Appearance of new variants of membrane skeletal protein 4.1 during terminal differentiation of avian erythroid and lenticular cells

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Abstract
The erythrocyte plasma membrane is lined with a network of extrinsic proteins, mainly spectrin and actin, which constitute a reticulum tethered to the intrinsic anion transport protein of the lipid bilayer through a linker protein, ankyrin1–3. Protein 4.1 forms a stable ternary complex with spectrin and actin4–7, thereby strengthening the reticulum and anchoring it directly to the lipid bilayer8 or to another intrinsic protein, glycophorin . It has been found recently that spectrin10–14, ankyrin15,16 and protein 4.1 (refs 17–20) are not erythrocyte-specific; this has elucidated further the mechanisms of plasma membrane assembly and modelling during the differentiation of diverse tissues. We have shown previously18 that protein 4.1 in chickens is most abundant in eryth-rocytes and lens cells, but is scarce or absent from other spectrin-rich cell types. In addition, it exists as a family of related polypeptides showing differential expression in these two tissues, suggesting variant-specific functions. Here we show that the pattern of protein 4.1 variants changes during the terminal differentiation of eryth-roid and lenticular cells, with novel variants appearing in post-mitotic cells. The accumulation of these variants may lead to the final stabilization of the plasma membrane skeletons of these cells.