The U-box is a highly conserved domain recently identified at the C terminu
s of yeast UFD2, an E4 ubiquitination factor. In yeast, UFD2 is the only U-
box-containing protein, but there are two UFD2 homologs and several other p
roteins containing a U-box domain in humans. Intriguingly, a database searc
h revealed 37 predicted proteins containing a U-box in Arabidopsis. The pla
nt U-box (PUB) proteins form five distinct subclasses, suggesting that they
play diverse roles. The ARC1 gene from Brassica, required for self-incompa
tibility, is currently the only PUB gene functionally characterized. Here,w
e discuss the characteristics and possible functions of the PUB gene family
.