The U-box protein family in plants

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 .