Orthobunyaviruses are enveloped viruses that can cause human and animal diseases. A novel and major member is the Schmallenberg virus(SBV), the etiological agent of an emerging disease of ruminants that has been spreading all over Europe since 2011. The glycoproteins Gn and Gc of orthobunyaviruses mediate the viral entry, and specifically Gc is a major target for the humoral immune response. For example, the N terminal subdomain of the SBV glycoprotein Gc is targeted by neutralizing monoclonal antibodies that recognize conformational epitopes. Here, we determined the structural features of the N terminus of Gc, and analysed its interaction with monoclonal antibodies. We were able to demonstrate that one of two N-glycosylation sites is essential for secretion and interaction with a subset of Gc-specific monoclonal antibodies. Furthermore, four disulfide bonds (S-S) were identified and the deletion of the third S-S blocked reactivity with another subset of mAbs with virus-neutralizing and non-neutralizing activity. The mutagenesis of the N-glycosylation sites and the disulfide bonds strongly indicated the independent folding of two subdomains within the SBV Gc N terminus. Further, the epitopes recognized by a panel of mAbs could be grouped into two clusters, as revealed by fine mapping using chimeric proteins. Combining the disulfide bonding and epitope mapping allowed us to generate a structural model of the SBV Gc N-terminus. This novel information about the role and structure of the amino terminal region of SBV Gc is of general relevance for the design of antivirals and vaccines against this virus.
Cell surface aminopeptidase N (APN) is a membrane-bound ectoenzyme that hydrolyzes proteins and peptides and regulates numerous cell functions. APN participates in tumor cell expansion and motility, and is a target for cancer therapies. Small drugs that bind to the APN active site inhibit catalysis and suppress tumor growth. APN is also a major cell entry receptor for coronavirus, which binds to a region distant from the active site. Three crystal structures that we determined of human and pig APN ectodomains defined the dynamic conformation of the protein. These structures offered snapshots of closed, intermediate and open APN, which represent distinct functional states. Coronavirus envelope proteins specifically recognized the open APN form, prevented ectodomain progression to the closed form and substrate hydrolysis. In addition, drugs that bind the active site inhibited both coronavirus binding to cell surface APN and infection; the drugs probably hindered APN transition to the virus-specific open form. We conclude that allosteric inhibition of APN functions occurs by ligand suppression of ectodomain motions necessary for catalysis and virus cell entry, as validated by locking APN with disulfides. Blocking APN dynamics can thus be a valuable approach to development of drugs that target this ectoenzyme.
Emerging and re-emerging pathogens represent a substantial threat to public health, as demonstrated with numerous outbreaks over the past years, including the 2013-2016 outbreak of Ebola virus in western Africa. Coronaviruses are also a threat for humans, as evidenced in2002/2003 with infection by the severe acute respiratory syndrome coronavirus (SARS-CoV), which caused more than 8000 human infections with 10% fatality rate in 37 countries. Ten years later, a novel human coronavirus (Middle East respiratory syndrome coronavirus, MERS-CoV), associated with severe pneumonia, arose in the Kingdom of Saudi Arabia. Until December 2016, MERS has accounted for more than 1800 cases and 35% fatality rate. Finding an animal model of disease is key to develop vaccines or antivirals against such emerging pathogens and to understand its pathogenesis. Knowledge of the potential role of domestic livestock and other animal species inthe transmission of pathogens is of importance to understand the epidemiology of the disease. Little is known about MERS-CoV animal host range. In this paper, experimental data on potential hosts for MERS-CoV is reviewed. Advantages and limitations of different animal models are evaluated in relation to viral pathogenesis and transmission studies. Finally, the relevance of potential new target species is discussed.