Gira Bhabha

Antibody Recognition of a Highly Conserved Influenza Virus Epitope

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.

A highly conserved neutralizing epitope on group 2 influenza A viruses.

An antibody against a conserved epitope broadly neutralizes group 2 influenza viruses. Current flu vaccines provide only limited coverage against seasonal strains of influenza viruses. The identification of VH1-69 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 VH1-69 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.

A dynamic knockout reveals that conformational fluctuations influence the chemical step of enzyme catalysis.

An Escherichia coli dihydrofolate reductase mutant is catalytically defective, because motions in the active site are impaired. 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.

Cross-neutralization of influenza A viruses mediated by a single antibody loop

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.

Activation of cytoplasmic dynein motility by dynactin-cargo adapter complexes

How dynein makes the right moves The molecular motor cytoplasmic dynein moves a wide range of different intracellular cargoes. Dyneins activity in vivo requires another protein, dynactin, but exactly why that should be has been very unclear. Although in vitro experiments have provided some evidence that dynactin increases dyneins processivity, the resulting dynein motility has never come close to matching dyneins cargo-transporting activity in living cells. Now, McKenney et al. show that tripartite complexes of dynein, dynactin, and an adaptor molecule are highly processive in vitro, moving the sort of distances that dynein transports cargo in vivo (see the Perspective by Allan). Science, this issue p. 337; see also p. 271 Single-molecule studies reveal a mechanism to activate the molecular motor cytoplasmic dynein in a cargo-specific manner. [Also see Perspective by Allan] Cytoplasmic dynein is a molecular motor that transports a large variety of cargoes (e.g., organelles, messenger RNAs, and viruses) along microtubules over long intracellular distances. The dynactin protein complex is important for dynein activity in vivo, but its precise role has been unclear. Here, we found that purified mammalian dynein did not move processively on microtubules in vitro. However, when dynein formed a complex with dynactin and one of four different cargo-specific adapter proteins, the motor became ultraprocessive, moving for distances similar to those of native cargoes in living cells. Thus, we propose that dynein is largely inactive in the cytoplasm and that a variety of adapter proteins activate processive motility by linking dynactin to dynein only when the motor is bound to its proper cargo.

A common solution to group 2 influenza virus neutralization

Significance The HA surface glycoprotein on influenza A viruses mediates viral entry into host cells. HA is highly variable and classified into 18 divergent subtypes, which cluster into two major phylogenetic groups. Antibody CR8043 has heterosubtypic neutralizing activity against group 2 viruses, including H3 viruses that currently circulate in humans. X-ray and EM structures of CR8043 Fab in complex with H3 HAs reveal that the antibody targets a conserved epitope on the HA stem. Compared with CR8020, the only other structurally characterized group 2 neutralizing antibody, CR8043 binds to HA with a different approach angle using different contact residues. The epitopes of both antibodies are very similar, which suggests that this conserved stem epitope has great potential for design of therapeutics and vaccines. The discovery and characterization of broadly neutralizing antibodies (bnAbs) against influenza viruses have raised hopes for the development of monoclonal antibody (mAb)-based immunotherapy and the design of universal influenza vaccines. Only one human bnAb (CR8020) specifically recognizing group 2 influenza A viruses has been previously characterized that binds to a highly conserved epitope at the base of the hemagglutinin (HA) stem and has neutralizing activity against H3, H7, and H10 viruses. Here, we report a second group 2 bnAb, CR8043, which was derived from a different germ-line gene encoding a highly divergent amino acid sequence. CR8043 has in vitro neutralizing activity against H3 and H10 viruses and protects mice against challenge with a lethal dose of H3N2 and H7N7 viruses. The crystal structure and EM reconstructions of the CR8043-H3 HA complex revealed that CR8043 binds to a site similar to the CR8020 epitope but uses an alternative angle of approach and a distinct set of interactions. The identification of another antibody against the group 2 stem epitope suggests that this conserved site of vulnerability has great potential for design of therapeutics and vaccines.

Automated identification of functional dynamic contact networks from X-ray crystallography.

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.

Exploring the repeat protein universe through computational protein design.

A central question in protein evolution is the extent to which naturally occurring proteins sample the space of folded structures accessible to the polypeptide chain. Repeat proteins composed of multiple tandem copies of a modular structure unit are widespread in nature and have critical roles in molecular recognition, signalling, and other essential biological processes. Naturally occurring repeat proteins have been re-engineered for molecular recognition and modular scaffolding applications. Here we use computational protein design to investigate the space of folded structures that can be generated by tandem repeating a simple helix–loop–helix–loop structural motif. Eighty-three designs with sequences unrelated to known repeat proteins were experimentally characterized. Of these, 53 are monomeric and stable at 95 °C, and 43 have solution X-ray scattering spectra consistent with the design models. Crystal structures of 15 designs spanning a broad range of curvatures are in close agreement with the design models with root mean square deviations ranging from 0.7 to 2.5 Å. Our results show that existing repeat proteins occupy only a small fraction of the possible repeat protein sequence and structure space and that it is possible to design novel repeat proteins with precisely specified geometries, opening up a wide array of new possibilities for biomolecular engineering.

Divergent evolution of protein conformational dynamics in dihydrofolate reductase.

Molecular evolution is driven by mutations, which may affect the fitness of an organism and are then subject to natural selection or genetic drift. Analysis of primary protein sequences and tertiary structures has yielded valuable insights into the evolution of protein function, but little is known about the evolution of functional mechanisms, protein dynamics and conformational plasticity essential for activity. We characterized the atomic-level motions across divergent members of the dihydrofolate reductase (DHFR) family. Despite structural similarity, Escherichia coli and human DHFRs use different dynamic mechanisms to perform the same function, and human DHFR cannot complement DHFR-deficient E. coli cells. Identification of the primary-sequence determinants of flexibility in DHFRs from several species allowed us to propose a likely scenario for the evolution of functionally important DHFR dynamics following a pattern of divergent evolution that is tuned by cellular environment.

How Dynein Moves Along Microtubules

Cytoplasmic dynein, a member of the AAA (ATPases Associated with diverse cellular Activities) family of proteins, drives the processive movement of numerous intracellular cargos towards the minus end of microtubules. Here, we summarize the structural and motile properties of dynein and highlight features that distinguish this motor from kinesin-1 and myosin V, two well-studied transport motors. Integrating information from recent crystal and cryoelectron microscopy structures, as well as high-resolution single-molecule studies, we also discuss models for how dynein biases its movement in one direction along a microtubule track, and present a movie that illustrates these principles.