Abstract: A novel class of fusion proteins based on the ubiquitin-conjugating enzyme, or E2, is described. The fusion proteins include, in addition to the E2 activity, a protein binding ligand having a specific affinity for a target protein. It has been discovered that under cytosolic conditions, such E2 fusions will add a ubiquitin moiety to a target protein. Since ubiquitin addition triggers the endogenous cellular protein degradation pathway, such E2 fusion proteins can be used to selectively target proteins in a host for degradation. Thus, E2 fusion proteins genes can be introduced into transgenic organisms to defeat or inhibit natural activities or traits. The E2 fusion proteins can also be used by introduction into hosts for similar effects.