| Summary: | The crystallins are the super-abundant structural proteins of vertebrate lens fibre cells, but certain of the crystallins are found at very much lower levels in some non-lens tissues. Chick α-, β and δ-crystallins are each encoded by a multi-gene family and their expression is developmentally regulated resulting in the changing pattern of crystallin composition of the successively formed lens fibre cells. The presence of α-, β- and δ-crystallins in extralenticular chick tissues, including the retina, is examined here at the level of individual crystallin RNAs by Northern, dot-blot and <i>in situ</i> hybridisation, and the accumulated crystallin polypeptides are examined by Western blotting and immunohistochemistry. αA-crystallin, several β-crystallins and both δ1 and δ2-crystallins are found to be expressed in non-lens chick tissues and the quantitative balance of δ-crystallin and individual β-crystallin polypeptides in non-lens tissues is found to be tissue-specific. The relative steady-stage levels of δ1- and δ2-crystallin mRNA are found to differ between lens and non-lens tissue and to be effected by transcriptional and post-transcriptional mechanisms. Selective inhibition of αA-crystallin synthesis using antisense oligonucleotides <i>in vitro</i> shows that αA-crystallin appears to be required for the correct differentiation of both developing lens and retina. The pattern of α-, β- and δ-crystallin expression in non-lens tissues is interpreted in terms of the possible status of crystallins as multifunctional proteins and is used to question current theories concerning the evolutionary origin, expression and current functions of these proteins.
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