Primary structures of the α‐crystallin A chains of twenty‐eight mammalian species, chicken and frog

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Abstract
The amino acid sequences of the α-crystallin A chains of 28 mammalian species, representing 14 different orders, have been analyzed, mainly on the basis of amino acid compositions of the composing peptides. The αA sequences of chicken and a frog have been completely determined by Edman degradation. A method is described to transport eye lenses, to be used for protein sequence studies, at ambient temperature in a solution of guanidine · HCl. The number of cysteine residues in different αA chains could be determined by alkaline urea gel electrophoresis after aminoethylation. In some cases the αA chains have been isolated from total lens extracts in a single ion-exchange chromatographic step. The average rate of substitutions in the evolution of the αA chains is moderately slow, approximately 3 amino acid substitutions per 100 residues in 100 million years, but varies considerably in different lineages. Substitutions involving changes in charge are strongly underrepresented; the αA chains tend to keep their net charge constant throughout evolution. Analysis of the types of substitutions suggests a directional trend leading to an increase in functional density of αA in the course of evolution.