The transmembrane proteins in the plasma membrane of normal human erythrocytes. Evaluation employing lactoperoxidase and proteases.

The molecular architecture of the human erythrocyte membrane has been probed using lactoperoxidase-catalyzed iodination in conjunction with Pronase hydrolysis. Resealed, hemoglobin-free ghosts were labeled at the cytoplasmic surface and the external membrane surface was subsequently digested with Pronase. Changes in size of the components labeled at the cytoplasmic surface were readily detected by sodium dodecyl sulfate gel electrophoresis. The protein 3 molecular weight class labeled at the cytoplasmic surface was extensively hydrolyzed at the external surface to produce a major 65000 molecular weight fragment and a minor 45000 molecular weight fragment. When resealed membranes were labeled on the external surface the same 65000 molecular weight labeled component is produced. These results unequivocally demonstrate that the same polypeptides in the protein 3 molecular weight class that can be labeled by lactoperoxidase at the cytoplasmic membrane surface are digested by Pronase at the external surface and are, therefore, transmembrane components. Where it is possible to label one surface of a membrane with lactoperoxidase and reseal the membrane this procedure represents an alternate method for establishing transmembrane configuration of membrane proteins.