Calmodulin stimulation of Ca2+-dependent ATP hydrolysis and ATP-dependent Ca2+ transport in synaptic membranes.
We report here characterization of calmodulin-stimulated Ca2+ transport activities in synaptic plasma membranes (SPM). The calcium transport activity consists of a Ca2+-stimulated, Mg2+-dependent ATP hydrolysis coupled with ATP-dependent Ca2+ uptake into membraneous sacs on the cytosolic face of the synaptosomal membrane. These transport activities have been found in synaptosomal subfractions to be located primarily in SPM-1 and SPM-2. Both Ca2+-ATPase and ATP-dependent Ca2+ uptake require calmodulin for maximal activity (KCm for ATPase = 60 nM; KCm for uptake = 50 nM). In the reconstituted membrane system, KCa was found to be 0.8 microM for Ca2+-ATPase and 0.4 microM for Ca2+ uptake. These results demonstrate for the first time the calmodulin requirements for the Ca2+ pump in SPM when Ca2+ ATPase and Ca2+ uptake are assayed under functionally coupled conditions. They suggest that calmodulin association with the membrane calcium pump is regulated by the level of free Ca2+ in the cytoplasm. The activation by calmodulin, in turn, regulates the cytosolic Ca2+ levels in a feedback process. These studies expand the calmodulin hypothesis of synaptic transmission to include activation of a high-affinity Ca2+ + Mg2+ ATPase as a regulator for cytosolic Ca2+.
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D001921:Brain / Q000378:metabolism*; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002135:Calcium-Binding Proteins / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013570:Synaptic Membranes / Q000378:metabolism*
D002135:Calcium-Binding Proteins; D002147:Calmodulin; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium