Sites of vulnerability in SARS-CoV-2: Antibodies that neutralize severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) could be an important tool in treating coronavirus disease 2019 (COVID-19). Brouwer et al. isolated 403 monoclonal antibodies from three convalescent COVID-19 patients. They show that the patients had strong immune responses against the viral spike protein, a complex that binds to receptors on the host cell. A subset of antibodies was able to neutralize the virus. Competition and electron microscopy studies showed that these antibodies target diverse epitopes on the spike, with the two most potent targeting the domain that binds the host receptor. Science , this issue p. 643

Significance: Coronaviruses such as Middle East respiratory syndrome coronavirus (MERS-CoV) cause severe respiratory distress with high fatality rates. The spike (S) glycoprotein is a determinant of host range and is the target of neutralizing antibodies and subunit vaccine development. We describe an engineering strategy for stabilization of soluble S proteins in the prefusion conformation, which results in greatly increased expression, conformational homogeneity, and elicitation of potent antibody responses. Cryo-EM structures of the stabilized MERS-CoV S protein in complex with a stem-directed neutralizing antibody provide a molecular basis for host-cell protease requirements and identify a site of immune pressure. We also defined four conformational states of the trimer wherein each receptor-binding domain is either packed together at the membrane-distal apex or rotated into a receptor-accessible conformation.

Knowing the Enemy: Infection of host cells by HIV-1 is mediated by an envelope glycoprotein (Env) trimeric spike on the surface of the virus. Proteins comprising the Env trimer must be cleaved for infectivity, and thus viral fusion involves three Env conformations. The flexibility of the Env trimer has made it a challenge to determine a high-resolution structure, although such a structure is key both for understanding trimer function and for guiding vaccine design. Lyumkis et al. (p. 1484 ) and Julien et al. (p. 1477 ) studied soluble cleaved trimers stabilized by specific mutations but that have kept a near-native antigenicity profile. Lyumkis et al. present a high-resolution structure of the trimer in complex with a broadly neutralizing antibody, and Julien et al. present a crystal structure of the trimer in complex with another broadly neutralizing antibody.

Recent history is punctuated by the emergence of highly pathogenic coronaviruses such as SARS- and MERS-CoV into human circulation. Upon infecting host cells, coronaviruses assemble a multi-subunit RNA-synthesis complex of viral non-structural proteins (nsp) responsible for the replication and transcription of the viral genome. Here, we present the 3.1 Å resolution structure of the SARS-CoV nsp12 polymerase bound to its essential co-factors, nsp7 and nsp8, using single particle cryo-electron microscopy. nsp12 possesses an architecture common to all viral polymerases as well as a large N-terminal extension containing a kinase-like fold and is bound by two nsp8 co-factors. This structure illuminates the assembly of the coronavirus core RNA-synthesis machinery, provides key insights into nsp12 polymerase catalysis and fidelity and acts as a template for the design of novel antiviral therapeutics.

Knowing the Enemy: Infection of host cells by HIV-1 is mediated by an envelope glycoprotein (Env) trimeric spike on the surface of the virus. Proteins comprising the Env trimer must be cleaved for infectivity, and thus viral fusion involves three Env conformations. The flexibility of the Env trimer has made it a challenge to determine a high-resolution structure, although such a structure is key both for understanding trimer function and for guiding vaccine design. Lyumkis et al. (p. 1484 ) and Julien et al. (p. 1477 ) studied soluble cleaved trimers stabilized by specific mutations but that have kept a near-native antigenicity profile. Lyumkis et al. present a high-resolution structure of the trimer in complex with a broadly neutralizing antibody, and Julien et al. present a crystal structure of the trimer in complex with another broadly neutralizing antibody.