Faculty Bibliography

Protein function often depends on the exchange between conformational substates. Allosteric ligand binding or distal mutations can stabilize specific active-site conformations and consequently alter protein function. Observing alternative conformations at low levels of electron density, in addition to comparison of independently determined X-ray crystal structures, can provide mechanistic insights into conformational dynamics. Here we report a new algorithm, CONTACT, that identifies contact networks of conformationally heterogeneous residues directly from high-resolution X-ray crystallography data. Contact networks determined for Escherichia coli dihydrofolate reductase (ecDHFR) predict the observed long-range pattern of NMR chemical shift perturbations of an allosteric mutation. A comparison of contact networks in wild-type and mutant ecDHFR suggests that mutations that alter optimized contact networks of coordinated motions can impair catalytic function. CONTACT-guided mutagenesis can exploit the structure-dynamics-function relationship in protein engineering and design.

Immune recognition of protein antigens relies on the combined interaction of multiple antibody loops, which provide a fairly large footprint and constrain the size and shape of protein surfaces that can be targeted. Single protein loops can mediate extremely high-affinity binding, but it is unclear whether such a mechanism is available to antibodies. Here we report the isolation and characterization of an antibody called C05, which neutralizes strains from multiple subtypes of influenza A virus, including H1, H2 and H3. X-ray and electron microscopy structures show that C05 recognizes conserved elements of the receptor-binding site on the haemagglutinin surface glycoprotein. Recognition of the haemagglutinin receptor-binding site is dominated by a single heavy-chain complementarity-determining region 3 loop, with minor contacts from heavy-chain complementarity-determining region 1, and is sufficient to achieve nanomolar binding with a minimal footprint. Thus, binding predominantly with a single loop can allow antibodies to target small, conserved functional sites on otherwise hypervariable antigens.

Dihydrofolate reductase (DHFR) is a well-studied drug target and a paradigm for understanding enzyme catalysis. Preparation of pure DHFR samples, in defined ligand-bound states, is a prerequisite for in vitro studies and drug discovery efforts. We use NMR spectroscopy to monitor ligand content of human and Escherichia coli DHFR (ecDHFR), which bind different co-purifying ligands during expression in bacteria. An alternate purification strategy yields highly pure DHFR complexes, containing only the desired ligands, in the quantities required for structural studies. Interestingly, ecDHFR is bound to endogenous THF while human DHFR is bound to NADP. Consistent with these findings, a designed "humanized" mutant of ecDHFR switches binding specificity in the cell.

Current flu vaccines provide only limited coverage against seasonal strains of influenza viruses. The identification of V(H)1-69 antibodies that broadly neutralize almost all influenza A group 1 viruses constituted a breakthrough in the influenza field. Here, we report the isolation and characterization of a human monoclonal antibody CR8020 with broad neutralizing activity against most group 2 viruses, including H3N2 and H7N7, which cause severe human infection. The crystal structure of Fab CR8020 with the 1968 pandemic H3 hemagglutinin (HA) reveals a highly conserved epitope in the HA stalk distinct from the epitope recognized by the V(H)1-69 group 1 antibodies. Thus, a cocktail of two antibodies may be sufficient to neutralize most influenza A subtypes and, hence, enable development of a universal flu vaccine and broad-spectrum antibody therapies.

Conformational dynamics play a key role in enzyme catalysis. Although protein motions have clear implications for ligand flux, a role for dynamics in the chemical step of enzyme catalysis has not been clearly established. We generated a mutant of Escherichia coli dihydrofolate reductase that abrogates millisecond-time-scale fluctuations in the enzyme active site without perturbing its structural and electrostatic preorganization. This dynamic knockout severely impairs hydride transfer. Thus, we have found a link between conformational fluctuations on the millisecond time scale and the chemical step of an enzymatic reaction, with broad implications for our understanding of enzyme mechanisms and for design of novel protein catalysts.

Beta-turns are common conformations that enable proteins to adopt globular structures, and their formation is often rate limiting for folding. Beta-turn mimics, molecules that replace the i + 1 and i + 2 amino acid residues of a beta-turn, are envisioned to act as folding nucleators by preorganizing the pendant polypeptide chains, thereby lowering the activation barrier for beta-sheet formation. However, the crucial kinetic experiments to demonstrate that beta-turn mimics can act as strong nucleators in the context of a cooperatively folding protein have not been reported. We have incorporated 6 beta-turn mimics simulating varied beta-turn types in place of 2 residues in an engineered beta-turn 1 or beta-bulge turn 1 of the Pin 1 WW domain, a three-stranded beta-sheet protein. We present 2 lines of kinetic evidence that the inclusion of beta-turn mimics alters beta-sheet folding rates, enabling us to classify beta-turn mimics into 3 categories: those that are weak nucleators but permit Pin WW folding, native-like nucleators, and strong nucleators. Strong nucleators accelerate folding relative to WW domains incorporating all alpha-amino acid sequences. A solution NMR structure reveals that the native Pin WW beta-sheet structure is retained upon incorporating a strong E-olefin nucleator. These beta-turn mimics can now be used to interrogate protein folding transition state structures and the 2 kinetic analyses presented can be used to assess the nucleation capacity of other beta-turn mimics.

Influenza virus presents an important and persistent threat to public health worldwide, and current vaccines provide immunity to viral isolates similar to the vaccine strain. High-affinity antibodies against a conserved epitope could provide immunity to the diverse influenza subtypes and protection against future pandemic viruses. Cocrystal structures were determined at 2.2 and 2.7 angstrom resolutions for broadly neutralizing human antibody CR6261 Fab in complexes with the major surface antigen (hemagglutinin, HA) from viruses responsible for the 1918 H1N1 influenza pandemic and a recent lethal case of H5N1 avian influenza. In contrast to other structurally characterized influenza antibodies, CR6261 recognizes a highly conserved helical region in the membrane-proximal stem of HA1 and HA2. The antibody neutralizes the virus by blocking conformational rearrangements associated with membrane fusion. The CR6261 epitope identified here should accelerate the design and implementation of improved vaccines that can elicit CR6261-like antibodies, as well as antibody-based therapies for the treatment of influenza.

NMR measurements can give important information on solution structure, without the necessity for a full-scale solution structure determination. The C-terminal protein binding domain of the ribosome-associated chaperone protein trigger factor is composed of non-contiguous parts of the polypeptide chain, with an interpolated prolyl isomerase domain. A construct of the C-terminal domain of Escherichia coli trigger factor containing residues 113-149 and 247-432, joined by a Gly-Ser-Gly-Ser linker, is well folded and gives excellent NMR spectra in solution. We have used NMR measurements on this construct, and on a longer construct that includes the prolyl isomerase domain, to distinguish between two possible structures for the C-terminal domain of trigger factor, and to assess the behavior of the trigger factor C-terminal domain in solution. Two X-ray crystal structures, of intact trigger factor from E. coli (Ferbitz et al., Nature 431:590-596, 2004), and of a truncated trigger factor from Vibrio cholerae (Ludlam et al., Proc Natl Acad Sci USA 101:13436-13441, 2004) showed significant differences in the structure of the C-terminal domain, such that the two structures could not be superimposed. We show using NMR chemical shifts and long range nuclear Overhauser effects that the secondary and tertiary structure of the E. coli C-terminal domain in solution is consistent with the crystal structure of the E. coli trigger factor and not with the V. cholerae protein. Given the similarity of the amino acid sequences of the E. coli and V. cholerae proteins, it appears likely that the structure of the V. cholerae protein has been distorted as a result of truncation of a 44-amino acid segment at the C-terminus. Analysis of residual dipolar coupling measurements shows that the overall topology of the solution structure is completely inconsistent with both structures. Dynamics analysis of the C-terminal domain using T1, T2 and heteronuclear NOE parameters show that the protein is overall rather flexible. These results indicate that the structure of this domain in solution resembles the X-ray crystal structure of the E. coli protein in secondary structure and at least some tertiary contacts, but that the overall topology differs in solution, probably due to structural fluctuation.

In humans, genetically diverse forms of muscular dystrophy are associated with a disrupted sarcoglycan complex. The sarcoglycan complex resides at the muscle plasma membrane where it associates with dystrophin. There are six known sarcoglycan proteins in mammals whereas there are only three in Drosophila melanogaster. Using imprecise P element excision, we generated three different alleles at the Drosophila delta-sarcoglycan locus. Each of these deletions encompassed progressively larger regions of the delta-sarcoglycan gene. Line 840 contained a large deletion of the delta-sarcoglycan gene, and this line displayed progressive impairment in locomotive ability, reduced heart tube function and a shortened life span. In line 840, deletion of the Drosophila delta-sarcoglycan gene produced disrupted flight muscles with shortened sarcomeres and disorganized M lines. Unlike mammalian muscle where degeneration is coupled with ongoing regeneration, no evidence for regeneration was seen in this Drosophila sarcoglycan mutant. In contrast, line 28 was characterized with a much smaller deletion that affected only a portion of the cytoplasmic region of the delta-sarcoglycan protein and left intact the transmembrane and extracellular domains. Line 28 had a very mild phenotype with near normal life span, intact cardiac function and normal locomotive activity. Together, these data demonstrate the essential nature of the transmembrane and extracellular domains of Drosophila delta-sarcoglycan for normal muscle structure and function.