The regulatory subunit has only two stable conformations, one in its native tetrameric holoenzyme form (R2C2), and the other with cAMP bound, seperate from the catalytic subunit. As of yet, neither the holoenzyme, nor the natural cAMP-Regulatory complex have had their crystal structures resolved. However, a crystal structure for a mutant of the regulatory subunit bound to cAMP has been elucidated. It is a deletion mutant, with the first 91 amino acid residues at the N-terminus taken out.
The first 21 residues of the mutant protein (91-112)are not seen in the x-ray diffraction and resolution, presumably because it is a disordered region. This region contains the inhibitor peptide that binds to the active site of the catalytic subunit. Since they couldn't be seen, these residues are not shown in the pdb coordinate files. But it is likely that this sequence is either the same or very analogous to the synthetic inhibitor peptide co-crystallized with the catalytic subunit.
Residue 113 is an Arg, which is followed by an alpha-helix (X:N). After this helix ends, domain A starts, and runs for about 100 residues. The carboxyl end domain A is linked directly to the amino side of domain B, which runs another 100 residues.