Molecular mechanisms for the assembly of the red cell membrane skeleton during erythroid cell development

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The "unit-assembly hypothesis" for red cell membrane skeleton during erythroid development was investigated.Stable transfectants of major constituents of red cell transmembrane proteins were developed in K562 and HEK293 cells using retroviral vectors. Band 3 and glycophorinCweretransportedtotheplasmamembranevia the ER/Golgi pathway. Band 3-binding domain of ankyrin (AnkN90) and protein 4.1 showed membranous localization in the cytoplasm with co-localization with markers for the ER and the Golgi apparatus in band 3-transfeted cells despite of the difference in erythroid differentiation. Endogenous membrane skeletal proteins including ankyrin, protein 4.1, and spectrin also exhibited membranous distribution. These results indicate that the assembly of membrane skeletal proteins occurs at an early stage of erythroidal development on the surface of the ER but not on the inner surface of the plasma membrane.Stable expression of band 3 caused a reduction in a proliferation rate of K562 cells to approximately 70% that of non-transfected cells. Reduced production of hemoglobin suggestive of erythroid differentiation was also observed. However, no significant changes in intracellular localization of skeletal proteins and cell morphology was detected Synthesis and assembly to the ER of skeletal proteins involving spectrin and subsequent synthesis and localization to the plasma membrane of band 3 were observed in erythroid precursor cells from peripheral blood of cattle in two phase liquid culture system, indicating that the assembly of membrane skeletal proteins to the ER occurs in physiolosical erythropoiesis.The present study demonstrated that the assembly of the red cell membrane skeleton is an event which takes place at early stages of erythroid development on the ER. The exact mechanism by which a small unit of the ekeleton formed on intracellular vesicles are integrated in the plasma membrane remained to be clarified.