Erythropoietin enhances the assembly of alpha,beta spectrin heterodimers on the murine erythroblast membranes by increasing beta spectrin synthesis.

Previous biosynthetic studies have revealed that in both mammalian and chicken erythroid cells, alpha spectrin is synthesized in 2-3-fold excess over beta spectrin. However, in the membrane skeleton, the two polypeptides are assembled in equimolar amounts, suggesting that the association of alpha spectrin with the membrane skeleton is rate-limited by the amount of beta spectrin synthesized. Here we have analyzed the synthesis and transcription of alpha and beta spectrin in Friend virus-infected murine erythroblasts (FVA cells) in vitro during the erythropoietin-dependent period of maturation. Erythropoietin (Epo) increases the synthesis of beta spectrin but not alpha spectrin, thereby altering the alpha/beta spectrin synthetic ratio. When immature FVA cells are exposed for 24 h to a standard dose of 0.2 unit/ml Epo for maximum effect, the mRNA content and the synthesis of beta spectrin are increased resulting in about 1.3-1.5-fold excess of beta spectrin over alpha spectrin in the cytosol. On the membrane, the incorporation of both alpha and beta spectrins is increased equally by about 30-35%. Furthermore, nuclear run-off transcription measurements indicate that the increased beta spectrin gene expression is regulated at the level of transcription. We conclude that in cells with strong Epo stimulus, the expression and synthesis of beta spectrin are increased resulting in an increase in the amount of alpha,beta spectrin heterodimers available for membrane assembly.