Crystallization studies of the catalytic subunit of cAMP-dependent protein kinase: crystals of murine recombinant catalytic subunit and a mutant, Cys 343----Ser, diffract to 2.7 A resolution.
The recombinant mouse catalytic subunit of cAMP-dependent protein kinase, expressed and purified from E. coli, has been successfully cocrystallized as a binary complex with an inhibitor peptide and as a ternary complex with an inhibitor peptide and MgATP. In contrast to the catalytic subunit obtained from porcine heart, the recombinant catalytic subunit lacks a myristoyl group at the amino terminus and differs in sequence at nine positions out of 350 amino acids. The catalytic activities of the two enzymes, however, are nearly identical. Both enzymes cocrystallized with a 20-amino-acid inhibitor and MgATP; however, the porcine-heart enzyme crystallized in a hexagonal space group (P6(1)22) while the recombinant murine catalytic subunit crystallized in an orthorhombic space group (P2(1)2(1)2(1), a = 73.70, b = 76.26, c = 80.74 A). The orthorhombic crystals of the recombinant catalytic subunit exhibit the best diffraction characteristics of all catalytic subunit crystals obtained so far: 2.7 A resolution. Unlike the mammalian porcine-heart enzyme, no crystals of the recombinant apo-enzyme were obtained under the same crystallization conditions. These results are consistent with earlier conclusions that the catalytic subunit exists in at least two distinct conformational states and furthermore suggests that the inhibitor peptide alone is sufficient to induce the major conformational changes that distinguish the two states. A mutant form of the catalytic subunit where Cys343 was replaced with Ser was also cocrystallized with the 20-amino-acid peptide inhibitor and MgATP, and resulted in an orthorhombic crystal isomorphous to crystals of the unmutated enzyme with a similar diffraction of 2.7 A.