The crystal structure of the tryptic core fragment of the lac repressor of Escherichia coli (LacR) complexed with the inducer isopropyl-beta-D-thiogalactoside was determined at 2.6 A resolution. The quaternary structure consists of two dyad-symmetric dimers that are nearly parallel to each other. This structure places all four DNA binding domains of intact LacR on the same side of the tetramer, and results in a deep, V-shaped cleft between the two dimers. Each monomer contributes a carboxyl-terminal helix to an antiparallel four-helix bundle that functions as a tetramerization domain. Some of the side chains whose mutation reduce DNA binding form clusters on a surface near the amino terminus. Placing the structure of the DNA binding domain complexed with operator previously determined by nuclear magnetic resonance onto this surface results in two operators being adjacent and nearly parallel to each other. Structural considerations suggest that the two dimers of LacR may flexibly alter their relative orientation in order to bind to the known varied spacings between two operators.

No abstract

The single-stranded DNA (ssDNA) binding protein gp32 from bacteriophage T4 is essential for T4 DNA replication, recombination and repair. In vivo gp32 binds ssDNA as the replication fork advances and stimulates replisome processivity and accuracy by a factor of several hundred. Gp32 binding affects nearly every major aspect of DNA metabolism. Among its important functions are: (1) configuring ssDNA templates for efficient use by the replisome including DNA polymerase; (2) melting out adventitious secondary structures; (3) protecting exposed ssDNA from nucleases; and (4) facilitating homologous recombination by binding ssDNA during strand displacement. We have determined the crystal structure of the gp32 DNA binding domain complexed to ssDNA at 2.2 A resolution. The ssDNA binding cleft comprises regions from three structural subdomains and includes a positively charged surface that runs parallel to a series of hydrophobic pockets formed by clusters of aromatic side chains. Although only weak electron density is seen for the ssDNA, it indicates that the phosphate backbone contacts an electropositive cleft of the protein, placing the bases in contact with the hydrophobic pockets. The DNA mobility implied by the weak electron density may reflect the role of gp32 as a sequence-independent ssDNA chaperone allowing the largely unstructured ssDNA to slide freely through the cleft.

No abstract

The structure of PIMT reveals a unique modification of the methyltransferase fold along with a site for specific recognition of isoaspartyl substrates. The sequence conservation among PIMTs suggests that the current structure should prove a reliable model for understanding the repair of isoaspartyl damage in all organisms.

We describe a method by which a single experiment can reveal both association model (pathway and constants) and low-resolution structures of a self-associating system. Small-angle scattering data are collected from solutions at a range of concentrations. These scattering data curves are mass-weighted linear combinations of the scattering from each oligomer. Singular value decomposition of the data yields a set of basis vectors from which the scattering curve for each oligomer is reconstructed using coefficients that depend on the association model. A search identifies the association pathway and constants that provide the best agreement between reconstructed and observed data. Using simulated data with realistic noise, our method finds the correct pathway and association constants. Depending on the simulation parameters, reconstructed curves for each oligomer differ from the ideal by 0.05-0.99% in median absolute relative deviation. The reconstructed scattering curves are fundamental to further analysis, including interatomic distance distribution calculation and low-resolution ab initio shape reconstruction of each oligomer in solution. This method can be applied to x-ray or neutron scattering data from small angles to moderate (or higher) resolution. Data can be taken under physiological conditions, or particular conditions (e.g., temperature) can be varied to extract fundamental association parameters (DeltaH(ass), DeltaS(ass)).

Spontaneous formation of isoaspartyl residues (isoAsp) disrupts the structure and function of many normal proteins. Protein isoaspartyl methyltransferase (PIMT) reverts many isoAsp residues to aspartate as a protein repair process. We have determined the crystal structure of human protein isoaspartyl methyltransferase (HPIMT) complexed with adenosyl homocysteine (AdoHcy) to 1.6-Å resolution. The core structure has a nucleotide binding domain motif, which is structurally homologous with the N-terminal domain of the bacterial Thermotoga maritima PIMT. Highly conserved residues in PIMTs among different phyla are placed at positions critical to AdoHcy binding and orienting the isoAsp residue substrate for methylation. The AdoHcy is completely enclosed within the HPIMT and a conformational change must occur to allow exchange with adenosyl methionine (AdoMet). An ordered sequential enzyme mechanism is supported because C-terminal residues involved with AdoHcy binding also form the isoAsp peptide binding site, and a change of conformation to allow AdoHcy to escape would preclude peptide binding. Modeling experiments indicated isoAsp groups observed in some known protein crystal structures could bind to the HPIMT active site.

In developing improved protein variants by site-directed mutagenesis or recombination, there are often competing objectives that must be considered in designing an experiment (selecting mutations or breakpoints): stability vs. novelty, affinity vs. specificity, activity vs. immunogenicity, and so forth. Pareto optimal experimental designs make the best trade-offs between competing objectives. Such designs are not “dominated”; i.e., no other design is better than a Pareto optimal design for one objective without being worse for another objective. Our goal is to produce all the Pareto optimal designs (the Pareto frontier), in order to characterize the trade-offs and suggest designs most worth considering, but to avoid explicitly considering the large number of dominated designs. To do so, we develop a divide-and-conquer algorithm, PEPFR (Protein Engineering Pareto FRontier), that hierarchically subdivides the objective space, employing appropriate dynamic programming or integer programming methods to optimize designs in different regions. This divide-and-conquer approach is efficient in that the number of divisions (and thus calls to the optimizer) is directly proportional to the number of Pareto optimal designs. We demonstrate PEPFR with three protein engineering case studies: site-directed recombination for stability and diversity via dynamic programming, site-directed mutagenesis of interacting proteins for affinity and specificity via integer programming, and site-directed mutagenesis of a therapeutic protein for activity and immunogenicity via integer programming. We show that PEPFR is able to effectively produce all the Pareto optimal designs, discovering many more designs than previous methods. The characterization of the Pareto frontier provides additional insights into the local stability of design choices as well as global trends leading to trade-offs between competing criteria.