Multifunctional lens crystallins and corneal enzymes. More than meets the eye.
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 alpha B-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 alpha B-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.
Piatigorsky
J
J
Laboratory of Molecular and Development Biology, National Eye Intitute, National Institutes of Health, Bethesda, Maryland 20892-2730, USA. joram@helix.nih.gov
D000818:Animals; D003315:Cornea / Q000201:enzymology* / Q000502:physiology; D003459:Crystallins / Q000235:genetics / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology; D019143:Evolution, Molecular; D005136:Eye Proteins / Q000235:genetics / Q000502:physiology; D005786:Gene Expression Regulation; D006360:Heat-Shock Proteins / Q000235:genetics; D018398:Homeodomain Proteins; D006801:Humans; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; D012097:Repressor Proteins; D012995:Solubility
D003459:Crystallins; D004268:DNA-Binding Proteins; D005136:Eye Proteins; D006360:Heat-Shock Proteins; D018398:Homeodomain Proteins; D000071841:PAX6 Transcription Factor; C000605730:PAX6 protein, human; D051761:Paired Box Transcription Factors; C000605732:Pax6 protein, mouse; D012097:Repressor Proteins