Valosin-containing protein (VCP) or transitional endoplasmic reticulum ATPase (TER ATPase) also known as p97 in mammals and CDC48 in S. cerevisiae, is an enzyme that in humans is encoded by the VCP gene. The TER ATPase is an ATPase enzyme present in all eukaryotes and archaebacteria. Its main function is to segregate protein molecules from large cellular structures such as protein assemblies, organelle membranes and chromatin, and thus facilitate the degradation of released polypeptides by the multi-subunit protease proteasome.
VCP/p97/CDC48 is a member of the AAA+ (extended family of ATPases associated with various cellular activities) ATPase family. Enzymes of this family are found in all species from bacteria to humans. Many of them are important chaperones that regulate folding or unfolding of substrate proteins. VCP is a type II AAA+ ATPase, which means that it contains two tandem ATPase domains (named D1 and D2, respectively) (Figure 1). The two ATPase domains are connected by a short polypeptide linker. A domain preceding the D1 domain (N-terminal domain) and a short carboxyl-terminal tail are involved in interaction with cofactors. The N-domain is connected to the D1 domain by a short N-D1 linker.
Most known substrates of VCP are modified with ubiquitin chains and degraded by the 26S proteasome. Accordingly, many VCP coenzymes and adaptors have domains that can recognize ubiquitin. It has become evident that the interplays between ubiquitin and VCP cofactors are critical for many of the proposed functions, although the precise role of these interactions remains to be elucidated.
Discovery
CDC48 was discovered in a genetic screen for genes involved in cell cycle regulation in budding yeast. The screen identified several alleles of Cdc48 that affect cell growth at non-permissive temperatures. A search for the mammalian homolog of CDC48 (valosin) revealed a 97 kDa protein precursor named "valosin-containing protein (VCP)" or p97, and also showed that it was only generated as an artefact of purification rather than during physiological processing. Even without evidence that valosin is a physiological product, the VCP nomenclature continues to be used in the literature.
Tissue and subcelluar distribution
VCP is one of the most abundant cytoplasmic proteins in eukaryotic cells. It is ubiquitously expressed in all tissues in multicellular organisms. In humans, the mRNA expression of VCP was found to be moderately elevated in certain types of cancer.
In mammalian cells, VCP is predominantly localized to the cytoplasm, and a significant fraction is associated to membranes of cellular organelles such as the endoplasmic reticulum (ER), Golgi, mitochondria, and endosomes. The subcellular localization of CDC48 has not been fully characterized, but is likely to be similar to the mammalian counterpart. A fraction of VCP was also found in the nucleus.
Structure
According to the crystal structures of full-length wild-type VCP, six VCP subunits assemble into a barrel-like structure, in which the N-D1 and D2 domains form two concentric, stacked rings (Figure 2). The N-D1 ring is larger (162 Å in diameter) than the D2 ring (113 Å) due to the laterally attached N-domains. The D1 and D2 domains are highly homologous in both sequence and structure, but they serve distinct functions. For example, the hexameric assembly of VCP only requires the D1 but not the D2 domain. Unlike many bacterial AAA+ proteins, assembly of VCP hexamer does not depend on the presence of nucleotide. The VCP hexameric assembly can undergo dramatic conformational changes during nucleotide hydrolysis cycle, and it is generally believed that these conformational changes generate mechanical force, which is applied to substrate molecules to influence their stability and function. However, how precisely VCP generates force is unclear.
The ATP hydrolysis cycle
The ATP hydrolyzing activity is indispensable for the VCP functions. The two ATPase domains of VCP (D1 and D2) are not equivalent because the D2 domain displays higher ATPase activity than the D1 domain in wild-type protein. Nevertheless, their activities are dependent of each other. For example, nucleotide binding to the D1 domain is required for ATP binding to the D2 domain and nucleotide binding and hydrolysis in D2 is required for the D1 domain to hydrolyze ATP.