Delta crystallins and their nucleic acids
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δ-Crystallin is a major structural protein of avian and reptilian lenses that is absent from the lenses of fish, amphibia and mammals. It appears to be a tetrameric protein with a native molecular weight near 200 000 (200K) and polypeptide molecular weights near (50K and 48K) (see Note added in proof). The δ-crystallin polypeptides are extremely similar, associate in various combinations of four and are held together by hydrophobic interactions. Although principally cytoplasmic, δ-crystallin may associate with the cell membranes. δ-Crystallin differs from other lens crystallins in its α-helical content, native and subunit molecular weights, antigenicity, low wavelength of maximum fluorescence emission (315 nm) after excitation at 280 nm and amino acid composition (high in leucine; low in aromatic residues en no cysteine). Analyses of peptides, native and subunit molecular weights, and circular dichroism spectra indicate that the primary, secondary, tertiary and subunit structures of δ-crystallin have been generally conserved during evolution. There are at least two tandemly arranged δ-crystallin genes containing 13–15 introns in the chicken; a similar structure exists for a cloned δ-crystallin gene in the duck. Experiments with chicken show that δ-crystallin synthesis occurs principally in the embryo, especially during lens fiber cell differentiation. δ-Crystallin synthesis also takes place during lens fiber cell differentiation in culture. There is evidence for both transcriptional and post-transcriptional regulation of δ-crystallin synthesis. Current studies on the crystallographic and primary structures of δ-crystallin, on the structure, evolution and expression of the δ-crystallin genes, and on the translation of δ-crystallin mRNAs make this specialized lens protein an active area of investigation.
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