Abstract
Band 3, the major intrinsic protein of the human erythrocyte membrane, was specifically labeled with the covalent fluorescent probe eosin isothiocyanate. The lateral mobility of labeled band 3 in the plane of the membrane under various conditions of ionic strength and temperature was examined by using the fluorescence photobleaching recovery technique. Low temperature (21 degrees C) and high ionic strength (46 mM NaPO(4)) favored immobilization of band 3(10% mobile) as well as slow diffusion of the mobile fraction (diffusion coefficient D = 4 x 10(-11) cm(2)sec(-1)). Increasing temperature (37 degrees C) and decreasing ionic strength (13 mM NaPO(4)) led to an increase in the fraction of mobile band 3(90% mobile) and a reversible increase in the diffusion rate of the mobile fraction (D = 200 x 10(-11) cm(2)sec(-1)). The increase in the fraction of mobile band 3 was markedly dissociated, however, from the increase in the diffusion rate of the mobile fraction. Thus, the fraction of mobile band 3 always increased at higher ionic strength and lower temperature than the ionic strength and temperature at which the diffusion rate increased. This dissociation was manifested kinetically on prolonged incubation of ghosts at constant ionic strength and temperature: the diffusion rate of the mobile fraction increased slowly at first and much more rapidly after the initial lag period, whereas the fraction of mobile band 3 increased almost immediately to 90% and remained maximal for the duration of the experiment. Further, changes in diffusion rate with temperature were promptly and totally reversible, whereas increases in the mobile fraction were only slowly and partially reversible. These effects were shown not to be due to complete dissociation of spectrin, the major protein of the erythrocyte cytoskeleton, from the membrane. This evidence suggests control of band 3 lateral mobility by at least two separate processes. The process that determines the diffusion coefficient of the mobile band 3 is completely reversible, and it probably involves a metastable state of cytoskeleton structure intermediate between tight binding to the membrane and complete dissociation from it.