Respiratory syncytial virus (RSV) is a human respiratory pathogen which remains a leading viral cause of hospitalizations and mortality among infants in their first year of life. Here, we review the biology of RSV, the primary laboratory isolates or strains which have been used to best characterize the virus since its discovery in 1956, and discuss the implications for genetic and functional variations between the established laboratory strains and the recently identified clinical isolates.

26Human coronaviruses are enveloped, positive-strand RNA viruses which cause respiratory 27 diseases ranging in severity from the seasonal common cold to SARS and COVID-19. Of the 7 28 human coronaviruses discovered to date, 3 emergent and severe human coronavirus strains 29 (SARS-CoV, MERS-CoV, and SARS-CoV-2) have recently jumped to humans in the last 20 30 years. The COVID-19 pandemic spawned by the emergence of SARS-CoV-2 in late 2019 has 31 highlighted the importance for development of effective therapeutics to target emerging 32 coronaviruses. Upon entry, the replicase genes of coronaviruses are translated and subsequently 33 proteolytically processed by virus-encoded proteases. Of these proteases, nonstructural protein 5 34 (nsp5, Mpro, or 3CLpro), mediates the majority of these cleavages and remains a key drug target 35 for therapeutic inhibitors. Efforts to develop nsp5 active-site inhibitors for human coronaviruses 36 have thus far been unsuccessful, establishing the need for identification of other critical and 37 conserved non-active-site regions of the protease. In this study, we describe the identification of 38 an essential, conserved horseshoe-shaped region in the nsp5 interdomain loop (IDL) of mouse 39 hepatitis virus (MHV), a common coronavirus replication model. Using site-directed 40 mutagenesis and replication studies, we show that several residues comprising this horseshoe-41 shaped region either fail to tolerate mutagenesis or were associated with viral temperature-42 sensitivity. Structural modeling and sequence analysis of these sites in other coronaviruses, 43 including all 7 human coronaviruses, suggests that the identified structure and sequence of this 44 horseshoe regions is highly conserved and may represent a new, non-active-site regulatory 45 region of the nsp5 (3CLpro) protease to target with coronavirus inhibitors. 46 47 48 49 In December 2019, a novel coronavirus (SARS-CoV-2) emerged in humans and triggered a 50 pandemic which has to date resulted in over 8 million confirmed cases of COVID-19 across 51 more than 180 countries and territories (June 2020). SARS-CoV-2 represents the third emergent 52 coronavirus in the past 20 years and the future emergence of new coronaviruses in humans 53 remains certain. Critically, there remains no vaccine nor established therapeutics to treat cases of 54 COVID-19. The coronavirus nsp5 protease is a conserved and indispensable virus-encoded 55 enzyme which remains a key target for therapeutic design. However, past attempts to target the 56 active site of nsp5 with inhibitors have failed stressing the need to identify new conserved non-57 active-site targets for therapeutic development. This study describes the discovery of a novel 58 conserved structural region of the nsp5 protease of coronavirus mouse hepatitis virus (MHV) 59 which may provide a new target for coronavirus drug development. 60 61 Introduction 62 63 Coronaviruses (CoVs) are enveloped, positive-strand RNA viruses which encode among the 64 largest RNA virus genomes on the planet ...