A HUMAN UBIQUITIN-CONJUGATING ENZYME HOMOLOGOUS TO YEAST UBC8

Ubiquitin-conjugating enzymes catalyze the covalent attachment of ubiq uitin to cellular substrates. Here we describe the isolation of a nove l ubiquitin-conjugating enzyme from human placenta and the cloning of the corresponding cDNA. DNA sequencing revealed that this gene, UbcH2, encodes a protein with significant sequence similarity to yeast UBC8. In contrast to a previous report (Qin, S., Nakajima, B., Nomura, M., and Arfin, S.M. (1991) J. Biol. Chem. 266, 15549-15554), we discovered that UBC8 is interrupted by a single intron bearing an unusual branch point sequence. The revised amino acid sequence of yeast UBC8 exhibit s 54% amino acid sequence identity to human UbcH2. Moreover, full-leng th UbcH2 and UBC8 enzymes expressed from their cDNAs show similar enzy matic activities in vitro by catalyzing the ubiquitination of histones , suggesting that the two enzymes may fulfill similar functions in viv o. Interestingly, comparison of the enzymatic activities of a truncate d UBC8 (Qin, S., Nakajima, B., Nomura, M. and Arfin, S.M. (1991) J. Bi ol. Chem. 266, 15549-15554) and of the full-length enzyme (this report ) suggests, that the first 12 amino-terminal residues of UBC8 are requ ired for ubiquitination of histones in vitro but not for thiolester fo rmation with ubiquitin. This suggests that the NH, terminus of UBC8 ma y be necessary either for substrate recognition or for the transfer of ubiquitin onto substrates. The UbcH2 gene is located on chromosome 7 and shows a complex expression pattern with at least five different mR NAs.