We have determined directly the effects of the inhibitory
peptide phospholamban (PLB) on the rotational dynamics of the
calcium pump (Ca-
ATPase) of cardiac sarcoplasmic reticulum (SR). This was accomplished by comparing mouse ventricular SR, which has PLB levels similar to those found in other mammals, with mouse atrial SR, which is effectively devoid of PLB and thus has much higher (unregulated)
calcium pump activity. To obtain sufficient quantities of atrial SR, we isolated the membranes from atrial
tumor cells. We used time-resolved phosphorescence anisotropy of an
erythrosin isothiocyanate label attached selectively and rigidly to the Ca-
ATPase, to detect the microsecond rotational motion of the Ca-
ATPase in the two preparations. The time-resolved phosphorescence anisotropy decays of both preparations at 25 degrees C were multi-exponential, because of the presence of different oligomeric species. The rotational correlation times for the different oligomers were similar for the two preparations, but the total decay amplitude was substantially greater for atrial
tumor SR, indicating that a smaller fraction of the Ca-
ATPase molecules exists as large aggregates. Phosphorylation of PLB in ventricular SR decreased the population of large-scale Ca-
ATPase aggregates to a level similar to that of atrial
tumor SR.
Lipid chain mobility (fluidity), detected by electron paramagnetic resonance of
stearic acid spin labels, was very similar in the two preparations, indicating that the higher
protein mobility in atrial
tumor SR is not due to higher
lipid fluidity. We conclude that PLB inhibits by inducing Ca-
ATPase lateral aggregation, which can be relieved either by phosphorylating or removing PLB.