A subset of GAF domains are evolutionarily conserved sodium sensors

Summary

Most organisms maintain a transmembrane sodium gradient for cell function. Despite the importance of Na⁺ in physiology, no directly Na⁺-responsive signalling molecules are known. The CyaB1 and CyaB2 adenylyl cyclases of the cyanobacterium Anabaena PCC 7120 are inhibited by Na⁺. A D360A mutation in the GAF-B domain of CyaB1 ablated cAMP-mediated autoregulation and Na⁺ inhibition. Na⁺ bound the isolated GAF domains of CyaB2. cAMP blocked Na⁺ binding to GAF domains but Na⁺ had no effect on cAMP binding. Na⁺ altered GAF domain structure indicating a mechanism of inhibition independent of cAMP binding. ΔcyaB1 and ΔcyaB2 mutant strains did not grow below 0.6 mM Na⁺ and ΔcyaB1 cells possessed defects in Na⁺/H⁺ antiporter function. Replacement of the CyaB1 GAF domains with those of rat phosphodiesterase type 2 revealed that Na⁺ inhibition has been conserved since the eukaryotic/bacterial divergence. CyaB1 and CyaB2 are the first identified directly Na⁺-responsive signalling molecules that function in sodium homeostasis and we propose a subset of GAF domains underpin an evolutionarily conserved Na⁺ signalling mechanism.