Limited tryptic digestion of Ca2+,Mg2+-adenosine triphosphatase of the sarcoplasmic reticulum: enzymatic properties of A1b + B complex

Sarcoplasmic reticulum membranes were treated with trypsin under conditions leading to accumulation of B and three other fragments a little smaller than A1, namely A1a, A1b, and C (Mr 27,000-28,000) (Saito, K. et al. (1984) J. Biochem. 95, 1297-1304), and enzymatic properties of trypsin-digested ATPase were investigated. The tryptic cleavage pattern of SR membranes in the presence of 1 M glycerol and 5 mM CaCl2 at 35 degrees C was qualitatively similar to that obtained in the presence of Ca2+ alone. However, considerably more A1-derived fragments, A1a and A1b, which are stabilized by the binding of Ca2+ to the enzyme, were accumulated. The sample digested under this condition for 60 min was mainly composed of A1b and B, and was designated as A1b + B complex. ATPase activity was lost in parallel with the formation of A1a and A1b. On the other hand, E-P forming activity was still retained by A1b + B complex. E-P formation with this complex was strictly dependent on the presence of Ca2+ ions at micromolar concentration. This indicates that Ca2+ binding site is well conserved in this complex. E-P formed with A1b + B complex was ADP-sensitive (E1-P), and was not further decomposed, since the transition from E1-P to E2-P was blocked.