Molecular Structure and Function of Crystallins

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Tissues of the eye depend on closely regulated developmental pathways and families of specialized proteins. These include the crystallins of the lens. We have shown that in humans and other species, crystallins have arisen by a process called Gene Recruitment. The origins and functions of many classes of crystallins have been elucidated. We have shown that gS-crystallin is the major bg-crystallin in the adult human lens. In mouse, the gS gene is the locus of the Opj cataract. Homologous recombination, "knock-out" mice for the gS gene have been created. In mice lacking gS the organization of lens cells is disrupted and several features of lens development are aberrant. Much of this is connected to a failure of organization of the lens cytoskeleton, suggesting a key functional role for gS in normal lens development. Other g-crystallins are associated with cataract in humans. We have made recombinant protein for a gD mutant that causes two kinds of human opacity. A small surface change in the protein leads to a marked loss of solubility. Structural and biophysical studies are in progress. We are also examining a new member of the g-crystallin family, gN, which has a gene structure that combines features of both b- and g- gene families. gN is expressed in mouse lens and retina and a new antibody detects a protein of the expected size in monkey lens. However there appear to have been changes in gN gene sequence in humans that suggest a change in function during mammalian evolution. Both gS and bA2 crystallins are abundantly expressed at the RNA level in lens in all mammals examined. However bA2 protein is very low in abundance in human lens. This may be due to translational control. We are investigating the effect of a hairpin loop in the 5' UTR of the human bA2 transcript. In addition there is now evidence that both gS and bA2 are expressed much early in lens development than previously thought and may be stress inducible. We have created cell lines and transgenic mice expressing the GFP marker protein under the control of the gS and bA2 gene promoters.