The three-dimensional structure of a yeast ubiquitin-conjugating enzyme, encoded by the Saccharomyces cerevisiae UBC4 gene, has been determined at 2.7 A. The structure was solved using molecular replacement techniques and refined by simulated annealing to an R-factor of 0.198. Bond lengths and angles in the molecule have root mean square deviations from ideal values of 0.018 A and 4.0 degrees, respectively. Ubc4 is an alpha/beta protein with four alpha-helices and a four-stranded antiparallel beta-sheet. The ubiquitin-accepting cysteine is located in a cleft between two loops. Comparison with the recently determined structure of a different plant enzyme suggests that class I ubiquitin-conjugating enzymes are highly conserved in their three-dimensional folding. Except for two extra residues at the N- and the C-terminus of the plant enzyme, the C alpha atoms of the two enzymes can be superimposed with a root mean square deviation of only 1.52 A. Greater variations are found between the surfaces of the two molecules, as most of the identical residues between the two enzymes are either buried or clustered on the surface that lies adjacent to the ubiquitin-accepting cysteine. We suggest that this conserved surface functions in protein-protein binding during ubiquitin thiol ester formation.