Structure of the mammalian catalytic subunit of cAMP-dependent protein kinase and an inhibitor peptide displays an open conformation

The crystal structure of a binary complex of the porcine heart catalytic (C) subunit of cAMP-dependent protein kinase (space group P4₁32; a = 171.5 Å) complexed with a di-iodinated peptide inhibitor, PKI(5–24), has been solved and refined to 2.9 Å resolution with an overall R of 21.1%. The r.m.s. deviations from ideal bond lengths and angles are 0.022 Å and 4.3°. A single isotropic B of 17 Ų was used for all atoms. The structure solution was carried out initially by molecular replacement of electron density followed by refinement against atomic coordinates from orthorhombic crystals of a binary complex of the mouse recombinant enzyme previously described [Knighton, Zheng, Ten Eyck, Ashford, Xuong, Taylor & Sowadski (1991). Science, 253, 407–414]. The most striking difference between the two crystal structures is a large displacement of the small lobe of the enzyme. In the cubic crystal, the β-sheet of the small lobe is rotated by 15° and translated by 1.9 Å with respect to the orthorhombic crystal. Possible explanations for why this binary complex crystallized in an open conformation in contrast to a similar binary complex of the recombinant enzyme are discussed. This study demonstrates that considerable information about parts of a crystal structure can be obtained without a complete crystal structure analysis. Specifically, the six rigid-group parameters of a poly alanine model of the β-structure were obtained satisfactorily from a crystal structure by refinement of difference Fourier coefficients based on an approximate partial structure model.