene structure, half beta and half gamma, and thus appear to be the 'missing link' between the beta and gamma crystallin lineages. Overall, there are four major classes of gamma-crystallin: the terrestrial group (including mammalian gamma A-F); the aquatic group (the fish gamma M-crystallins); the gamma S group; and the novel gamma N group. Like the evolutionarily ancient beta-crystallins (but unlike the terrestrial gamma A-F and aquatic gamma M groups), both the gamma S and gamma N crystallins form distinct clades with members in fish, reptiles, birds and mammals. In rodents, gamma N is expressed in nuclear fibers of the lens and, perhaps hinting at an ancestral role for the gamma-crystallins, also in the retina. Although well conserved throughout vertebrate evolution, gamma N in primates has apparently undergone major changes and possible loss of functional expression.