The sequence of human and rat βA4-crystallin and possible implications for stability

Purpose. To determine the sequence of human and rat βA4-crystallin cDNA, determine the homology between the deduced amino acid sequences, and measure the relative amounts of βA4-crystallin in human and rat lens. Methods. Lens mRNA was reverse transcribed and amplified using 5′ and 3′ RACE PCR with gene specific primers. The resulting products were cloned and the cDNA sequence determined. Soluble proteins from newborn to 3-year-old human and 11-day to 4-month-old rats were separated using two-dimensional electrophoresis. Gels were then stained with Coomassie blue for densitometry. Results. Human and rat βA4 cDNA both encoded proteins of 196 amino acids in length. The N-terminal extensions were each 11 amino acids long and were 13 amino acids shorter than the reported sequence of the bovine βA4 N-terminal extension. A similar length extension has been found for chicken βA4, thus the reported sequence of the bovine βA4 N-terminal extension may contain a cloning artifact. Excluding the N-terminal 13 amino acid residues of the reported bovine sequence, human, rat, and bovine βA4 sequences shared approximately 90% identity, but only 64-67% identity to chicken βA4. βA4 contributed 3.9% and 4.5% to the total soluble protein of newborn human and 11-day-old rat lenses, respectively. By three years of age in the human and 4 months of age in the rat the amount of βA4 was 2.5% and 5.2% of the total soluble protein, respectively. The amounts of βA4 were thus relatively stable during maturation, in marked contrast to the sharp decreases in βA3 and βB1 during the same time period. Conclusion. βA4 is highly homologous between human, rat, and bovine. The relative stability of βA4 compared to βB1 and βA3 during maturation may result from its shorter N-terminal extension. β-crystallins with shorter N-termini may be less susceptible to proteolysis.