Rat alpha-Crystallin A Chain with an Insertion of 22 Residues

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Abstract
Rat lens .alpha.-crystallin contains a minor chain in addition to the usual .alpha.A and .alpha.B subunits. This subunit was purified by ion-exchange chromatography and its primary structure studied. It appeared to be an elongated .alpha.A-like chain with an insertion of 22 residues between position 63 and 64 of an otherwise normal .alpha.A2 chain. This subunit was called .alpha.AIns, i.e., an .alpha.A chain with an inserted sequence. This inserted region containing 3 methionyl, 5 basic and no acidic residues, apparently results in an adequately functioning .alpha.AIns chain. The .alpha.AIns chain may be the product of a gene which has originated after duplication of the .alpha.A gene by insertion in 1 of the copies of a stretch of 66 nucleotides of unknown origin. Alternatively, it may be the result of unusual transcription or processing of precursor mRNA (pre-mRNA) leaving an extra 66 nucleotides internally in the mRNA to be translated.