FUNCTIONAL AND PHYSICAL CHARACTERIZATION OF THE CELL-CYCLE UBIQUITIN-CONJUGATING ENZYME CDC34 (UBC3) - IDENTIFICATION OF A FUNCTIONAL DETERMINANT WITHIN THE TAIL THAT FACILITATES CDC34 SELF-ASSOCIATION

Like several other ubiquitin-conjugating enzymes, the yeast cell cycle enzyme CDC34 (UBCS) has a carboxyl terminal extension or tail. These tails appear to carry out unique functions that can vary from one ubiq uitin-conjugating enzyme to the next. Using biophysical techniques we have determined that the tail of CDC34 constitutes a highly structured and extended domain. Although the tail of CDC34 is the largest tail i dentified to date (125 residues), we have found that only 39 residues lying adjacent to the catalytic domain are necessary and sufficient fo r full cell cycle function and that this region fulfills a novel funct ion that may be common to the tails of other ubiquitin-conjugating enz ymes. Cross-linking studies demonstrate that this region facilitates a physical interaction between CDC34 monomers in vitro. Furthermore, ph enotypic analysis of various CDC34 derivatives expressed in different cdc34 mutant strains indicates that this region facilitates the same i nteraction in vivo. Based on these findings, it appears that the cell cycle function of CDC34 is dependent upon the ability of CDC34 monomer s to interact with one another and that this interaction is mediated b y a small region of the CDC34 tail. The similarity of this region with sequences contained within the tails of the UBC1 and UBC6 enzymes sug gests that these tails may function in a similar manner.