Proteolytic activation of the canine cardiac sarcoplasmic reticulum calcium pump

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Abstract
Mild trypsin treatment of canine cardiac microsomes consisting largely of sarcoplasmic reticulum vesicles produced a severalfold activation of oxalate-facilitated calcium uptake. The increase in calcium uptake was associated with an increase in ATP hydrolysis. Proteases other than trypsin were also effective although to a lesser degree. Trypsin produced a shift of the Ca2+ concentration dependency curve for calcium uptake toward lower Ca2+ concentrations, which was almost identical with that produced by phosphorylation of microsomes by cyclic AMP dependent protein kinase when the trypsin and the protein kinase were present at maximally activating concentrations. The Hill numbers (.+-. SD) of the Ca2+ dependency after treatment of microsomes with trypsin (1.5 .+-. 0.1) or protein kinase (1.7 .+-. 0.1) were similar and were not significantly different from those for untreated control microsomes (1.6 .+-. 0.1 and 1.8 .+-. 0.1, respectively). Autoradiograms of sodium dodecyl sulfate-polyacrylamide electrophoretic gels indicate that 32P incorporation into phospholamban (Mr 27.3K) or its presumed monomeric subunit (Mr 5.5 K) was markedly reduced when trypsin-treated microsomes were incubated in the presence of cyclic AMP dependent protein kinase and [.gamma.-32P]ATP compared to control microsomes incubated similarly but pretreated with trypsin inhibitor inactivated trypsin. The activation of calcium uptake by increasing concentrations of trypsin was paralleled by the reduction of phosphorylation of phospholamban. Trypsin treatment of microsomes previously thiophosphorylated in the presence of cyclic AMP dependent protein kinase and [.gamma.-35S]thio-ATP did not result in a loss of 35S label from phospholamban, which suggests that phosphorylation of phospholamban protects against trypsin attack. Trypsin treatment of microsomes prepared from rabbit fast skeletal muscle, which does not contain phospholamban, did not stimulate calcium uptake. However, autoradiograms of gels of rabbit skeletal muscle and canine cardiac microsomes incubated under conditions favorable for the formation of the 100-kilodalton acylphosphoprotein intermediate of the (Ca2+ + Mg2+)-activated ATPase reaction showed an identical pattern of effects of different concentrations of trypsin. These data suggest that trypsin cleaves similar sites on both calcium pump proteins. Therefore, the stimulatory effect of trypsin on cardiac microsomal calcium uptake does not appear to be due to a direct effect on the calcium pump protein. The data are consistent with a model in which a segment of phospholamban is in communication with the cytoplasm. If the segment is cleaved proteolytically, the basal rate of calcium transport is increased. The proteolytic activation of calcium transport would suggest that the presence of this unphosphorylated cytosolic segment has an inhibitory effect on the calcium pump.