Membrane intercalated particles in human erythrocyte ghosts: sites of preferred passage of water molecules at low temperature.

Although hydrophilic pores have been inferred to account for characteristics of the movement of water molecules across erythrocyte membranes, no direct evidence has associated such pores with actual structural differentiations within the membrane. Freeze-fracture and freeze-etch studies of isolated erythrocyte membranes show that they are comprised of fluid bilayer domains tranversed by protein-containing intercalations ("membrane intercalated particles"). It has previously been hypothesized that the topology of the polar and apolar spaces of the membrane-intercalated particles was not concentric the hydrophobic spaces being equatorially distributed. In consequence, axial organization of the hydrophilic regions could result in hydrophilic continuity across the membrane and might provide a structural basis for passage of hydrophilic molecules. The present experiments support this hypothesis. It is shown that sublimation at -100 degrees of erythrocyte membrane suspensions (that had been incubated at pH 5.5 to cause aggregation of the membrane particles) results in progressive and selective sinking of the membrane regions comprised of aggregates of intercalated particles, i.e., that sublimation of water molecules occurs preferentially across these membrane regions. These results indicate that, under these experimental conditions, the membrane-intercalated particles provide a preferential structural pathway for passage of water molecules across erythrocyte ghost membranes.