The ubiquitin-mediated degradation of cellular proteins requires the s
equential activity of E1, E2 and, in some cases, E3 enzymes. Using the
yeast two-hybrid system, we have cloned 1.0- and 2.5-kb cDNAs encodin
g the identical murine E2, or ubiquitin conjugating enzyme by virtue o
f its interaction with the E2A transcription factor. This cDNA encodes
the 158-amino-acid protein, mUBC9, which has considerable sequence ho
mology to UBC9 from Saccharomyces cerevisiae and HUS5 from Schizosacch
aromyces pombe and is identical to the human UBC9 protein. HUS5 is ess
ential for DNA damage repair, whereas UBC9 is necessary for G2/M progr
ession. The human protein has been shown to correct the UBC9 defect in
yeast. Antisera raised against bacterially expressed mUBC9 fusion pro
tein recognize a murine cellular protein of approximately 18 kDa, corr
esponding to the predicted mobility. Unlike E2A, the mUBC9 protein lev
el is not regulated by serum growth factors. The activity of the appar
ent homologues UBC9 and HUS5 suggests that mUBC9 may be involved in th
e degradation of key nuclear proteins that regulate cell cycle progres
sion. (C) 1997 Elsevier Science B.V.