A cyclic AMP binding protein of rat liver interaction with an histone kinase

Abstract

Summary Cyclic AMP binding to different subcellular fractions of rat liver was studied. Some binding was found in nucleoplasm, but cyclic AMP bound primarily to the postmicrosomal supernatant; thus, its cellular distribution was parallel to that of cyclic AMP dependent protein kinase. From the postmicrosomal supernatant, a protein which hound cyclic AMP was purified 150 fold. The dissociation constant (Kd) was 10 −8 M, i.e. in the range of the physiological concentrations of this nucleotide in rat liver. Only cyclic GMP and cyclic IMP were partial competitors. The binding protein was associated with a protein kinase activated by cyclic AMP. The two proteins could be partially separated after DEAE cellulose chromatography. The separation was greatly improved in presence of cyclic AMP which appeared to induce the dissociation of the two proteins. The protein kinase free of receptor was then no longer activated by the nucleotide. Ageing of a crude preparation also allowed separation between the two proteins, together with the loss of activation of the protein kinase by cyclic AMP. Addition of the receptor to the purified protein kinase suppressed its activity. This inhibition was partially reversed by cyclic AMP. The present results confirm and extend to rat liver the hypothesis that cyclic AMP binds to an inhibitor of a protein kinase and promotes, through the dissociation of the inactive complex, the activation of the kinase.