Ubxd1 is a novel co-factor of the human p97 ATPase

The AAA ATPase complex known as p97 or VCP in mammals and Cdc48 in yeast is connected to a multitude of cellular pathways, including membrane fusion, protein folding, protein degradation and activation of membrane-bound transcription factors. The mechanism by which p97 participates in such a broad spectrum of cellular functions appears to be via recruiting certain specific co-factors. Here we isolate and characterize the human protein Ubxd1, a novel co-factor of p97. We show that Ubxd1 is a stable protein that localizes to the cytoplasm and nucleus and is highly enriched in centrosomes. In mice Ubxd1 is widely expressed, but especially abundant in brain. Curiously, Ubxd1 does not associate with p97 via its UBX domain, but via its PUB domain which binds the extreme C-terminus of p97. Phosphorylation of the penultimate tyrosine residue in p97 completely abolishes Ubxd1 interaction. Ternary complexes of Ubxd1, p47, and p97 were detected in vitro. Inhibition of Ubxd1 expression by siRNA did not affect the degradation of bulk protein or a model substrate of the ERAD pathway, indicating that Ubxd1 directs p97 activity to specialized functions in vivo.