Calmodulin alters the cation requirement of membrane-bound guanylate cyclase in Tetrahymena

Guanylate cyclase [EC 4,6,2,1] was found to be localized principally in the surface membrane (pellicle) of Tetrahymena pyriformis and to some extent in cilia, mitochondria and microsomes. In the pellicle fraction, guanylate cyclase activity with Mn2+ was higher than that with Mg2+. Addition of calmodulin and Ca2+ to the reaction mixture resulted in marked activation of guanylate cyclase with Mg2+ and Mn2+, but the extent of enhancement by calmodulin was greater with Mg2+ than with Mn2+. The activated enzyme could use either Mg2+ or Mn2+ as an equipotent cofactor.