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Canine distemper virus (CDV) is an enveloped RNA morbillivirus that triggers respiratory, enteric, and high incidence of severe neurological disorders. CDV induces devastating outbreaks in wild and endangered animals as well as in domestic dogs in countries associated with suboptimal vaccination programs. The receptor-binding tetrameric attachment (H)-protein is part of the morbilliviral cell entry machinery. Here, we present the cryo-electron microscopy (cryo-EM) structure and supramolecular organization of the tetrameric CDV H-protein ectodomain. The structure reveals that the morbilliviral H-protein is composed of three main domains: stalk, neck, and heads. The most unexpected feature was the inherent asymmetric architecture of the CDV H-tetramer being shaped by the neck, which folds into an almost 90° bent conformation with respect to the stalk. Consequently, two non-contacting receptor-binding H-head dimers, which are also tilted toward each other, are located on one side of an intertwined four helical bundle stalk domain. Positioning of the four protomer polypeptide chains within the neck domain is guided by a glycine residue (G158), which forms a hinge point exclusively in two protomer polypeptide chains. Molecular dynamics simulations validated the stability of the asymmetric structure under near physiological conditions and molecular docking showed that two receptor-binding sites are fully accessible. Thus, this spatial organization of the CDV H-tetramer would allow for concomitant protein interactions with the stalk and head domains without steric clashes. In summary, the structure of the CDV H-protein ectodomain provides new insights into the morbilliviral cell entry system and offers a blueprint for next-generation structure-based antiviral drug discovery.

Multiple enveloped RNA viruses of the family Paramyxoviridae and Pneumoviridae, like measles virus (MeV), Nipah virus (NiV), canine distemper virus (CDV), or respiratory syncytial virus (RSV), are of high clinical relevance. Each year a huge number of lives are lost as a result of these viral infections. Worldwide, MeV infection alone is responsible for over a hundred thousand deaths each year despite available vaccine. Therefore, there is an urgent need for treatment options to counteract these viral infections. The development of antiviral drugs in general stands as a huge challenge due to the rapid emergence of viral escape mutants. Here, we disclose the discovery of a small-molecule antiviral, compound 1 (ZHAWOC9045), active against several pneumo-/paramyxoviruses, including MeV, NiV, CDV, RSV, and parainfluenza virus type 5 (PIV-5). A series of mechanistic characterizations revealed that compound 1 targets a host factor which is indispensable for viral genome replication. Drug resistance profiling against a paramyxovirus model (CDV) demonstrated no detectable adaptation despite prolonged time of investigation, thereby mitigating the rapid emergence of escape variants. Furthermore, a thorough structure-activity relationship analysis of compound 1 led to the invention of 100-times-more potent-derivatives, e.g., compound 2 (ZHAWOC21026). Collectively, we present in this study an attractive host-directed pneumoviral/paramyxoviral replication inhibitor with potential therapeutic application. IMPORTANCE Measles virus, respiratory syncytial virus, canine distemper virus, and Nipah virus are some of the clinically significant RNA viruses that threaten substantial number of lives each year. Limited to no availability of treatment options for these viral infections makes it arduous to handle the outbreaks. This highlights the major importance of developing antivirals to fight not only ongoing infections but also potential future epidemics. Most of the discovered antivira