Effect of MgATP on cAMP-Dissociation Kinetics of Lactating Rat Mammary Gland

Abstract

Mammary gland cytosols exhibit temperature-dependent inter-conversion of cAMP-dissociation rates from low to high affinity (k-1 = 0.14 min⁻¹ at 0C to k-1 = 0.02 min⁻¹ at 24 association). This interconversion corresponds to a change from a site 2 to a site 1 cAMP-dissociation rate for the type II cAMP-dependent protein kinase in mammary gland cytosols. This report presents data which indicates a requirement for MgATP in the temperature-dependent interconversion of cAMP-dissociation rates. The effect of MgATP on the generation of the high affinity state was observed at 24C but not 0C association. The effect of MgATP was not mimicked by equimolar MgAMP-PNP, but did require an intact type II protein kinase holoenzyme which can undergo autophosphorylation of its regulatory subunit. The effect of MgATP was reproduced with partially purified preparations of beef heart type II protein kinase. These results suggest that MgATP may act through autophosphorylation of the type II holoenzyme. The data suggest a novel role of MgATP in the regulation of cAMP binding to cAMP-dependent protein kinase II.