Remarkable homology among the internal repeats of erythroid and nonerythroid spectrin

A cDNA clone for nonerythroid alpha-spectrin was identified by direct immunological screening of a chicken smooth muscle cDNA library. A library prepared in the expression plasmids pUC8 and pUC9 was screened with an antiserum specific for chicken alpha-spectrin. Blots of poly(A)+ RNA from various tissues of chicken and mouse show that the cDNA hybridizes to an 8-kilobase mRNA. The cDNA hybridizes to a single-copy sequence on Southern blots of chicken genomic DNA. The complete nucleic acid sequence of the clone has a single 1419-base open reading frame. The derived amino acid sequence is organized into two partial and three complete 106-amino-acid repeats that show homology to the repeats described for human erythroid alpha- and beta-spectrin. Immunological and biochemical data indicate that chicken nonerythroid and human erythroid alpha-spectrin are two of the more widely diverged members of the spectrin family of proteins. In this respect, the degree of homology found between them was unexpected. Our data suggest a common evolutionary origin for these two alpha-spectrins and allow some predictions concerning spectrin gene structure.