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. This protein is related to cellular retinol-binding protein type I (CRBP I) but has 3,4-didehydroretinol, rather than retinol, as a ligand. The 3,4-didehydroretinol gives the lens a yellow color, thus protecting the retina by absorbing short-wave radiation. iota-Crystallin could be either the gecko's housekeeping CRBP I, recruited for an additional function in the lens, or the 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 picturatus now supports the former option. Comparison with iota-crystallin of a distantly related gecko, Gonatodes vittatus, and with mammalian CRBP I, suggests that acquiring the additional lens function is associated with increased amino acid changes. Compared with the rat CRBP I structure, the iota-crystallin model shows reduced negative surface charge, which might facilitate 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 number of replacements in the ligand pocket, recombinant iota-crystallin binds 3,4-didehydroretinol and retinol with similar and high affinity (approximately 1.6 nM). Availability of ligand thus determines whether it binds 3,4-didehydroretinol, as in the lens, or retinol, in other tissues. iota-Crystallin presents a striking example of exploiting the potential of an existing gene without prior duplication.
Werten
P J
PJ
Department of Biochemistry, University of Nijmegen, P.O. Box 9101, NL-6500 HB Nijmegen, The Netherlands.
Röll
B
B
van Aalten
D M
DM
de Jong
W W
WW
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003459:Crystallins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D018076:DNA, Complementary; D019143:Evolution, Molecular; D005123:Eye / Q000528:radiation effects*; D006801:Humans; D008024:Ligands; D008116:Lizards; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012177:Retinol-Binding Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D054840:Retinol-Binding Proteins, Cellular; D017386:Sequence Homology, Amino Acid; D014466:Ultraviolet Rays
D003459:Crystallins; D018076:DNA, Complementary; D008024:Ligands; C518456:RBP1 protein, human; C518459:RBP2 protein, human; C518458:Rbp1 protein, rat; C518461:Rbp2 protein, rat; D011994:Recombinant Proteins; D012177:Retinol-Binding Proteins; D054840:Retinol-Binding Proteins, Cellular; C406730:iota crystallin