Highly prone to frequent gene rearrangement within a genome and horizontal gene transfer among bacterial and archaeal species. Proteins are often subject to post-translational modification as part of signal transduction pathways to rapidly regulate their activity or abundance in response to changes in the environment. In addition to post-translational modification by small chemical groups such as phosphate, acetyl, or methyl groups, proteins can also be Cinoxacin modified by the Epimedoside-A ubiquitin family of proteins. Ubiquitin is a highly conserved 76-amino acid protein that is covalently attached to substrates either as 
single ubiquitin moieties or as ubiquitin chains, with a poly-ubiquitin chain typically targeting its substrate for degradation via the 26S proteasome. Conjugation requires C-terminal processing of the ubiquitin precursor, followed by a cascade of E1, E2, and E3 enzymes that successively activate ubiquitin and direct its attachment to the substrate. Approximately ten other ubiquitin-like proteins, including Small Ubiquitin-like MOdifier, are conjugated to substrates via similar mechanisms, although each pathway has its own dedicated set of enzymes. The functions of SUMO and the other ubiquitin-like proteins are distinct from that of ubiquitin, with frequent cross-pathway regulation or interactions being uncovered. As the final enzyme in the conjugation pathway, the identification of ubiquitin ligase and understanding their substrate specificity, regulation, and functions remains a major goal of ubiquitin research. Three groups of E3s exist, each distinguishable by their sequence motifs and conjugation mechanism. RING and U-box E3s facilitate the transfer of ubiquitin from the E2 to the target by functioning as a scaffold or adaptor that recruits the E2 and substrate together, whereas HECT E3s form a thioester bond with activated ubiquitin, which is subsequently transferred from the HECT active cysteine residue directly onto the target substrate. Most E3s can polymerize chains onto substrates by ligating additional ubiquitin monomers onto any one of seven lysine residues of a previously conjugated ubiquitin. A final class of ubiquitin ligases, E4s, can only ligate ubiquitin onto other ubiquitin residues and therefore requires a pre-conjugated residue to generate a ubiquitin chain on a substrate protein. Ubiquitylation has multiple functions, with its most common role being the targeting of the modified substrate to the proteasome for degradation. For some proteins, however, conjugation to ubiquitin is not sufficient to target them to the proteasome. One of the key intermediaries in the pathway is Cdc48/p97, a highly conserved essential AAA-ATPase that bridges ubiquitylation and proteasomal degradation by using its “segregase” activity to extract a ubiquitylated substrate from membranes or from protein complexes. Cdc48/p97 is required for diverse biological processes including endoplasmic reticulum-associated degradation, processing of the transcription factor Spt23, and the proteolytic ubiquitin fusion degradation and N-end rule pathways. Cdc48 participates in these diverse pathways through its intrinsic ubiquitin-binding activity and by associating with an array of co-factors, including ubiquitin-binding proteins that recruit conjugated proteins to Cdc48, multiple RING finger E3s such as Ufd2, Hrd1, and Doa10, and a deubiquitinating enzyme that regulate chain length. Through this association with a wide variety of co-factors, Cdc48 is involved in a diverse array of cellular processes. The substrate recruitment factors typically bind to the Cdc48 N-terminal domain while the Ufd2 and Ufd3 substrate processing factors bind at the Cdc48 C-terminus.