Beta-2 microglobulin (beta2m) is a globular protein that self-associates into fibrillar amyloid deposits in patients undergoing hemodialysis therapy. Formation of these beta-sheet-rich assemblies is a fundamental property of polypeptides that can be triggered by diverse conditions. For beta2m, oligomerization into pre-amyloidogenic states occurs in specific response to coordination by Cu2+. Here we report the basis for this self-association at atomic resolution. Metal is not a direct participant in the molecular interface. Rather, binding results in distal alterations enabling the formation of two new surfaces. These interact to form a closed hexameric species. The origins of this include isomerization of a buried and conserved cis-proline previously implicated in the beta2m aggregation pathway. The consequences of this isomerization are evident and reveal a molecular basis for the conversion of this robust monomeric protein into an amyloid-competent state.