The armadillo repeats of the Ufd4 ubiquitin ligase recognize ubiquitin-fusion proteins

Armadillo (ARM) repeats have been identified in proteins regulating cell junction assembly, nuclear transport and transcription activation. Recent studies showed that a large number of ubiquitin (Ub) ligases (or E3s) also contain ARM repeats. The function of the ARM repeats in these E3s, however, remains unknown. Here we report that the ARM repeats of Ufd4, the E3 component of the UFD (Ub fusion degradation) pathway, recognize the Ub degradation signal of the UFD substrates. Disruption of the ARM repeats abolishes the ubiquitylating activity of Ufd4. This study uncovers a new role of the ARM repeats in protein ubiquitylation.