Homology between the primary structures of the major bovine β‐crystallin chains

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Abstract
Partial amino acid sequences of 6 major subunits of bovine .beta.-crystallin were determined by automatic liquid-phase Edman degradation and the dansyl-Edman procedure, complemented by amino acid analyses of peptides. The results show that, including the previously established .beta.Bp sequence [Driessen et al. (1981)] there exists at least 7 primary gene products in bovine .beta.-crystallin, which exhibit 40% or more sequence homology. Two of the gene products are completely identical except for the presence in 1 of them of 17 additional residues at the N terminus, possibly caused by differential splicing of the same primary RNA transcript. The rate of evolutionary change of the .beta. chains (4% sequence change/100 .times. 106 yr) is about equally slow as that of .alpha.-crystallin, and the gene duplications giving rise to the different chains must have occurred very early in vertebrate evolution. The .beta. chains can be divided into 2 groups, according to sequence homology and presence of deletions/insertions and C-terminal extension, on which basis a new, rational nomenclature for the .beta. subunits is introduced. The N-terminal extensions of all .beta. chains are very different in length and sequence, even between homologous .beta. chains in different species. Possible explanations for this finding are discussed.