Mechanistic mathematical modeling supports that RII phosphorylation occurs before opening of PKA-II holoenzymes. (A) Illustration of the core PKA-II model. Colored arrows indicate accessible epitopes recognized by pRII (red) and Cα (green) antibodies, respectively. (B) Illustration of model extensions allowing for differences between cAMP analogues (H1 and H3) and feedback regulation (H2 and H4). (C) Illustration of an alternative model structure assuming that cAMP induces RII phosphorylation. (D) Ranking of model hypotheses according to the AIC value.