Membrane Dynamics of the Water Transport Protein Aquaporin-1 in Intact Human Red Cells

Abstract

Aquaporin-1 (AQP1) is the prototype integral membrane protein water channel. Although the three-dimensional structure and water transport function of the molecule have been described, the physical interactions between AQP1 and other membrane components have not been characterized. Using fluorescein isothiocyanate–anti-Co3 (FITC-anti-Co3), a reagent specific for an extracellular epitope on AQP1, the fluorescence photobleaching recovery (FPR) and fluorescence imaged microdeformation (FIMD) techniques were performed on intact human red cells. By FPR, the fractional mobility of fluorescently labeled AQP1 (F- αAQP1) in the undeformed red cell membrane is 66 ± 10% and the average lateral diffusion coefficient is (3.1 ± 0.5) × 10 −11 cm 2/s. F- αAQP1 fractional mobility is not significantly affected by antibody-induced immobilization of the major integral proteins band 3 or glycophorin A, indicating that AQP1 does not exist as a complex with these proteins. FIMD uses pipette aspiration of individual red cells to create a constant but reversible skeletal density gradient. F- αAQP1 distribution, like that of lipid-anchored proteins, is not at equilibrium after microdeformation. Over time, ∼50% of the aspirated F- αAQP1 molecules migrate toward the membrane portion that had been maximally dilated, the aspirated cap. Based on the kinetics of migration, the F- αAQP1 lateral diffusion coefficient in the membrane projection is estimated to be 6 × 10 −10 cm 2/s. These results suggest that AQP1 lateral mobility is regulated in the unperturbed membrane by passive steric hindrance imposed by the spectrin-based membrane skeleton and/or by skeleton-linked membrane components, and that release of these constraints by dilatation of the skeleton allows AQP1 to diffuse much more rapidly in the plane of the membrane.