Calmodulin selectively modulates the guanylate cyclase activity by repressing the lipid phase separation temperature in the inner half of the bilayer of rat brain synaptosomal plasma membranes.
The association of [125I-]calmodulin with rat brain synaptosomal plasma membranes, when incubated for 1 h at 25 degrees in the presence or in absence of 20 microM Ca2+, follows a sigmoid path with a Hill coefficient h = 1.79 +/- 0.12 and h = 1.72 +/- 0.11, respectively. The total association of calmodulin with the membrane increased approx. 60%-80% at all the range of calmodulin concentrations used in the presence of 20 microM Ca2+. A three fold increase of guanylate cyclase activity was shown in the presence of low concentrations of calmodulin (up to 10 nM); higher concentrations (up to 40 nM) however, led to a progressive inhibition of the enzyme activity with respect to maximal stimulation. Calmodulin increased the lipid fluidity of synaptosomal plasma membranes labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH), as indicated by the steady-state fluorescence anisotropy [(ro/r)-1]-1. Arrhenius-type plots of [(ro/r)-1]-1 indicated that the lipid separation of the membrane at 22.7 +/- 1.2 degrees was perturbed by calmodulin such that the temperature was reduced to 16.3 +/- 0.9 degrees and 15.5 +/- 0.8 degrees in the absence or in the presence of 20 microM Ca2+. Arrhenius plots of guanylate cyclase and acetylcholinesterase activities exhibited break points at 26.7 +/- 1.4 degrees and 22.3 +/- 1.0 degrees in control synaptosomal plasma membranes, respectively. The break point for the guanylate cyclase was reduced to 16.3 +/- 0.9 degrees in calmodulin treated synaptosomal plasma membranes whereas that of acetylcholinesterase remained unaffected (21.1 +/- 0.9 degrees).(ABSTRACT TRUNCATED AT 250 WORDS)
Kopeikina-Tsiboukidou
L
L
Department of Experimental Physiology, University of Athens Medical School, Greece.
Deliconstantinos
G
G
D000110:Acetylcholinesterase / Q000378:metabolism; D000818:Animals; D001921:Brain / Q000201:enzymology*; D002118:Calcium / Q000494:pharmacology; D002147:Calmodulin / Q000378:metabolism / Q000494:pharmacology*; D002462:Cell Membrane / Q000201:enzymology; D006162:Guanylate Cyclase / Q000378:metabolism*; D007700:Kinetics; D008051:Lipid Bilayers; D008563:Membrane Lipids / Q000502:physiology*; D011485:Protein Binding; D051381:Rats; D013574:Synaptosomes / Q000201:enzymology*; D013816:Thermodynamics
D002147:Calmodulin; D008051:Lipid Bilayers; D008563:Membrane Lipids; D000110:Acetylcholinesterase; D006162:Guanylate Cyclase; D002118:Calcium