Determination of the oligomeric structure of the Ca2+ pump protein in canine cardiac sarcoplasmic reticulum membranes using radiation inactivation analysis.

The size of the Ca2+ pump protein in canine cardiac sarcoplasmic reticulum (SR) membranes was determined using target theory analysis of radiation-inactivation data. Samples of cardiac SR were irradiated in the frozen state with increasing doses of high energy electrons from a Van de Graaff accelerator. The loss of Ca2+-dependent ATPase activity and of ATP-dependent, oxalate-facilitated Ca2+ uptake (loading) with increasing irradiation dosage paralleled one another. Also, the loss of staining intensity of the predominant polypeptide (Mr = 110,000) in sodium dodecyl sulfate-polyacrylamide gel electrophoresis correlated with the loss of ATPase and loading activities. The target size by all three measurements varied between 213,000 and 229,000 Da. We conclude that the Ca2+ pump protein in canine cardiac SR is a dimer.