Physiological calcium concentrations regulate calmodulin binding and catalysis of adenylyl cyclase exotoxins.
Edema factor (EF) and CyaA are calmodulin (CaM)-activated adenylyl cyclase exotoxins involved in the pathogenesis of anthrax and whooping cough, respectively. Using spectroscopic, enzyme kinetic and surface plasmon resonance spectroscopy analyses, we show that low Ca(2+) concentrations increase the affinity of CaM for EF and CyaA causing their activation, but higher Ca(2+) concentrations directly inhibit catalysis. Both events occur in a physiologically relevant range of Ca(2+) concentrations. Despite the similarity in Ca(2+) sensitivity, EF and CyaA have substantial differences in CaM binding and activation. CyaA has 100-fold higher affinity for CaM than EF. CaM has N- and C-terminal globular domains, each binding two Ca(2+) ions. CyaA can be fully activated by CaM mutants with one defective C-terminal Ca(2+)-binding site or by either terminal domain of CaM while EF cannot. EF consists of a catalytic core and a helical domain, and both are required for CaM activation of EF. Mutations that decrease the interaction of the helical domain with the catalytic core create an enzyme with higher sensitivity to Ca(2+)-CaM activation. However, CyaA is fully activated by CaM without the domain corresponding to the helical domain of EF.
Shen
Yuequan
Y
Ben-May Institute for Cancer Research, The University of Chicago, Chicago, IL 60637, USA.
Lee
Young-Sam
YS
Soelaiman
Sandriyana
S
Bergson
Pamela
P
Lu
Dan
D
Chen
Alice
A
Beckingham
Kathy
K
Grabarek
Zenon
Z
Mrksich
Milan
M
Tang
Wei-Jen
WJ
D000262:Adenylyl Cyclases / Q000378:metabolism*; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000378:metabolism*; D002384:Catalysis; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D004926:Escherichia coli / Q000378:metabolism; D005098:Exotoxins / Q000378:metabolism*; D008274:Magnesium / Q000494:pharmacology; D008958:Models, Molecular; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000378:metabolism; D020349:Surface Plasmon Resonance; D014757:Viper Venoms / Q000378:metabolism
D002147:Calmodulin; D005098:Exotoxins; D011994:Recombinant Proteins; D014757:Viper Venoms; C416405:edema factor; D000262:Adenylyl Cyclases; D008274:Magnesium; D002118:Calcium