SERCA clears calcium from the muscle cytoplasm after each contraction. It binds two calcium ions on the cytoplasmic side and closes a gate. It phosphorylates itself with ATP, then opens a second gate to the lumen. Protons move the other way. One protein chain does all of this. It swings three cytoplasmic domains and rearranges ten membrane helices.
2
Two calcium sites sit side by side inside the membrane domain.
1 H+ per Ca2+
Proton counter-transport
16Measured directly in reconstituted proteoliposomes.
10
Two of them unwind to coordinate the calcium ions.
3
A, N and P domains. They separate widely without nucleotide.
1-2 umol Ca2+ per mg protein per min
Reconstituted proteoliposomes, proportional to protein content.
10-20 mM
Lumenal calcium reached
16Asymptotic level in reconstituted vesicles.
0.1 nM
Thapsigargin inhibition
13Free thapsigargin at 1e-10 M already inhibits calcium uptake.
1
Chains in the working unit
14One polypeptide pumps on its own. Regulators bind from the membrane.