Effect of heat treatment on the ATPase activity of various sarcoplasmic reticulum preparations

Involvement of the L6-7 loop in SERCA1a Ca2+-ATPase activation by Ca2+ (or Sr2+) and ATP

Wild-type (WT) and the double mutant D813A,D818A (ADA) of the L6-7 loop of SERCA1a were expressed in yeast, purified, and reconstituted into lipids. This allowed us to functionally study these ATPases by both kinetic and spectroscopic means, and to solve previous discrepancies in the published literature about both experimental facts and interpretation concerning the role of this loop in P-type ATPases. We show that in a solubilized state, the ADA mutant experiences a dramatic decrease of its calcium-dependent ATPase activity. On the contrary, reconstituted in a lipid environment, it displays an almost unaltered maximal calcium-dependent ATPase activity at high (millimolar) ATP, with an apparent affinity for Ca(2+) altered only moderately (3-fold). In the absence of ATP, the true affinity of ADA for Ca(2+) is, however, more significantly reduced (20-30-fold) compared with WT, as judged from intrinsic (Trp) or extrinsic (fluorescence isothiocyanate) fluorescence experiments. At low ATP, transient kinetics experiments reveal an overshoot in the ADA phosphorylation level primarily arising from the slowing down of the transition between the nonphosphorylated "E2" and "Ca(2)E1" forms of ADA. At high ATP, this slowing down is only partially compensated for, as ADA turnover remains more sensitive to orthovanadate than WT turnover. ADA ATPase also proved to have a reduced affinity for ATP in studies performed under equilibrium conditions in the absence of Ca(2+), highlighting the long range interactions between L6-7 and the nucleotide-binding site. We propose that these mutations in L6-7 could affect protonation-dependent winding and unwinding events in the nearby M6 transmembrane segment.

Effect of temperature on the reversal of the calcium ion pump in sarcoplasmic reticulum

Sarcoplasmic-reticulum vesicles were actively loaded with Ca2+ in the presence of phosphate, and the ADP-induced Ca2+ efflux and ATP synthesis were measured as a function of temperature. Arrhenius plots show break points for both processes at about 18 and 37 degrees C. Between 18 and 37 degrees C, Ca2+ efflux and ATP synthesis occur with an activation energy of 67.2-71.4 kJ/mol, whereas it is about 189-210 kJ/mol for temperatures below 18 degrees C. Above 37 degrees C, the rates of ADP-induced Ca2+ release and of ATP synthesis sharply decline until the temperature reaches about 42 degrees C. Above this temperature, the Ca2+ efflux increases again even in absence of ADP, although the synthesis of ATP is inhibited, which reflects leakiness of the vesicles. The results show that the transition temperatures for ADP-induced Ca2+ efflux and for ATP synthesis resemble those for active Ca2+ uptake, which indicates that the same coupling mechanism is involved during the inward and outward Ca2+ translocations across the membrane.