Post-translational modifications of the regulatory subunits of cAMP-dependent protein kinases during G1/S progression.

During the G1/S transition of the cell cycle variations in the labelling by 8-N3-[32P]cAMP of the protein kinase A regulatory subunits RI and RII, used as a probe to monitor post-translational modifications that may regulate cAMP binding, were observed in synchronized HeLa cells. A decrease in 8-N3-[32P]cAMP labelling of RI, RII and RII phosphorylated by the catalytic subunit of PKA was correlated with the increased percentage of cells in phases G1. An increase in 8-N3-[32P]cAMP incorporated into the 54-kDa RII subunit during progression from G1 to S was correlated with an increase in intracellular cAMP. A transient increase in Mn-SOD activity was detected in cells arrested at the G1/S transition using two different techniques, suggesting that oxidative modulation of regulatory subunits by free radicals may modify cAMP binding sites during the cell cycle. Decreased photoaffinity labelling by 8-N3-[32P]cAMP of RI, RII and autophosphorylated RII subunits was found to be an inherent characteristic of PKA in the G1/S transition.