CyaC, a redox‐regulated adenylate cyclase of <i>Sinorhizobium meliloti</i> with a quinone responsive diheme‐B membrane anchor domain

Abstract

The nucleotide cyclase CyaC of Sinorhizobium meliloti is a member of class III adenylate cyclases (AC), a diverse group present in all forms of life. CyaC is membrane-integral by a hexahelical membrane domain (6TM) with the basic topology of mammalian ACs. The 6TM domain of CyaC contains a tetra-histidine signature that is universally present in the membrane anchors of bacterial diheme-B succinate-quinone oxidoreductases. Heterologous expression of cyaC imparted activity for cAMP formation from ATP to Escherichia coli, whereas guanylate cyclase activity was not detectable. Detergent solubilized and purified CyaC was a diheme-B protein and carried a binuclear iron-sulfur cluster. Single point mutations in the signature histidine residues caused loss of heme-B in the membrane and loss of AC activity. Heme-B of purified CyaC could be oxidized or reduced by ubiquinone analogs (Q₀ or Q₀ H₂ ). The activity of CyaC in bacterial membranes responded to oxidation or reduction by Q₀ and O₂ , or NADH and Q₀ H₂ respectively. We conclude that CyaC-like membrane anchors of bacterial ACs can serve as the input site for chemical stimuli which are translated by the AC into an intracellular second messenger response.