Involvement of Valosin-Containing Protein in Ubiquitin-Proteasome Mediated Degra

Basic Sciences

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Abstract

The ubiquitin-proteasome (Ub-Pr) mediated degradation pathway has been shown to regulate many important cellular processes. The pathway consists of two sequential steps: the target protein is first conjugated with multiple ubiquitin molecules, then transferred to the 26S proteasome, unfolded and degraded. The multisubunit proteasome has ATPase activity which presumably provides the energy needed for the chaperoning and unfolding of the substrate before its degradation. We previously identified Valosin-containing protein (VCP), a highly conserved ATPase, physically and functionally associated with both the ubiquitinated Ub-Pr substrates and the highly purified proteasome. We hypothesize that VCP is an ATPase chaperone that carries the ubiquitinated substrates to the proteasome. VCP and its orthologs, Cdc48p and p97, belong to a highly conserved AAA (ATPases associated with a variety of cellular activities) family. Despite the high sequence and structural similarities among VCP/Cdc48p/p97, paradoxically these proteins are involved in many seemingly unrelated functions, e.g. cell cycle control, T and B cell activation, homotypic membrane fusion, vesical-mediated transport, and the Ub-Pr degradation. The apparent paradox suggests that VCP may function in a common mechanism that underlies these activities. Since Ub-Pr pathways have been shown to regulate many substrates that are also involved in these same activities, our hypothesis of VCP being a chaperone in the Ub-Pr pathways would provide the underlying mechanism explaining the paradox for VCP. We demonstrate that VCP preferentially binds the ubiquitinated substrates, and this binding is mediated through a direct interaction between N-terminal 200 residues of VCP and the poly-ubiquitin chains. Preliminary data also reveal that VCP prevents aggregate formation, an indicative characteristic of molecular chaperones. Our current and future research focuses on demonstrating the required chaperone role of VCP in Ub-Pr-mediated degradation, and to elucidate the structure-function relationships in VCP.