Fine structure of the band 3 protein in human red cell membranes: Freeze‐fracture studies

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Abstract
The major red cell membrane protein, band 3, is a glycoprotein which extends across the membrane from the extracellular space into the cytoplasmic compartment. It is widely held that band 3 is a component of the intramembrane particles (IMP) which can be demonstrated by freeze‐fracture electron microscopy. In this study, we find that the outer surface poles of the IMP can be seen by freeze‐etching after they are unmasked by proteolysis under conditions which excise the surrounding sialopeptides from the membrane. The poles appear as distinctive projections, 30–50 Å in diameter, the “ES particles.” The ES particles remain associated with the outer surface of the membrane following cleavage of the band 3 polypeptide by chymotrypsin or pronase. This is consistent with previous biochemical studies which have shown that the 38,000‐dalton outer surface segment of band 3 is intercalated in the lipid bilayer. A granulofibrillar component at the inner surface of the membrane is provisonally identified as the 40,000‐dalton inner‐surface domain of band 3.