Reconstitution of Ca(2+)-dependent K+ transport in erythrocyte membrane vesicles requires a cytoplasmic protein.

We have demonstrated that calcium-dependent potassium transport in erythrocytes requires the participation of a cytoplasmic protein. Activation of calcium-dependent potassium transport causes an increase in the membrane-bound levels of this protein which is dependent on the calcium concentration and which is highly correlated (r = 0.791, p less than 0.0001) with the loss of potassium. Reconstitution of this transport pathway in sonicated erythrocyte membrane vesicles was achieved only in vesicles containing the cytoplasmic protein indicating a causal relationship in this transport system. The protein is found in high levels within the cytoplasm of erythrocytes (5.6 mg/ml red blood cells) and yet less than 1% of the protein located in the cytoplasm is required to bind to the membrane in order to initiate the potassium efflux. The analysis of rat organ homogenates demonstrated that this protein is located in most tissues with particular enrichment in adrenal glands, brain, lung, and blood. These results demonstrate that there is a cytoplasmic protein, herein named calpromotin, which is a necessary and sufficient cytoplasmic component of calcium-dependent potassium transport in erythrocytes and perhaps other tissues.