THE STRUCTURE AND INTERACTIONS OF CA2+-ATPASE

Electron crystallographic studies on membrane crystals of Ca2+-ATPase reveal different patterns of ATPase-ATPase interactions depending on e nzyme conformation. Physiologically relevant changes in Ca2+ concentra tion and membrane potential affect these interactions. Ca2+ induced di fference FTIR spectra of Ca2+-ATPase triggered by photolysis of caged Ca2+ are consistent with changes in secondary structure and carboxylat e groups upon Ca2+ binding; the changes are reversed during ATP hydrol ysis suggesting that a phosphorylated enzyme form of low Ca2+ affinity is the dominant intermediate during Ca2+ transport. A two-channel mod el of Ca2+ translocation is proposed involving the membrane-spanning h elices M2-M5 and M4, M5, M6 and M8 respectively, with separate but int eracting Ca2+ binding sites.