Polymorphous crystallization and diffraction of threonine deaminase from Escherichia coli

The biosynthetic threonine deaminase from Escherichia coli, an allosteric tetramer with key regulatory functions, has been crystallized in several crystal forms. Two distinct forms, both belonging to either space group P3121 or P3221, with different sized asymmetric units that both contain a tetramer, grow under identical conditions. Diffraction data sets to 2.8 A resolution (native) and 2. 9 A resolution (isomorphous uranyl derivative) have been collected from a third crystal form in space group I222.

Structure and control of pyridoxal phosphate dependent allosteric threonine deaminase

BACKGROUND

Feedback inhibition of biosynthetic threonine deaminase (TD) from Escherichia coli provided one of the earliest examples of protein-based metabolic regulation. Isoleucine, the pathway end-product, and valine, the product of a parallel pathway, serve as allosteric inhibitor and activator, respectively. This enzyme is thus a useful model system for studying the structural basis of allosteric control mechanisms.

RESULTS

We report the crystal structure of TD at 2.8 A resolution. The tetramer has 222 symmetry, with C-terminal regulatory domains projecting out from a core of catalytic PLP-containing N-terminal domains. The subunits, and especially the regulatory domains, associate extensively to form dimers, which associate less extensively to form the tetramer. Within the dimer, each monomer twists approximately 150 degrees around a thin neck between the domains to place its catalytic domain adjacent to the regulatory domain of the other subunit.

CONCLUSIONS

The structure of TD and its comparison with related structures and other data lead to the tentative identification of the regulatory binding site and revealed several implications for the allosteric mechanism. This work prepares the way for detailed structure/function studies of the complex allosteric behaviour of this enzyme.

Monomer-heptamer equilibrium of the Escherichia coli chaperonin GroES

In an effort to clarify the role of GroES in chaperonin-facilitated protein folding, a plasmid-encoding expression system for GroES incorporating a histidine-tagged, thrombin-cleavable, N-terminal sequence was constructed. This approach facilitated the rapid purification of native-like, histidine-cleaved GroES (HC-GroES). The addition of NaSCN to purification buffers to mildly promote subunit dissociation enabled the complete separation of chromosomally encoded, wild-type GroES chains from recombinant chains, allowing the production of homogeneous mutant variants of GroES. A substitution of histidine-7 to tryptophan in GroES was used to demonstrate the concentration-dependent modulation of the heptameric quaternary structure of the chaperonin. Fluorescence and light scattering studies of this mutant suggest that GroES heptamers dissociate to monomers upon dilution with half-times of 2-4 min. Sedimentation equilibrium experiments using either wild-type or HC-GroES can best be described by a monomer--heptamer equilibrium, yielding dissociation constants of 1 x 10(-38) M6 for native GroES and 2 x 10(-32) M6 for HC-GroES. These results are supported by subunit exchange experiments using mixtures of native or HC-GroES and GroES containing the complete N-terminal histidine tail. Native polyacrylamide gel electrophoresis demonstrates that these mixtures form an eight-membered hybrid set within minutes. The studies described here suggest a dynamic equilibrium for the quaternary structure of GroES, which may be an important feature for its role in GroEL-mediated protein folding reactions.