Segmental dynamics of the cytoplasmic domain of erythrocyte band 3 determined by time-resolved fluorescence anisotropy: sensitivity to pH and ligand binding

Abstract

Interactions between the erythrocyte membrane and its skeleton are mediated primarily by binding of cytoskeletal components to a conformationally sensitive structure, the cytoplasmic domain of band 3 (cdb3). To examine the nanosecond segmental motions of cdb3, band 3 was labeled selectively by fluorescein maleimide at Cys-201 near the proposed hinge in cdb3 about which pH-dependent conformational changes occur. Time-resolved anisotropy of labeled cdb3 in isolated form and in stripped erythrocyte membranes was measured by parallel-acquisition frequency-domain microfluorimetry. Samples had a single-component fluorescein lifetime of ≈4 ns. Multifrequency phase and modulation data (5-200 MHz) fitted well to a segmental motion model containing two correlation times (τ_1c and τ_2c) and two limiting anisotropies (r1infinity and r2infinity). Measurements in protease-cleaved and denatured samples indicated that τ_1c(100-150 ps) corresponded to rapid rotation of bound fluorescein and τ_2c(2-5 ns) corresponded to segmental motion of cdb3. Both motions were hindered as quantified by nonzero r_1∞ and r_2∞. The strong pH dependence of segmental motion correlated with that of cdb3 conformation measured by intrinsic tryptophan fluorescence. Significant changes in cdb3 segmental motion occurred upon interactions with the small ligands 2,3-bisphosphoglycerate and calcium and several glycolytic enzymes known to bind to the N terminus of band 3. These time-resolved fluorescence measurements of the nanosecond segmental dynamics of a labeled membrane protein provide evidence for the sensitivity of cdb3 conformation to ligand binding and suggest long-range structural communication through cdb3.