Oligomeric regulation of the later reaction steps of the sarcoplasmic reticulum calcium ATPase.

The phosphorylated intermediate of the sarcoplasmic reticulum Ca2+ ATPase (EP) was formed from [gamma-32P] ATP, and its decomposition was monitored by chasing with either ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or an excess amount of [gamma-31P]ATP. Chasing in the initial phase of E32P formation leads to monophasic and rapid decomposition, whereas chasing after the maximal level of E32P has been reached leads to biphasic decomposition. If a portion of EPtotal is formed from [gamma-31P]ATP (E31P) and the rest from [gamma-32P]ATP (E32P), decomposition of the later portion (E32P) is roughly monophasic and the rate of decomposition is about the same as that of the slow phase of the biphasic decomposition. Initiation of the EP reaction by concurrent addition of [gamma-32P]ATP and EGTA, conditions that permit the selective monitoring of the EP reaction via one of the subunits of the putative dimer (Ikemoto, N., Garcia, A. M., Kurobe, Y., and Scott, T. L. (1981) J. Biol. Chem. 256, 8593-8601) leads to monophasic decomposition at a rate comparable with the fast phase of biphasic decomposition. The kinetics of biphasic EP decomposition is reversibly affected by the nonionic detergent ethylene glycol monoether and the cleavable cross-linker dithiobissuccinimidyl propionate, both of which presumably affect subunit-subunit interactions. The results suggest that the biphasic EP decomposition is due to sequential liberation of Pi from different subunits (EI and EII) forming an oligomer as shown in the following model.