Multifunctional Lens Crystallins and Corneal Enzymes: More than Meets the Eye

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Abstract
Abstract: The abundant water‐soluble proteins, called crystallins, of the transparent, refractive eye lens have been recruited from metabolic enzymes and stress‐protective proteins by a process called “gene sharing.” Many crystallins are also present at lower concentration in nonocular tissues where they have nonrefractive roles. The complex expression pattern of the mouse αB‐crystallin/small heat shock protein gene is developmentally controlled at the transcriptional level by a combinatorial use of shared and lens‐specific regulatory elements. A number of crystallin genes, including that for αB‐crystallin, are activated by Pax‐6, a conserved transcription factor for eye evolution. Aldehyde dehydrogenase class 3 and transketolase are metabolic enzymes comprising extremely high proportions of the water‐soluble proteins of the cornea and may have structural as well as enzymatic roles, reminiscent of lens enzyme‐crystallins. Inductive processes appear to be important for the corneal‐preferred expression of these enzymes. The use of the same protein for entirely different functions by a gene‐sharing mechanism may be a general strategy based on evolutionary tinkering at the level of gene regulation.