Valosin-containing protein is a multiubiquitin chain targeting factor required in ubiquitin-proteasome degradation

The ubiquitin-proteasome (Ub-Pr) degradation pathway regulates many cellular activities(1,2), but how ubiquitinated substrates are targeted to the proteasome is not understood. We have shown previously that valosin-containing protein (VCP) physically and functionally targets the ubiquitinated nuclear factor kappaB inhibitor, I kappaB alpha, to the proteasome for degradation(3). VCP is an abundant and a highly conserved member of the AAA (ATPases associated with a variety of cellular activities) family(5-7). Besides acting as a chaperone in membrane fusions, VCP has been shown to have a role in a number of seemingly unrelated cellular activities. Here we report that loss of VCP function results in an inhibition of Ub-Pr-mediated degradation and an accumulation of ubiquitinated proteins. VCP associates with ubiquitinated proteins through the direct binding of its amino-terminal domain to the multi-ubiquitin chains of substrates. Furthermore, its N-terminal domain is required in Ub-Pr-mediated degradation. We conclude that VCP is a multi-ubiquitin chain-targeting factor that is required in the degradation of many Ub-Pr pathway substrates, and provide a common mechanism that underlies many of the functions of VCP.