We dissected physical and functional interactions between the ubiquitin-conjugating (E2) enzyme Ubc2p and Ubr1p, the E3 component of the N-end rule pathway in Saccharomyces cerevisiae. The binding of the 20 kDa Ubc2p by the 225 kDa Ubr1p is shown to be mediated largely by the basic residue-rich (BRR) region of Ubr1p. However, mutations of the BRR domain that strongly decrease the interaction between Ubr1p and Ubc2p do not prevent the degradation of N-end rule substrates. In contrast, this degradation is completely dependent on the RING-H2 finger of Ubr1p adjacent to the BRR domain. Specifically, the first cysteine of RING-H2 is required for the ubiquitylation activity of the Ubr1p-Ubc2p complex, although this cysteine plays no detectable role in either the binding of N-end rule substrates by Ubr1p or the physical affinity between Ubr1p and Ubc2p. These results defined the topography of the Ubc2p-Ubr1p interaction and revealed the essential function of the RING-H2 finger, a domain that is present in many otherwise dissimilar E3 proteins of the ubiquitin system.