Gecko iota-crystallin: How cellular retinol-binding protein became an eye lens ultraviolet filter

Eye lenses of various diurnal geckos contain up to 12% iota-crystallin. Thi s protein is related to cellular retinol-binding protein type I (CRBP I) bu t has 3,4-didehydroretinol, rather than retinol, as a ligand. The 3,4-dideh ydroretinol gives the lens a yellow color, thus protecting the retina by ab sorbing short-wave radiation, iota-Crystallin could be either the gecko's h ousekeeping CRBP I, recruited for an additional function in the lens, or th e specialized product of a duplicated CRBP I gene, The finding of the same CRBP I-like sequence in lens and liver cDNA of the gecko Lygodactylus pictu ratus now supports the former option. Comparison with iota-crystallin of a distantly related gecko, Gonatodes vittatus, and with mammalian CRBP I, sug gests that acquiring the additional lens function is associated with increa sed amino acid changes, Compared with the rat CRBP I structure, the iota-cr ystallin model shows reduced negative surface charge, which might facilitat e the required tight protein packing in the lens. Other changes may provide increased stability, advantageous for a long-living lens protein, without frustrating its role as retinol transporter outside the lens. Despite a num ber of replacements in the ligand pocket recombinant iota-crystallin binds 3,4-didehydroretinol and retinol with similar and high affinity (approximat e to 1.6 nM). Availability of ligand thus determines whether it binds 3,4-d idehydroretinol, as in the lens, or retinol, in other tissues, iota-Crystal lin presents a striking example of exploiting the potential of an existing gene without prior duplication.