Spectrin is the major protein of the membrane erythrocyte skeleton. More recently, homologous but non-identical spectrins (fodrins) were also found in various non-erythroid tissues. Spectrin mRNA in erythroid and various non-erythroid cells was examined by direct hybridization with human a-spectrin, p-spectrin (erythroid spectrins), and a-fodrin (non-erythroid spectrin) cDNA probes. Northern blot analysis of poly (A)' RNA revealed a distinct pattern of expression in erythroid vs. non-erythroid cells. Erythroid cells from early erythroblasts to reticulocyte stage expressed two mRNA species of P-spectrin, whereas they expressed only a single species of a-spectrin, and no a-fodrin mRNA. In contrast, nonerythroid cells (platelets, myeloid cells, liver, muscle, heart, cerebellum, and eye lens) expressed either no a-spectrin mRNA or a different molecular weight transcript(s) of this gene, and a single species of a-fodrin mRNA. Additionally, they also expressed from none to multiple species of P-spectrin, and these were of different molecular size(s) from that found in erythroid cells (with the exception of platelets). Transcripts of non-erythroid spectrin, a-fodrin, were found as a single copy only in non-erythroid tissues. Human and murine erythroleukemia cells expressed both erythroid spectrin transcripts in addition to a-fodrin and raise the possibility that erythroid progenitors may have the potential to express both erythroid and non-erythroid species. These data indicated that several mRNA species of P-spectrin could be detected in both erythroid and some non-erythroid cells.
Whether multiple spectrin peptides could also be found with functional heterogeneity is unclear. However, in each case, the pattern combination observed appeared to be tissue-specific Spectrin is the major protein of the membrane skeleton which underlies the internal side of the erythrocyte membrane (3,12,35,41). Erythroid spectrin is the fibre-like protein composed of two 240,000 and 220,000 dalton homologous but non-identical polypeptides, a and p, which are twisted along each other into a het-