Comparison of spectrin isolated from erythroid and non-erythroid sources

Abstract

Spectrin from erythrocytes and 2 other tissues (brain and intestine) were isolated from 2 distant species, pig and chicken; some structural and functional properties were compared. A quantitative antibody inhibition assay was used to determine that antibodies to mammalian red cell spectrin cross-react very poorly, if at all, with their non-erythroid (brain) counterpart and similarly antibodies to pig brain spectrin (fodrin) cross-react very weakly with erythroid spectrin. Antibodies which were directed against the 240,000-MW subunit of avian fodrin were completely inhibited with avian spectrin and vice versa. To analyze the structural relatedness of these molecules further, the chymotryptic iodinated peptide maps generated from each individual subunit were compared. Consistent with the antibody results, little (< 10%) homology was found between peptides derived from mammalian fodrin and spectrin, but there was complete homology (100%) of the peptides derived from the 240,000-MW subunits of chicken fodrin, spectrin and another related molecule from intestine, TW260/240. Whereas the peptide maps of fodrin (brain spectrin) revealed striking similarity between divergent species, suggesting a high degree of structural conservation, the peptide maps of erythrocyte spectrin were highly variable between species, indicating that it has diverged considerably in mammalian evolution. A functional activity of mammalian spectrins, the ability to bind calmodulin, was compared using 2 different assays. Whereas fodrin-calmodulin interaction can be readily demonstrated, the binding to mammalian erythroid spectrin is negligible. The high-affinity calmodulin site present on fodrin probably has been lost from spectrin in mammalian evolution.