Rat alpha-crystallin A chain with an insertion of 22 residues.

Rat lens alpha-crystallin contains, besides the usual alphaA and alphaB subunits, an additional minor chain. This subunit was purified by ion-exchange chromatography and its primary structure studied. It appeared to be an elongated alphaA-like chain, having an insertion of 22 residues between position 63 and 64 of an otherwise normal alphaA2 chain. Therefore this subunit was called alphaAIns, i.e. an alphaA chain with an inserted sequence. This inserted region, which contains three methionyl, five basic and no acidic residues, apparently results in an adequately functioning alphaAIns chain. The alphaAIns chain may be the product of a gene which has originated, after duplication of the alphaA gene, by insertion in one of the copies of a stretch of 66 nucleotides of unknown origin, or alternatively 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.