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

The β and γ 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 γ-crystallins expressed in mammal, reptile, bird and fish species (particularly in the zebrafish, Danio rerio) has led to the discovery of γN-crystallin, an evolutionary bridge between the β and γ families. In all species examined, γN-crystallins have a hybrid gene structure, half β and half γ, and thus appear to be the 'missing link' between the β and γ crystallin lineages. Overall, there are four major classes of γ-crystallin: the terrestrial group (including mammalian γA-F); the aquatic group (the fish γM-crystallins); the γS group; and the novel γN group. Like the evolutionarily ancient β-crystallins (but unlike the terrestrial γA-F and aquatic γM groups), both the γS and γN crystallins form distinct clades with members in fish, reptiles, birds and mammals. In rodents, γN is expressed in nuclear fibers of the lens and, perhaps hinting at an ancestral role for the γ-crystallins, also in the retina. Although well conserved throughout vertebrate evolution, γN in primates has apparently undergone major changes and possible loss of functional expression.