Abstract
γ-Crystallins, abundant proteins of vertebrate lenses, were thought to be absent from birds. However, bird genomes contain well-conserved genes for γS- and γN-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 γS in chicken lens was relatively very low even though the chicken crygs gene promoter had lens-preferred activity similar to that of mouse. Chicken γS was detected by a peptide antibody in lens, but not in other ocular tissues. Low levels of γS and γN 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 γS is well conserved in protein sequence, it has one notable difference in leucine 16, replacing a surface glutamine conserved in other γ-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 Tm ∼ 65 °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 γ-crystallins in bird lenses. The conservation of genes for γS- and γN-crystallins suggests they may have been preserved for reasons unrelated to the bulk properties of the lens. Two genes for lens γ-crystallins are conserved in birds, although their expression is low compared with other vertebrates. The gene for γS maintains a lens-preferred promoter. γS protein sequence is well conserved. However, all birds have one unusual residue change present for over 100 Myr. This has no effect on basic structure or stability; it likely evolved for a species-specific protein interaction.
Original language | English (US) |
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Pages (from-to) | 1516-1530 |
Number of pages | 15 |
Journal | FEBS Journal |
Volume | 283 |
Issue number | 8 |
DOIs | |
State | Published - Apr 1 2016 |
Keywords
- crystallin
- evolution
- eye
- promoter
- protein folding
- proteomics
ASJC Scopus subject areas
- Biochemistry
- Molecular Biology
- Cell Biology