The cAMP-binding domains of the regulatory subunit of cAMP-dependent protein kinase and the catabolite gene activator protein are homologous.

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Abstract
Comparison of the recently determined amino acid sequences of the regulatory subunit of cAMP-dependent protein kinase (RII) from bovine cardiac muscle and the Escherichia coli catabolite gene activator protein (CAP) shows significant homology. This homology extends over most of the amino-terminal domain in CAP and is particularly good for the region of the .beta.-roll structure. The RII sequence contains 2 adjacent and internally homologous regions, both of which have high resemblance to the cAMP-binding domain in CAP. The protein kinase regulatory subunit evidently contains 2 cAMP-binding domains in the carboxyl-terminal region, each having a .beta.-roll structure similar to that in CAP. The cAMP molecule is expected to bind to the RII within a pocket formed by residues from the .beta.-roll, as is the case with CAP. One cAMP molecule would interact with residues from about 163-220, and the other cAMP would interact with amino acids in the stretch 285-350 of the RII protein kinase sequence. As the carboxyl-terminal domain of CAP shows homologies to the DNA-binding domains of other regulatory proteins, the protein appears to be of modular construction: a DNA-binding domain joined to a cAMP-binding domain.

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