alpha A-crystallin is expressed in non-ocular tissues

Abstract

alpha-Crystallin, the predominant structural protein of the ocular lens, has been considered to be composed of two subunits, alpha A-crystallin and alpha B-crystallin. Of these two, alpha B-crystallin has been previously shown to be an extralenticular protein while alpha A-crystallin has been considered to be a lens-specific polypeptide. Using an antiserum directed against an N-terminal peptide of alpha-crystallin, we have detected a 20-kDa protein in various rat tissues including the brain, liver, lung, spleen, skin, and small intestine and in a number of established epithelial and fibroblast cell lines. PCR analysis of poly(A)-enriched RNA and Southern blot analysis indicated the presence of alpha A-crystallin mRNA sequences in different non-lenticular tissues. Among the non-ocular tissues examined, spleen showed the highest levels of alpha A-crystallin protein and mRNA. The identity of alpha A-crystallin sequences in the spleen was established by cloning and sequencing a polymerase chain reaction-amplified region of alpha A-crystallin mRNA. Sequences derived from spleen and eye revealed almost 100% identity at the nucleotide level. Interestingly, alpha A-crystallin and alpha B-crystallin seem to exist in an inverse quantitative relationship in the spleen and the heart, the two non-ocular tissues where they show highest concentrations, respectively. The known conserved evolution of alpha A-crystallin and the definitive demonstration of the non-ocular expression of this polypeptide suggest important non-crystallin functions for this protein.