γ-Crystallins of the chicken lens: remnants of an ancient vertebrate gene family in birds

gamma-Crystallins, abundant proteins of vertebrate lenses, were thought to be absent from birds. However, bird genomes contain well-conserved genes for S- and N-crystallins. Although expressed sequence tag analysis of chicken eye found no transcripts for these genes, RT-PCR detected spliced transcripts for both genes in chicken lens, with lower levels in cornea and retina/retinal pigment epithelium. The level of mRNA for S in chicken lens was relatively very low even though the chicken crygs gene promoter had lens-preferred activity similar to that of mouse. Chicken S was detected by a peptide antibody in lens, but not in other ocular tissues. Low levels of S and N proteins were detected in chicken lens by shotgun mass spectroscopy. Water-soluble and water-insoluble lens fractions were analyzed and 1934 proteins (< 1% false discovery rate) were detected, increasing the known chicken lens proteome 30-fold. Although chicken S is well conserved in protein sequence, it has one notable difference in leucine 16, replacing a surface glutamine conserved in other -crystallins, possibly affecting solubility. However, L16 and engineered Q16 versions were both highly soluble and had indistinguishable circular dichroism, tryptophan fluorescence and heat stability (melting temperature T-m similar to 65 degrees C) profiles. L16 has been present in birds for over 100 million years and may have been adopted for a specific protein interaction in the bird lens. However, evolution has clearly reduced or eliminated expression of ancestral -crystallins in bird lenses. The conservation of genes for S- and N-crystallins suggests they may have been preserved for reasons unrelated to the bulk properties of the lens.