Accelerating the Association of the Most Stable Protein-Ligand Complex by More than Two Orders of Magnitude.

The complex between the bacterial type 1 pilus subunit FimG and the peptide corresponding to the N-terminal extension (termed donor strand, Ds) of the partner subunit FimF (DsF) shows the strongest reported noncovalent molecular interaction, with a dissociation constant (KD ) of 1.5×10(-20) m. However, the complex only exhibits a slow association rate of 330 m(-1) s(-1) that limits technical applications, such as its use in affinity purification. Herein, a structure-based approach was used to design pairs of FimGt (a FimG variant lacking its own N-terminal extension) and DsF variants with enhanced electrostatic surface complementarity. Association of the best mutant FimGt/DsF pairs was accelerated by more than two orders of magnitude, while the dissociation rates and 3D structures of the improved complexes remained essentially unperturbed. A KD  value of 8.8×10(-22) m was obtained for the best mutant complex, which is the lowest value reported to date for a protein/ligand complex.

Architecture and function of human uromodulin filaments in urinary tract infections.

Uromodulin is the most abundant protein in human urine, and it forms filaments that antagonize the adhesion of uropathogens; however, the filament structure and mechanism of protection remain poorly understood. We used cryo-electron tomography to show that the uromodulin filament consists of a zigzag-shaped backbone with laterally protruding arms. N-glycosylation mapping and biophysical assays revealed that uromodulin acts as a multivalent ligand for the bacterial type 1 pilus adhesin, presenting specific epitopes on the regularly spaced arms. Imaging of uromodulin-uropathogen interactions in vitro and in patient urine showed that uromodulin filaments associate with uropathogens and mediate bacterial aggregation, which likely prevents adhesion and allows clearance by micturition. These results provide a framework for understanding uromodulin in urinary tract infections and in its more enigmatic roles in physiology and disease.

Determinants of the assembly and function of antibody variable domains.

The antibody Fv module which binds antigen consists of the variable domains VL and VH. These exhibit a conserved ß-sheet structure and comprise highly variable loops (CDRs). Little is known about the contributions of the framework residues and CDRs to their association. We exchanged conserved interface residues as well as CDR loops and tested the effects on two Fvs interacting with moderate affinities (KDs of ~2.5 µM and ~6 µM). While for the rather instable domains, almost all mutations had a negative effect, the more stable domains tolerated a number of mutations of conserved interface residues. Of particular importance for Fv association are VLP44 and VHL45. In general, the exchange of conserved residues in the VL/VH interface did not have uniform effects on domain stability. Furthermore, the effects on association and antigen binding do not strictly correlate. In addition to the interface, the CDRs modulate the variable domain framework to a significant extent as shown by swap experiments. Our study reveals a complex interplay of domain stability, association and antigen binding including an unexpected strong mutual influence of the domain framework and the CDRs on stability/association on the one side and antigen binding on the other side.

Catch-bond mechanism of the bacterial adhesin FimH.

Ligand-receptor interactions that are reinforced by mechanical stress, so-called catch-bonds, play a major role in cell-cell adhesion. They critically contribute to widespread urinary tract infections by pathogenic Escherichia coli strains. These pathogens attach to host epithelia via the adhesin FimH, a two-domain protein at the tip of type I pili recognizing terminal mannoses on epithelial glycoproteins. Here we establish peptide-complemented FimH as a model system for fimbrial FimH function. We reveal a three-state mechanism of FimH catch-bond formation based on crystal structures of all states, kinetic analysis of ligand interaction and molecular dynamics simulations. In the absence of tensile force, the FimH pilin domain allosterically accelerates spontaneous ligand dissociation from the FimH lectin domain by 100,000-fold, resulting in weak affinity. Separation of the FimH domains under stress abolishes allosteric interplay and increases the affinity of the lectin domain. Cell tracking demonstrates that rapid ligand dissociation from FimH supports motility of piliated E. coli on mannosylated surfaces in the absence of shear force.

Resistance to peloruside A and laulimalide: functional significance of acquired ßI-tubulin mutations at sites important for drug-tubulin binding.

Cancer cell lines selected for resistance to microtubule targeting agents (MTA) often have acquired mutations in the ß-tubulin binding sites for these agents. Despite strong correlational evidence, the functional and quantitative significance of such mutations in the resistance to MTA remains unknown. We recently showed that peloruside A (PLA) and laulimalide (LAU)-resistant cancer cell lines, 1A9-R1 (R1) and 1A9-L4 (L4), generated through multi-step selection of human 1A9 ovarian cancer cells with high concentrations of either PLA (for R1) or LAU (for L4) have single distinct mutations in their ßI-tubulin gene. The R1 cells have a mutation at amino acid position 296 (A296T), and the L4 cells have a mutation at position 306 (R306H/C), both of which lie at the putative binding sites of PLA and LAU. To gain insights on the functional role of these mutations in the resistance phenotype, R1 and L4 cells were transfected with wild type ßI-tubulin. MTT cell proliferation assays revealed that restoration of wild type ßI-tubulin expression partially sensitized the R1 and L4 cells to PLA and LAU. Cell cycle analysis and intracellular tubulin polymerization assays demonstrated that the increased sensitivity was correlated with an increased ability of PLA and LAU to induce G2-M arrest and tubulin polymerization in the cells. Unlike paclitaxel-selected clones of 1A9 cells, both R1 and L4 cells exhibited a functional p53 gene, and the abundance of the mismatch repair gene hMSH2 (human mutS homolog 2) was comparable to the parental 1A9 cells. This study provides the first direct evidence that A296 and R306 of ßI-tubulin are important determinants of the PLA and LAU response in cancer cells.