Alternative splicing of alpha A-crystallin RNA. Structural and quantitative analyses of the mRNAs for the alpha A2- and alpha Ains-crystallin polypeptides.

The eye lens contains a structural protein (alpha-crystallin) composed of two homologous primary gene products, alpha A2 and alpha B2. In certain rodents, there is another alpha-crystallin polypeptide, alpha Ains which is identical with alpha A2 except that it contains an additional peptide between residues 63 and 64 of the alpha A2 chain. The alpha A-crystallin gene encodes the alpha Ains peptide in a separate 69-base pair exon, suggesting that the alpha A2 and alpha Ains mRNAs are derived by alternative RNA splicing. In the present study, we report the isolation of a cloned cDNA (pM alpha AinsCr1) from a cDNA library constructed in the bacterial plasmid pBR322. The nucleotide sequence of pM alpha AinsCr1 in the region of the insert peptide provides compelling evidence that the alpha Ains mRNA is derived from the same gene as the alpha A2 mRNA. S1 nuclease protection experiments, using a DNA fragment from pM alpha AinsCr1, showed that the alternative splicing gives 5 to 10 times more alpha A2 than alpha Ains mRNA. This ratio is comparable to that of the respective polypeptides in the lens. We did not detect an age-related or a differentiation-related difference in the ratio of the alpha A2 to the alpha Ains mRNA or their polypeptides. These results indicate that when the alpha A-crystallin gene is expressed in the lens, it splices the RNA sequences from the insert exon into functional mRNA 10 to 20% of the time.