Molecular characterization of the in situ red cell membrane calcium pump by limited proteolysis.

In inside-out red cell membrane vesicles active calcium transport and the formation of the enzyme-phosphate complex (EP) of the calcium pump were simultaneously investigated and the effects of a limited proteolytic digestion examined. In order to visualize the proteolyzed EP forms we have induced the formation of a maximum level EP from [gamma-32P]ATP in the presence of Ca2+ + La3+ and applied a good-resolution acidic discontinuous sodium dodecyl sulfate-polyacrylamide gel electrophoresis system. Proteolysis of inside-out vesicle membranes by trypsin, Pronase, papain, or chymotrypsin produces a calmodulin-like activation of the calcium pump, abolishes its calmodulin sensitivity, and decreases the original 140-kDa EP complex to a limit polypeptide of 80 kDa. Trypsin digestion produces another major intermediary fragment of 90 kDa, which is still a low-activity calmodulin-sensitive form of the pump. The red cell calcium pump is activated by trypsin both in the absence and presence of Ca2+ during digestion although the rate of activation and the appearance of the 80-kDa polypeptide are enhanced by Ca2+. If proteolytic digestion is carried out by chymotrypsin, a calmodulin-insensitive maximum activation of the calcium pump coincides with the formation of a 125-130-kDa EP-forming polypeptide. Chymotrypsin and carboxypeptidase A have synergistic effects on the formation of this latter high-activity species. Based on these data we suggest a probable molecular arrangement for the functional parts of the red cell membrane calcium pump.