Sarcoplasmic reticulum Ca?ATPase is cleaved by trypsin at two sites, TI and T2. Cleavage at T1 is complete, whereas only about 50% of the Ca2+ATPase is digested at the T2 site. In the absence of Ca2+ ionophor, Ca2+-ATPase activity of the digested enzyme remains virtually unchanged. In the presence of Ca2 + ionophor, however, the calculated specific activity of the doubly cleaved Ca2+ATPase is decreased by about 40%. The decrease in Ca2+ transport activity is much more rapid than cleavage of the T2 site, and could be correlated with an increased leak of Ca2+ from the digested vesicles. We obtained evidence that this leakiness is independent of the digestion of the Ca2+ATPase itself and is presumably due to the digestion of some other components of the sarcoplasmic reticulum vesicles. Examination of steady-state phosphoenzyme levels resulting from phosphorylation by ATP and Pi, or dephosphorylation by ADP or ADP/EGTA revealed no difference between the digested and the undigested Ca2+ATPase indicating no change in the equilibria caused by the T2 cleavage. Analysis of the substrate concentration dependence of the CaZ+ATPase activity also led to the conclusion that the digestion at T2 reduced the V,,, of ATP hydrolysis but leaves the K, unchanged. The above results are consistent with the model that cleavage at the T2 site reduces the turnover rate of the Ca2+ATPase reaction cycle by about 40% by slowing down or altering the rate-limiting step without affecting the equilibrium constants of the examined steps. We found no evidence of true uncoupling of Ca2+ transport from ATP hydrolysis correlated with cleavage at the T2 site.The Ca2+ATPase of the sarcoplasmic reticulum can be selectively digested at two sites, TI (Arg-505) and T2 [l]. Each of the resulting large tryptic fragments (A2, A1 and B) possesses a different functional site: the A2 fragment has Ca2+-binding properties [2, 31, the A1 fragment bears the phosphorylation site [4] and the B fragment, labelled with fluorescein 5'-isothiocyanate [5], carries the nucleotide-binding site. Comparison of the sequences of different ion pumps shows that the T1 site is located in a region of variable sequence that may mark the boundary between the nucleotide binding and the phosphorylation domain, whereas the T2 site resides in another variable segment within the transduction domain [6,7]. The effects of the tryptic cleavages may give an insight into domain interactions required for the function of the Ca2+ATPase.Susceptibility of the Ca2+ATPase to trypsin depends on the conditions of digestion. In the presence of vanadate/EGTA [8] or inorganic phosphate/EGTA [9] the cleavage by trypsin is limited to the T1 site. In the presence of Ca2+ this is followed by a slower cleavage at the T2 site. This second cleavage is incomplete. Saito et al. [lo, 111 demonstrated that, depending on the presence or absence of nucleotides, on the concentration of Ca2+ and sucrose and on the temperature, the selectivity and extent of the T2 cleavage can be affected, i.e. sucrose reduces the ...