The present invention is based on the discovery of a composition that provides targeted ubiquitination. Specifically the composition contains a ubiquitin pathway protein binding moiety which recognizes a ubiquitin pathway protein and a targeting moiety which recognizes a target protein. In addition, the present invention provides libraries of compositions, where each composition contains a ubiquitin pathway protein binding moiety and a member of a molecular library. The libraries of the present invention can be used to identify proteins involved in a predetermined function of cells.