Isolation and partial characterization of a high molecular weight red cell membrane protein complex normally removed by the spleen

Red cell membranes from all splenectomized and some functionally asplenic persons contained a small proportion of a very high molecular weight, sodium dodecyl sulfate (SDS)-insoluble protein aggregate not found in normals. This complex averaged 3.6% ± 1.7% (SD) of the total red cell membrane protein (range 1.2%-6.3%). It was identified in the marrow reticulocytes, but not the circulating red cells, of a patient with a normal spleen. In splenectomized individuals it was present equally in young and old circulating erythrocytes. When ghosts were disrupted and banded in a sucrose density gradient, the high molecular weight protein complex remained with the membrane fraction, indicating that it was not a particulate cytoplasmic contaminant. It was accessible to iodination by lactoperoxidase only at the inner membrane surface. The complex was remarkably resistant to dissociation. It was partially disaggregated by reduction in SDS with the release of the membrane protein, spectrin. Judging from its amino acid composition, however, the complex must contain one or more additional proteins which remain to be identified. These studies suggest that reticulocyte membranes contain a high molecular weight aggregate of spectrin and other membrane proteins and that splenic “surface remodeling” includes the removal of this protein complex as well as membrane lipids. The complex may prove to be a useful tool for further study of this poorly understood splenic function and for clinical assessment of splenic hypofunction.