A new side to ubiquitin.

Mono-ubiquitination is a common mechanism of protein regulation, and more than ten ubiquitin-interacting domains that recognize the hydrophobic region centered on Ile44 of ubiquitin have been characterized. Two recent reports describe the crystal structure of the Rab5 guanine-nucleotide-exchange factor Rabex-5 and show that it contains two novel ubiquitin-binding domains. One of these is an A20 zinc finger that binds to a polar interaction interface of ubiquitin centered on Asp58. The discovery of an alternative interaction face of ubiquitin opens new avenues for understanding how this small protein regulates protein function.