20 21 22SARS-CoV-2 is a newly emerged coronavirus responsible for the current COVID-2319 pandemic that has resulted in more than one million infections and 73,000 24 deaths 1,2 . Vaccine and therapeutic discovery efforts are paramount to curb the 25 pandemic spread of this zoonotic virus. The SARS-CoV-2 spike (S) glycoprotein 26 promotes entry into host cells and is the main target of neutralizing antibodies. 27Here we describe multiple monoclonal antibodies targeting SARS-CoV-2 S 28 identified from memory B cells of a SARS survivor infected in 2003. One 29 antibody, named S309, potently neutralizes SARS-CoV-2 and SARS-CoV 30 pseudoviruses as well as authentic SARS-CoV-2 by engaging the S receptor-31 binding domain. Using cryo-electron microscopy and binding assays, we show 32 that S309 recognizes a glycan-containing epitope that is conserved within the 33 sarbecovirus subgenus, without competing with receptor attachment. Antibody 34 cocktails including S309 along with other antibodies identified here further 35 enhanced SARS-CoV-2 neutralization and may limit the emergence of 36 neutralization-escape mutants. These results pave the way for using S309 and 37 S309-containing antibody cocktails for prophylaxis in individuals at high risk of 38 exposure or as a post-exposure therapy to limit or treat severe disease. 39 40 : bioRxiv preprint 2 Coronavirus entry into host cells is mediated by the transmembrane spike (S) 41 glycoprotein that forms homotrimers protruding from the viral surface 3 . The S 42 glycoprotein comprises two functional subunits: S1 (divided into A, B, C and D domains) 43 that is responsible for binding to host cell receptors and S2 that promotes fusion of the 44 viral and cellular membranes 4,5 . Both SARS-CoV-2 and SARS-CoV belong to the 45 sarbecovirus subgenus and their S glycoproteins share 80% amino acid sequence 46 identity 6 . SARS-CoV-2 S is closely related to the bat SARS-related CoV (SARSr-CoV) 47RaTG13 with which it shares 97.2% amino acid sequence identity 1 . We and others 48 recently demonstrated that human angiotensin converting enzyme 2 (hACE2) is a 49 functional receptor for SARS-CoV-2, as is the case for SARS-CoV 1,6-8 . The S domain 50 B (S B ) is the receptor binding domain (RBD) and binds to hACE2 with high-affinity, 51possibly contributing to the current rapid SARS-CoV-2 transmission in humans 6,9 , as 52 previously proposed for SARS-CoV 10 . 53As the coronavirus S glycoprotein mediates entry into host cells, it is the main 54 target of neutralizing antibodies and the focus of therapeutic and vaccine design 55 efforts 3 . The S trimers are extensively decorated with N-linked glycans that are 56 important for protein folding 11 and modulate accessibility to host proteases and 57 neutralizing antibodies 12-15 . Cryo-electron microscopy (cryoEM) structures of SARS-58CoV-2 S in two distinct functional states 6,9 along with cryoEM and crystal structures of 59 SARS-CoV-2 S B in complex with hACE2 16-18 revealed dynamic states of S B domains, 60 providing a blueprint for the desig...