CRYSTAL-STRUCTURE OF A CLASS-I UBIQUITIN-CONJUGATING ENZYME (UBC7) FROM SACCHAROMYCES-CEREVISIAE AT 2.9 ANGSTROM RESOLUTION

Ubiquitin-conjugating enzymes are a family of related proteins that pa rticipate in the ubiquitination of proteins. Previous studies on the c rystal structures of Saccharomyces cerevisiae Ubc4 and Arabidopsis tha liana Ubc1 indicated that the smallest enzymes (class I), which consis t entirely of the conserved core domain, share a common tertiary fold. Here we report the three-dimensional structure of the S. cerevisiae c lass I enzyme encoded by the UBC7 gene. The crystal structure has been solved using molecular replacement techniques and refined by simulate d annealing to an R-factor of 0.183 at 2.93 Angstrom resolution. Bond lengths and angles in the molecule have root-mean-square deviations fr om ideal values of 0.016 Angstrom and 2.3 degrees, respectively. Ubc7 is an alpha/beta protein with four alpha-helices and a four-stranded a ntiparallel beta-sheet. With the exception of two regions where extra residues are present, the tertiary folding of Ubc7 is similar to those of the other two enzymes. The ubiquitin-accepting cysteine is located in a cleft between two loops. One of these loops is nonconserved, as this region of the Ubc7 molecule differs from the other two enzymes by having 13 extra residues. There is also a second single amino acid in sertion that alters the orientation of the turn between the first two beta-strands. Analysis of the 13 ubiquitin-conjugating enzyme sequence s in S. cerevisiae indicates that there may be two other regions where extra residues could be inserted into the common tertiary fold. Both of these other regions exhibit significant deviations in the superposi tion of the three structures and, like the two insertion regions in Ub c7, may represent hypervariable regions within a common tertiary fold. As ubiquitin-conjugating enzymes interact with different substrates o r other accessory proteins in the ubiquitination pathway, these variab le surface regions may confer distinct specificity to individual enzym es.