The structure of a ternary complex of the catalytic subunit of cAMP-dependent protein kinase, MgATP, and a 20-residue inhibitor peptide was determined at a resolution of 2.7 A using the difference Fourier technique starting from the model of the binary complex (Knighton et al., 1991a). The model of the ternary complex was refined using both X-PLOR and TNT to an R factor of 0.212 and 0.224, respectively. The orientation of the nucleotide and the interactions of MgATP with numerous conserved residues at the active site of the enzyme are clearly defined. The unique protein kinase nucleotide binding site consists of a five-stranded antiparallel beta-sheet with the base buried in a hydrophobic site along beta-strands 1 and 2 and fixed by hydrogen bonds to the N6 amino and N7 nitrogens. The small lobe secures the nucleotide via a glycine-rich loop and by ion pairing with Lys72 and Glu91. While the small lobe fixes the nontransferable alpha- and beta-phosphates in this inhibitor complex, the gamma-phosphate is secured by two Mg2+ ions and interacts both directly and indirectly with several residues in the large lobe--Asp184, Asn171, Lys168. Asp166 is positioned to serve as a catalytic base. The structure is correlated with previous chemical evidence, and the features that distinguish this nucleotide binding motif from other nucleotide binding proteins are delineated.

Our comparison reveals that the overall structure of OmpF is not influenced by crystal lattice constraints and, thus, presumably bears close resemblance to the in vivo structure. The tetragonal crystal structure has provided the starting model for the phasing of neutron diffraction data obtained from this crystal form, as described in an accompanying article.

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The crystal structure of a binary complex of the porcine heart catalytic (C) subunit of cAMPdependent protein kinase (space group P4132; a = 171.5A) complexed with a di-iodinated peptide inhibitor, PKI(5-24), has been solved and refined to 2.9A resolution with an overall R of 21.1% . The r.m.s, deviations from ideal bond lengths and angles are 0.022 A and 4.3 °. A single isotropic B of 17 A 2 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][408][409][410][411][412][413][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 fl-sheet of the small lobe is rotated by lY and translated by 1.9 A, 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 recombi-* To whom all correspondence should be addressed.© 1993 International Union of Crystallography Printed in Great Britain --all rights reserved nant 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 rigidgroup parameters of a poly-alanine model of the fl-structure were obtained satisfactorily from a crystal structure by refinement of difference Fourier coefficients based on an approximate partial structure model.

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