Protein 7.2b of human erythrocyte membranes binds to calpromotin

Calpromotin is a soluble cytoplasmic protein of human red blood cells which is involved in the activation of the charybdotoxin-sensitive calcium-dependent potassium channel. This activation is associated with increased binding of calpromotin to the red cell membrane. To elucidate this mechanism we tested different fractions of red cell membrane proteins to bind to a calpromotin affinity column. Proteins, which bound specifically to the column, were eluted and identified by SDS polyacrylamide gel electrophoresis. This procedure demonstrated that spectrin and actin, from a low salt extraction of the membrane, bound weakly to the column and a portion of this could be attributed to non-specific binding. Glyceraldehyde-3-phosphate dehydrogenase (band 6) and a 40K molecular weight band, from a high salt extraction of the membrane, bound strongly to the affinity column. Several minor integral membrane proteins, obtained by Triton X-100 treatment of the membrane, bound specifically to the calpromotin affinity column. The molecular weight of these proteins ranged from 95k to 23K. We further demonstrated that the 31.5K band from this fraction is protein 7.2b (stomatin) by staining with a monoclonal antibody. Protein 7.2b is believed to have a role in regulating monovalent cation transport through the erythrocyte membrane.