Review: A case for corneal crystallins.

It is established that the diverse, multifunctional crystallins are responsible for the optical properties of the cellular, transparent lens of the complex eyes of vertebrates and invertebrates. Lens crystallins often differ among species and may be enzymes or stress proteins. I present here the idea that abundant water-soluble enzymes and other proteins may also be used for cellular transparency in the epithelial cells and, possibly, stromal keratocytes of the cornea. Aldehyde dehydrogenases and transketolase are among the putative "corneal crystallins" in mammals, and gelsolin may be a corneal crystallin in the zebrafish. In invertebrates, the glutathione S-transferase-related S-crystallins of the lens appear to be used also as corneal crystallins in the squid, and an aldehyde dehydrogenase-related protein is the crystallin in the lens and, possibly, cornea of the scallop. The use of abundant, taxon-specific water-soluble proteins as crystallins for cellular transparency in the cornea would provide a new conceptual link between this tissue and the lens.