A binary complex of the catalytic subunit of cAMP-dependent protein kinase and adenosine further defines conformational flexibility

Background: cAMP-dependent protein kinase (cAPK), a ubiquitous protein in eukaryotic cells, is one of the simplest members of the protein kinase family. It was the first protein kinase to be crystallized and continues to serve as a biochemical and structural prototype for this family of enzymes. To further understand the conformational changes that occur in different liganded and unliganded states of cAPK, the catalytic subunit of cAPK was crystallized in the absence of peptide inhibitor. Results: The crystal structure of the catalytic subunit of mouse recombinant cAPK (rC) complexed with adenosine was solved at 2.6 Angstrom resolution and refined to a crystallographic R factor of 21.9% with good stereochemical parameters. This is the first structure of the rC subunit that lacks a bound inhibitor or substrate peptide. The structure was solved by molecular replacement and comprises two lobes (large and small) which contain a number of conserved loops. Conclusions: The binary complex of rC and adenosine adopts an 'intermediate' conformation relative to the previously described 'closed' and 'open' conformations of other rC complexes. Based on a comparison of these structures, the induced fit that is necessary for catalysis and closing of the active-site cleft appears to be confined to the small robe, as in the absence of the peptide the conformation of the large lobe, including the peptide-docking surface, does not change. Three specific components contribute to the closing of the cleft: rotation of the small lobe; movement of the C-terminal tail; and closing of the so-called glycine-rich loop. There is no induced fit in the large robe to accommodate the peptide and the closing of the cleft. A portion of the C-terminal tail, residues 315-334, serves as a gate for the entry or exit of the nucleotide into the hydrophobic active-site cleft.