The alternative splicing product alpha Ains-crystallin is structurally equivalent to alpha A and alpha B subunits in the rat alpha-crystallin aggregate.

In rodents and some other unrelated mammals, alternative splicing of the alpha A-crystallin gene transcript results in the synthesis of the elongated alpha Ains-crystallin chain. This polypeptide is identical to the normal alpha A-crystallin chain of 173 residues, but contains an additional sequence of 23 amino acid residues inserted between positions 63 and 64. To determine the effects of this insert peptide, the structure of the rat alpha-crystallin aggregate and its subunits alpha A-, alpha Ains- and alpha B-crystallin was studied using fluorescence spectra, partial urea dissociation, and lactoperoxidase-catalysed iodination of surface residues. The data suggest that all alpha-crystallin subunits occupy equivalent positions in the protein aggregate, and that the insert peptide merely elongates the connecting peptide between the putative amino- and carboxyl-terminal domain of the alpha A-crystallin subunit.