The escherichia coli adenylyl cyclase complex: Stimulation by GTP and other nucleotides

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Abstract
Escherichia coli cells permeabilized by treatment with low concentrations of toluene contain an adenylyl cyclase activity that can be stimulated 3.6–7.6‐fold by GTP. The stimulatory effect of GTP is maximal at concentrations of the nucleotide in the physiological range (above 0.7 mM). Studies of the dependence of velocity on substrate (ATP) concentration indicate that the velocity vs. substrate plots are sigmoid in the absence of GTP but hyperbolic in the presence of GTP, suggesting an allosteric regulatory site that can be occupied by either ATP or GTP. Replacement of ATP by AMPPNP as substrate results in velocity vs. substrate plots that are hyperbolic in the absence or presence of GTP, although GTP increases the Vmax by a factor of 2.2; these findings indicate that AMPPNP specifically occupies the substrate site and GTP exclusively occupies the regulatory site. A test of the capacity of other guanine nucleotides to stimulate adenylyl cyclase activity showed that 2′‐deoxy‐GTP was almost as effective as GTP, but that GDP, GMP, ppGpp, and 3′, 5′‐cGMP were not stimulatory effectors; GTP‐γ‐S and GMPPNP stimulated adenylyl cyclase activity but to a lesser degree than did GTP. In addition to the previous indication that ATP can occupy the regulatory site on adenylyl cyclase, it was found that CTP and UTP were potent stimulators. Thus, all the naturally occurring RNA precursor nucleoside triphosphates are capable of stimulating adenylyl cyclase activity. In contrast, PPPi inhibits adenylyl cyclase activity. Additional studies showed that there is only one regulatory site for all the nucleotide stimulators and that the nucleotide occupying the substrate site can influence the properties of the regulatory site. These observations suggest a mechanism by which adenylyl cyclase activity can be regulated by the availability of nucleic acid precursors.