Formation and properties of tetramers of band 3 protein from human erythrocyte membranes in planar lipid bilayers.

Lipid bilayer experiments were performed in the presence of solubilized band 3 protein from human red cell membranes. Band 3 protein increased the conductance of the lipid membranes by several orders of magnitude. Membrane conductance was found to be dependent on the fourth power of protein concentration. This shows that four band 3 subunits form an ion permeable pathway in the lipid bilayer membranes. It also shows that, in the membranes, the protein molecules undergo an association equilibrium which involves at least the monomer and the tetramer of the protein, relaxation towards equilibrium being rapid on the time scale of the experiment. The increase in bilayer conductance induced by the band 3 tetramer could be inhibited by pretreatment of the protein with several SH-reagents (pCMB, pCMBS, DTNB) which also inhibit water transport across the human red cell membrane. Other SH-reagents which do not influence water transport (iodoacetamide, N-ethylmaleimide) did not show any influence on the band 3 induced conductance increase. A band 3-mediated exchange of anions comparable to that in the erythrocyte membrane did not occur in the system studied by us. Our results suggest that, in the human erythrocyte membrane, a pore formed by the band 3 tetramer could be the pathway responsible for the protein-mediated part of water transport.