Protein 4.1 is a peripheral membrane protein that strengthens the actin-spectrin based membrane skeleton of the red blood cell and also serves to attach this structure to the plasma membrane. In avian erythrocytes it exists as a family of closely related polypeptides that are differentially expressed during erythropoiesis. We have analyzed the synthesis and assembly onto the membrane skeleton of protein 4.1 and in this paper we show that its assembly is extremely rapid and highly efficient since greater than 95% of the molecules synthesized are assembled in less than 1 min. The remaining minor fraction of unassembled protein 4.1 differs kinetically and is either degraded or assembled with slower kinetics. All protein 4.1 variants exhibit a similar kinetic behavior irrespective of the stage of erythroid differentiation. Thus, the amount and the variants ratio of protein 4.1 assembled are determined largely at the transcriptional or at the translational level and not posttranslationally. During erythroid terminal differentiation the molar amounts of protein 4.1 and spectrin assembled change. In postmitotic cells, as compared with proliferative cells, far more protein 4.1 than spectrin is assembled onto the membrane-skeleton. This modulation may permit the assembly of an initially flexible membrane skeleton in mitotic erythroid cells. As cells become postmitotic and undergo the final steps of maturation the membrane skeleton may be gradually stabilized by the assembly of protein 4.1.

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