A, Cyclic nucleotide binding was determined by equilibrium dialysis. Shown is the [3H] cAMP binding of the PKA bovine RIα subunit (■), the human RIβ subunit (●), and the T. equiperdum R-like protein (▲). Also shown is the [3H] cGMP binding of the parasite R-like protein (Inset). B, cAMP binding was also analyzed by chromatography throughout a cAMP-Sepharose affinity column. Gel lanes contained the following samples: 1, original protein; 2, flow through fraction; 3, bound protein. The bovine RIα subunit and human RIβ subunit were included as controls. Arrows indicate the molecular masses of the three proteins used in the experiment.