cAMP-dependent protein kinase from Mucor rouxii: Physical evidence of a ternary complex holoenzyme-cAMP

Gel electrophoresis and sucrose density gradient centrifugation techniques permitted the visualization for the first time of the ternary complex formed by the binding of cAMP to Mucor rouxii cAMP-dependent protein kinase holoenzyme. The addition of 0.5 M NaCl or histone plus ATP-Mg⁺⁺, together with cAMP, dissociates the holoenzyme into free regulatory (R) and catalytic (C) subunits. At 4°C, cAMP bound to the holoenzyme is readily exchangeable with unlabeled cAMP (half life 2.5 min), while the nucleotide bound to the R subunit has a very slow exchange rate (half life 210 min). The amount of cAMP bound to R subunit is approximately twice the amount bound to holoenzyme at saturation.