The arenavirus family includes >50 known viruses, including both severe human pathogens and laboratory models such as lymphocytic choriomeningitis virus (LCMV). All of the Old World arenaviruses and some of the New World arenaviruses use alpha- dystroglycan as a cellular receptor. This molecule is bound by the single glycoprotein expressed on the viral surface, termed GP. GP thus mediates attachment, fusion and entry into host cells, and also determines tropism and often, pathogenicity. Understanding the structure of GP, especially the structure of oligomeric, prefusion GP as it exists on the viral surface, is essential to understanding virus entry and tropism and to developing vaccines, antibodies, and antivirals. However, no structure yet exists for any prefusion GP of any arenavirus. Further, no structure yet exists for the GP1 receptor-binding subunit of GP for any arenavirus that uses alpha-dystroglycan as a receptor. In our preliminary data, we describe our newly determined, hard-won crystal structure of the fully glycosylated, prefusion arenavirus GP from the ?-dystroglycan- binding Old World virus LCMV. This LCMV GP ectodomain forms a trimer in the crystals. In Aim 1, we propose to determine if this arrangement represents the biologically relevant trimer on the surface of arenaviruses. In Aim 2, we propose to map the ?- dystroglycan-binding site of the Old World arenavirus GP. An R21 is intended for novel studies that break new ground and extend previous results in new directions. The research proposed here will indeed provide transformative models for understanding assembly and entry of this very large family of human pathogens. It will also provide new templates for design of vaccines, therapeutics, and immunotherapeutic cocktails. Aim1 will provide the first biologically supported model of prefusion, oligomeric GP, which is likely applicable to the entire arenavirus family. Aim 2 will elucidate the receptor-binding site for the many Old World arenaviruses and those of clade C New World arenaviruses as well. Aim 2 will also provide templates for understanding viral tropism, and will explain historical mutations that determine disease course and that have been used extensively to illuminate virus immunobiology.