Evidence for a Guanosine‐3′: 5′‐Monophosphate‐Binding Protein from Rat Cerebellum

Guo‐3′:5′‐P and Ado‐3′:5′‐P (guanosine and adenosine 3′: 5′‐monophosphate) are bound by a supernatant fraction of homogenates from rat cerebellum. Treatment of the supernatant fraction with trypsin destroyed the ability of the supernatant fraction to bind either of the cyclic nucleotides. For Ado‐3′:5′‐P a hyperbolic saturation curve was obtained and an apparent Kd of 8.5 nM was estimated. At a saturating concentration of 0.1 μM between 6 and 8 pmol per mg extracted protein was bound. For Guo‐3′: 5′‐P two different types of binding sites were observed; one nonspecific site from which Guo‐3′: 5′‐P could be displayed by low concentrations of Ado‐ 3′:5′‐P and one, which was specific for Guo‐3′: 5′‐P. The apparent Kd of the specific site for Guo‐3′:5′‐P was 12 nM and at a saturating concentration of 0.1 μM 1.5 pmol per mg extracted protein were bound. The Guo‐3′:5′‐P binding protein was partially separated from an Ado‐3′: 5′‐P‐dependent protein kinase by ammonium sulfate precipitation.