Cloning and characterization of a cow beta crystallin cDNA

We have isolated and sequenced a beta crystallin cDNA clone derived from the mRNA of cow lenses. Comparisons of the deduced amino acid sequence with the amino acid sequences of the principal beta crystallins of the cow, Beta Bp (1) and the mouse (2,3), confirm the general homology within the beta crystallin family and with the gamma crystallins. This beta crystallin cDNA, designated pBeta25/23, has 96% amino acid homology with the murine beta23 and only 43% amino acid homology with the cow beta Bp. The N-terminal 14 amino acids of the murine beta23, which are composed of a high percentage of hydrophobic amino acid residues, bear no similarity to the predicted amino acid sequence of the cow beta crystallin cDNA clone which we have isolated. The remaining amino acid sequences show greater homology between the mouse and cow beta crystallins than the corresponding alpha and beta globins between the two species. The degree of homology is comparable to that of the alpha crystallins for the cow and mouse. The pBeta25/23 cDNA is interesting for the presence of two potential translation initiation sites which are in phase and, if both are used, would code for two polypeptides of MW 25,100 and 23,200. These correspond to the 25,000 and 23,000 dalton beta crystallins that have been partially sequenced by Berbers et al. (4). This raises the possibility that the beta 25 and the beta23 synthesized in the cow lens are not encoded by two separate genes but may be derived from single mRNA using two translation initiation sites or by post-translational processing of the larger peptide. It is possible also that the two proteins are from two mRNA's derived from a single hnRNA by differential splicing of a intervening sequence at the 5' end of the gene.